Twins are two offspring In biology, offspring is the product of reproduction, a new organism produced by one or more parents resulting from the same pregnancy Pregnancy is the carrying of one or more offspring, known as a fetus or embryo, inside the uterus of a female. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets. Human pregnancy is the most studied of all mammalian pregnancies. Obstetrics is the surgical field that studies and cares for high risk pregnancy, usually born Childbirth is the culmination of a human pregnancy or gestation period with birth of one or more newborn infants from a woman's uterus. The process of normal human childbirth is categorized in three stages of labour: the shortening and dilation of the cervix, descent and birth of the infant, and birth of the placenta.. In some cases, childbirth is in close succession. They can be the same or different sex. Twins can either be monozygotic (MZ, colloquially "identical") or dizygotic (DZ, colloquially "fraternal" or "non-identical").

Contents

Definitions

The general term for more than one offspring in the same pregnancy (multiple birth A multiple birth occurs when more than one fetus is carried to term in a single pregnancy. Different names for multiple births are used, depending on the number of offspring. Common multiples are two and three, known as twins and triplets. These and other multiple births occur to varying degrees in most animal species, although the term is most) is multiples; a fetus A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth. The plural is fetuses which develops alone in the womb is called a singleton.

Statistics

The number of living human twins in the world has been estimated to be approximately 125 million in 2006[1] (roughly 1.9% of the world population The world population is the total number of living humans on Earth at a given time. As of 8 December 2009, the Earth's population is estimated by the United States Census Bureau to be 6.802 billion. The world population has been growing continuously since the end of the Black Death around 1400. The fastest rates of world population growth were), with just 10 million monozygotic twins (roughly 0.2% of the world population and 8% of all twins). The twin birth rate in the United States in 2004, 2005 and 2006 was slightly above 32 twin live births per 1,000 live births[2].

Due to the limited size of the mother's womb, multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting only 37 weeks (3 weeks less than full term) on average.[3] Since premature births In humans, preterm birth refers to the birth of a baby of less than 37 weeks gestational age. Premature birth, commonly used as a synonym for preterm birth, refers to the birth of a premature infant. The child may commonly be referred to throughout their life as being born a "preemie" or "preemie baby". Because it is by far the can have health consequences for the babies, twin births are often handled with special precautions.

The Yoruba Christianity 60%, Islam 30%, Orisha veneration and Ifá 10%, a large Nigerian ethnic group, have the highest rate of twinning in the world, at 45 twins per 1,000 live births.[4][5][6] Some researchers have claimed this may be because of high consumption of a specific type of yam Yam is the common name for some species in the genus Dioscorea . These are perennial herbaceous vines cultivated for the consumption of their starchy tubers in Africa, Asia, Latin America and Oceania. There are many cultivars of yam, Dioscorea rotundata or white yam containing a natural hormone A hormone is a chemical released by one or more cells that affects cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. It is essentially a chemical messenger that transports a signal from one cell to another. All multicellular organisms produce hormones; plant hormones are also called phytoestrogen Phytoestrogens, sometimes called "dietary estrogens", are a diverse group of naturally occurring non steroidal plant compounds that, because of their structural similarity with estradiol , have the ability to cause estrogenic or/and antiestrogenic effects which may stimulate the ovaries The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in females are homologous to testes in males, in that they are both gonads and endocrine glands to release an egg from each side.[7][8]

Zygosity

Zygosity Zygosity is the similarity of alleles of a gene for a trait in an organism. If both alleles are the same, the organism is homozygous for the trait. If both alleles are different, the organism is heterozygous for that trait. If one allele is missing, it is hemizygous, and if both alleles are missing, it is nullizygous is the degree of identity in the genome In modern molecular biology, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA of twins. There are five common variations of twinning. The three most common variations are all dizygotic:

The other two variations are monozygotic twins:

Among non-twin births, male singletons are slightly (about five percent) more common than female singletons. The rates for singletons vary slightly by country. For example, the sex ratio of birth in the US is 1.05 males/female,[9] while it is 1.07 males/female in Italy.[10] However, males are also more susceptible than females to death in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.

Eight month old dizygotic twin girls napping

Dizygotic twins

Dizygotic twins (commonly known as fraternal twins, but also referred to as non-identical twins or biovular twins) usually occur when two fertilized Fertilization , is the fusion of gametes to produce a new organism. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation eggs are implanted in the uterine wall at the same time. When two eggs are independently fertilized by two different sperm cells, DZ twins result. The two eggs, or ova, form two zygotes A zygote , or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development. Zygotes are usually produced by a fertilization event between two haploid cells — an ovum from a female and, hence the terms dizygotic and biovular.

Dizygotic twins, like any other siblings, have an extremely small chance of having the same chromosome profile. Like any other siblings In most societies throughout the world, siblings usually grow up together and spend a good deal of their childhood with each other. This genetic and physical closeness may be marked by the development of strong emotional associations such as love or enmity. The sibling bond is often complicated and is influenced by factors such as parental, DZ twins may look similar, particularly given that they are the same age. However, DZ twins may also look very different from each other. They may be of different sexes or the same sex. The same holds true for brothers and sisters from the same parents, meaning that DZ twins are simply brothers and/or sisters who happen to be the same age.

Studies show that there is a genetic basis for DZ twinning. However, it is only their mother that has any effect on the chances of having DZ twins; there is no known mechanism for a father to cause the release of more than one ovum An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization. In lower plants and algae, the ovum is also often called oosphere. Dizygotic twinning ranges from six per thousand births in Japan (similar to the rate of monozygotic twins) to 14 and more per thousand in some African countries.[7]

DZ twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.[11] With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly. For example, in New York City New York is the most populous city in the United States, and the center of the New York metropolitan area, which is one of the most populous urban areas in the world. A leading global city, New York exerts a powerful influence over worldwide commerce, finance, culture, fashion and entertainment. As host of the United Nations headquarters, it is's Upper East Side Before the arrival of Europeans, the mouths of streams that eroded gullies in the East River bluffs are conjectured to have been the sites of fishing camps used by the Lenape, whose controlled burns once a generation or so kept the dense canopy of oak-hickory forest open at ground level there were 3,707 twin births in 1995; there were 4,153 in 2003; and there were 4,655 in 2004. Triplet births have also risen, from 60 in 1995 to 299 in 2004.

Monozygotic twins

Comparison of zygote development in monozygotic and dizygotic twins. In the uterus, a majority of monozygotic twins (60–70%) share the same placenta The placenta is an organ unique to mammals that connects the developing fetus to the uterine wall. The placenta supplies the fetus with oxygen and food, and allows fetal waste to be disposed via the maternal kidneys. The word placenta comes from the Latin for cake, from Greek plakóenta/plakoúnta, accusative of plakóeis/plakoús – πλακόε but have separate amniotic sacs The amniotic sac and its filling provide a liquid which surrounds and cushions the fetus. It allows the fetus to move freely without the walls of the uterus being too tight against its body. Buoyancy is also provided. In 18–30% of monozygotic twins each fetus has a separate placenta and a separate amniotic sac. A small number (1–2%) of monozygotic twins share the same placenta and amniotic sac. Dizygotic twins each have their own placenta and own amniotic sac.

Monozygotic twins, frequently referred to as identical twins, occur when a single egg An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization. In lower plants and algae, the ovum is also often called oosphere is fertilized to form one zygote A zygote , or zygocyte, is the initial cell formed when a new organism is produced by means of sexual reproduction. A zygote is synthesized from the union of two gametes, and constitutes the first stage in a unique organism's development. Zygotes are usually produced by a fertilization event between two haploid cells — an ovum from a female and (monozygotic) which then divides into two separate embryos An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination. In humans, it is called an embryo until about eight weeks after fertilization (i.e. ten weeks LMP), and from then it is instead called a fetus.

Monozygotic twins are almost always the same sex and their traits and physical appearances are very similar but not exactly the same; although they have nearly identical DNA,[12] environmental conditions both inside the womb and throughout their lives influence the switching on and off of various genes. Division of the zygote into two embryos is not considered to be a hereditary trait, but rather an anomaly that occurs in birthing at a rate of about three in every 1000 deliveries worldwide,[13] regardless of race. The two embryos develop into fetuses A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth. The plural is fetuses sharing the same womb. When one egg is fertilized by one sperm cell, and then divides and separates, two identical cells will result.

Monozygotic twins are almost genetically identical (unless there has been a mutation during development) and they are almost always the same sex. On rare occasions, monozygotic twins may express different phenotypes A phenotype is any observable characteristic or trait of an organism: such as its morphology, development, biochemical or physiological properties, or behavior. Phenotypes result from the expression of an organism's genes as well as the influence of environmental factors and possible interactions between the two. The genotype of an organism is the (normally due to an environmental factor or the deactivation of different X chromosomes in monozygotic female twins), and in some extremely rare cases, due to aneuploidy Aneuploidy is an abnormal number of chromosomes, and is a type of chromosome abnormality. An extra or missing chromosome is a common cause of genetic disorders . Some cancer cells also have abnormal numbers of chromosomes. Aneuploidy occurs during cell division when the chromosomes don't separate properly between the two cells. Chromosome, twins may express different sexual phenotypes A phenotype is any observable characteristic or trait of an organism: such as its morphology, development, biochemical or physiological properties, or behavior. Phenotypes result from the expression of an organism's genes as well as the influence of environmental factors and possible interactions between the two. The genotype of an organism is the, normally due to an XXY Klinefelter's syndrome Klinefelter's syndrome, 47, XXY or XXY syndrome is a condition in which males have an extra X sex chromosome. While females have an XX chromosomal makeup, and males an XY, affected individuals have at least two X chromosomes and at least one Y chromosome. Klinefelter's syndrome is the most common sex chromosome disorder and the second most common zygote splitting unevenly[14][15]. Monozygotic twins look alike, although they do not have the same fingerprints A fingerprint is an impression of the friction ridges on all parts of the finger. A friction ridge is a raised portion of the epidermis on the palmar or digits (fingers and toes) or plantar (sole) skin, consisting of one or more connected ridge units of friction ridge skin. These are sometimes known as "epidermal ridges" which are caused (which are environmental as well as genetic). As they mature, MZ twins often become less alike because of lifestyle choices or external influences. Genetically speaking, the children of MZ twins are half-siblings rather than cousins. It is estimated that there are around 10 million monozygotic twins and triplets in the world.

The likelihood of a single fertilisation resulting in MZ twins appears to be a random event, not a hereditary trait, and is uniformly distributed in all populations around the world.[11] This is in marked contrast to DZ twinning, which ranges from about six per thousand births in Japan (almost similar to the rate of MZ twins, which is around 4–5) to 15 and more per thousand in some parts of India[16] and up to 24 in the US, which might mainly be due to IVF (in vitro fertilisation The term in vitro, from the [Latin] root meaning within the glass, is used, because early biological experiments involving cultivation of tissues outside the living organism from which they came, were carried out in glass containers such as beakers, test tubes, or petri dishes. Today, the term in vitro is used to refer to any biological procedure). The exact cause for the splitting of a zygote or embryo is unknown.

Monozygotic twins have nearly identical DNA Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information. DNA is often compared to a set of blueprints or a recipe, or a code, since it contains the instructions needed, but differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigenetic modification In biology, the term epigenetics refers to changes in phenotype or gene expression caused by mechanisms other than changes in the underlying DNA sequence, hence the name epi- (Greek: over; above) -genetics. These changes may remain through cell divisions for the remainder of the cell's life and may also last for multiple generations. However,. A study of 80 pairs of human twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences between MZ twins increases with age. Fifty-year-old twins had over three times the epigenetic difference of three-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference.[17] However, certain characteristics become more alike as twins age, such as IQ and personality.[18][19] This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behaviour.

A recent theory posits that monozygotic twins are formed after a blastocyst The blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula, but before implantation. It possesses an inner cell mass , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast, which later forms the placenta. The trophoblast surrounds the inner cell mass and essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half. That leaves the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop. The research was presented at a meeting of the European Society of Human Reproduction and Embryology The European Society of Human Reproduction and Embryology was created in 1985 by both Professor R.G. Edwards from Cambridge, and Dr. J. Cohen from Paris, who felt that the study and research in the field of reproduction needed to be encouraged and recognized in Lyon Lyon , often Anglicized as Lyons, is a city in east-central France in the region Rhône-Alpes, situated between Paris and Marseille. Its name is pronounced [ljɔ̃] ( listen) in French and Arpitan, and /liːˈɒn/ or /ˈlaɪ.ənz/ in English. Lyon is located at 470 km from Paris, 320 km from Marseille, 160 km from Geneva, 280 km from Turin, 450 km, France France (pronounced /ˈfræns/ or /ˈfrɑːns/; French pronunciation (help·info): [fʁɑ̃s]), officially the French Republic (French: République française, pronounced: [ʁepyblik fʁɑ̃sɛz]), is a member state of the European Union located in its western region, with several overseas territories and islands located on other continents. France. Utilizing computer software to take photos every two minutes of 33 embryos growing in a laboratory, Dr. Dianna Payne, a visiting research fellow at the Mio Fertility Clinic in Japan, documented for the first time the early days of twin development. Payne also discovered explanation for why in-vitro fertilization techniques are more likely to create twins. Only about three pairs of twins per 1,000 deliveries occur as a result of natural conception, while for IVF deliveries, there are nearly 21 pairs of twins for every 1,000.[20][21]

Half twin

Main article: Half twin Semi-identical twins are the result of a very rare form of twinning in which the twins inherit exactly the same genes from their mother but different genes from their father. The exact mechanism of their conception is not well-understood, but could theoretically occur in in polar body twinning where sperm cells fertilize both the ovum and the

The extremely rare half twins or semi-identical twins are twins that inherit exactly the same genes from their mother but different genes from their father. The exact mechanism of their conception is not well understood, but could theoretically occur in polar body twinning A polar body is a cell structure found inside an ovum. Both animal and plant ova possess it. It is also known as a polar cell where sperm cells fertilize both the ovum An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization. In lower plants and algae, the ovum is also often called oosphere and the second polar body A polar body is a cell structure found inside an ovum. Both animal and plant ova possess it. It is also known as a polar cell.

Degree of separation

Various types of chorionicity and amniosity (how the baby's sac looks) in monozygotic (one egg/identical) twins as a result of when the fertilized egg divides

In very rare cases, twins become conjoined twins Conjoined/Siamese twins are identical twins whose bodies are joined in utero. A rare phenomenon, the occurrence is estimated to range from 1 in 50,000 births to 1 in 200,000 births, with a somewhat higher incidence in Southwest Asia and Africa. Approximately half are stillborn, and a smaller fraction of pairs born alive have abnormalities. Furthermore, there is a various degree of shared environment of twins in the womb, potentially leading to pregnancy complications.

Dichorionic-Diamniotic

Normally, twins have two separate (di- being a numerical prefix Numerical prefixes are prefixes usually derived from the words for numbers in various languages, most commonly Greek and Latin, although this is not always the case for two) chorions The chorion is one of the membranes that exists during pregnancy between the developing fetus and mother. It is formed by extraembryonic mesoderm and the two layers of trophoblast and surrounds the embryo and other membranes. The chorionic villi emerge from the chorion, invade the endometrium, and allow transfer of nutrients from maternal blood to and amniotic sacs The amniotic sac and its filling provide a liquid which surrounds and cushions the fetus. It allows the fetus to move freely without the walls of the uterus being too tight against its body. Buoyancy is also provided, termed Dichorionic-Diamniotic or "DiDi". It occurs in almost all cases of dizygotic twins (except in very rare cases of fusion between their blastocysts[22] ), in 99.7% of all pregnancies,[23] and in 18–36%[24] (or around 25%[22]) of monozygotic Twins are two offspring resulting from the same pregnancy, usually born in close succession. They can be the same or different sex. Twins can either be monozygotic or dizygotic (DZ, colloquially "fraternal" or "non-identical") (identical) twins. Dichorionic-Diamniotic twins form when splitting takes place after the third day after fertilization Fertilization , is the fusion of gametes to produce a new organism. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo. Depending on the animal species, the process can occur within the body of the female in internal fertilisation, or outside in the case of external fertilisation.[22]

DiDi twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons. [25]

Monochorionic

Main article: Monochorionic twins Monochorionic twins are twins that share the same placenta. The condition occurs in 0.3% of all pregnancies, and in 75% of monozygotic twins, when the split takes place after the third day after fertilization. The remaining 25% of monozygous twins become dichorionic diamniotic. The condition may affect any type of multiple birth, resulting in

Monochorionic twins Monochorionic twins are twins that share the same placenta. The condition occurs in 0.3% of all pregnancies, and in 75% of monozygotic twins, when the split takes place after the third day after fertilization. The remaining 25% of monozygous twins become dichorionic diamniotic. The condition may affect any type of multiple birth, resulting in share the same placenta.

Monochorionic twins generally have two amniotic sacs (called Monochorionic-Diamniotic "MoDi"), which occurs in 60–70% of the pregnancies with monozygotic twins.[24] Sometimes, however, in the case of monoamniotic twins (Monochorionic-Monoamniotic "MoMo"), they also share the same amniotic sac. Monoamniotic twins occur when the split takes place after the ninth day after fertilization. [22] Monoamniotic twins are always monozygotic (identical twins).[26] Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused. [22]

Monochorionic-Monoamniotic

Main article: Monoamniotic twins

Monoamniotic twins also share the same amniotic sac. Monoamniotic twins are always monozygotic.[26] Monochorionic twins, on the other hand, may be dizygotic. Furthermore, two placentas (dichorionic twins) does not necessarily exclude monozygocity since monozygotic twins can have two placentas. Chorionicity and amniocity are a result of the division time. Dichorionic twins divide within the first 4 days. Monoamnionic twins divide after the first week.

It occurs in 1–2% of monozygotic twin pregnancies.[24]

The survival rate for monoamniotic twins is somewhere between 50%[26] to 60%[27].

Consequently, if twins are monoamniotic that means that the two babies will be sharing a placenta and as a result, due to the small capacity of sharing a sac, the umbilical cord has an increase chance of being tangled around the babies. Because of this, there is an increase chance that the newborns may be a miscarriage due to the lack of oxygen, or maybe be born mentally challenged due to the lack of oxygen reaching the brain.

Conjoined

Main article: Conjoined twins

When the division of the developing zygote into 2 embryos occurs, 99% of the time it is within 8 days of fertilization. If the division of the zygote occurs later than the 8 days then conjoined twins are usually the result.

Mortality is highest for conjoined twins due to the many complications resulting from shared organs.

Demographics

A recent study found that vegan mothers are one-fifth as likely to have twins as vegetarian or omnivore mothers, and concluded that "Genotypes favoring elevated IGF and diets including dairy products, especially in areas where growth hormone is given to cattle, appear to enhance the chances of multiple pregnancies due to ovarian stimulation."[28]

From 1980–97, the number of twin births in the United States rose 52%.[29] This rise can at least partly be attributed to the increasing popularity of fertility drugs like Clomid and procedures such as in vitro fertilization, which result in multiple births more frequently than unassisted fertilizations do. It may also be linked to the increase of growth hormones in food.[28]

Ethnicity

About 1 in 90 human births (1.1%) results from a twin pregnancy.[30] The rate of dizygotic twinning varies greatly among ethnic groups, ranging as high as about 45 per 1000 births for the Yoruba to 10% for Linha São Pedro, a tiny Brazilian settlement which belongs to the city of Cândido Godói.[31] In Cândido Godói, one in five pregnancies have resulted in twins.[32] The Argentine historian Jorge Camarasa has put forward a theory that experiments of the Nazi doctor Josef Mengele could be responsible for the high ratio of twins in the area.[33] His theory was rejected by Brazilian scientists who had studied twins living in Linha São Pedro; they suggested genetic factors within that community as a more likely explanation.[34] A high twinning rate has also been observed in other places of the world, including Igbo-Ora in Nigeria[35] and Kodinji in India.[36]

The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of multiple births". In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. Nevertheless, the rate of monozygotic twins remains at about 1 in 333 across the globe.

In a study on the maternity records of 5750 Hausa women living in the Savannah zone of Nigeria, there were 40 twins and 2 triplets per 1000 births. Twenty-six per cent of twins were monozygous. The incidence of multiple births, which was about five times higher than that observed in any western population, was significantly lower than that of other ethnic groups, who live in the hot and humid climate of the southern part of the country. The incidence of multiple births was related to maternal age but did not bear any association to the climate or prevalence of malaria.[37]

Predisposing factors

The predisposing factors of monozygotic twinning are unknown.

Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:

Women undergoing certain fertility treatments may have a greater chance of dizygotic multiple births. This can vary depending on what types of fertility treatments are used. With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples.

Delivery interval

A 15-year German study[38] of 8,220 vaginally delivered twins (that is, 4,110 pregnancies) in Hesse yielded a mean delivery time interval of 13.5 minutes.[39] The delivery interval between the twins was measured as follows:

  • Within 15 minutes: 75.8%
  • 16-30 minutes: 16.4%
  • 31-45 minutes: 4.3%
  • 46-60 minutes: 1.7%
  • Over 60 minutes: 1.8% (72 instances)

The study stated that the occurrence of complications "was found to be more likely with increasing twin-to-twin delivery time interval" and suggested that the interval be kept short, though it noted that the study did not examine causes of complications and did not control for factors such as the level of experience of the obstetrician, the wish of the women giving birth, or the "management strategies" of the procedure of delivering the second twin.

Complications of twin pregnancy

Vanishing twins

Main article: Vanishing twin

Researchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term, because the other has died very early in the pregnancy and has not been detected or recorded.[40] Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. This is known as vanishing twin syndrome.

Chang and Eng Bunker, born in Siam (now Thailand) in 1811, were the origin of the term "Siamese twins".

Conjoined twins

Main article: Conjoined twin

Conjoined twins (or the deprecated term "Siamese twins") are monozygotic twins whose bodies are joined together during pregnancy. This occurs where the single zygote of MZ twins fails to separate completely, and the zygote starts to split after day 12[22] following fertilization. This condition occurs in about 1 in 50,000 human pregnancies. Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as the brain, heart or liver.

Chimerism

Main article: Chimera (genetics)

A chimera is an ordinary person or animal except that some of their parts actually came from their twin or from the mother. A chimera may arise either from monozygotic twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which can be identified by chromosomal comparisons from various parts of the body. The number of cells derived from each fetus can vary from one part of the body to another, and often leads to characteristic mosaicism skin coloration in human chimeras. A chimera may be intersex, composed of cells from a male twin and a female twin. In addition, in certain cases the person or chimera may have two sets of DNA.

Parasitic twins

Main article: Parasitic twin

Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a parasite towards the other. Sometimes the parasitic twin becomes an almost indistinguishable part of the other, and sometimes this needs to be medically dealt with.

Partial molar twins

A very rare type of parasitic twinning is one where a single viable twin is endangered when the other zygote becomes cancerous, or molar. This means that the molar zygote's cellular division continues unchecked, resulting in a cancerous growth that overtakes the viable fetus. Typically, this results when one twin has either triploidy or complete paternal uniparental disomy, resulting in little or no fetus and a cancerous, overgrown placenta, resembling a bunch of grapes.

Miscarried twin

Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. This occurrence is similar to the vanishing twin syndrome, but typically occurs later than the vanishing twin syndrome.

Low birth weight

Twins typically suffer from the lower birth weights and greater likelihood of prematurity that is more commonly associated with the higher multiple pregnancies. Throughout their lives twins tend to be smaller than singletons on average.

Twin-to-twin transfusion syndrome

Main article: Twin-to-twin transfusion syndrome

Monozygotic twins who share a placenta can develop twin-to-twin transfusion syndrome. This condition means that blood from one twin is being diverted into the other twin. One twin, the 'donor' twin, is small and anemic, the other, the 'recipient' twin, is large and polycythemic. The lives of both twins are endangered by this condition.

Human twin studies

Main article: Twin study

Twin studies are utilized in an attempt to determine how much of a particular trait is attributable to either genetics or environmental influence. These studies compare monozygotic and dizygotic twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from epigenetic and environmental influence. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been used widely in the exploration of human nature. However, the utility and accuracy of these twin studies has been called into question and remains controversial. Classical twin studies have largely been replaced in favor of modern molecular genetic methodologies.

Unusual twinnings

Among dizygotic twins, in rare cases, the eggs are fertilized at different times with two or more acts of sexual intercourse, either within one menstrual cycle (superfecundation) or, even more rarely, later on in the pregnancy (superfetation). This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). This phenomenon is known as heteropaternal superfecundation. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details.

Dizygotic twins from biracial couples can sometimes be mixed twins, which exhibit differing ethnic and racial features. One such pairing was born in Germany in 2008 to a white father from Germany and a black mother from Ghana.[41]

Heterotopic pregnancy is an exceedingly rare type of dizygotic twinning in which one twin implants in the uterus as normal and the other remains in the fallopian tube as an ectopic pregnancy. Ectopic pregnancies must be resolved because they can be life-threatening to the mother. However, in most cases, the intrauterine pregnancy can be salvaged.

Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). The probability of this is so vanishingly small (only 3 documented cases[42]) that multiples having different sexes is universally accepted as a sound basis for a clinical determination that in utero multiples are not monozygotic. When monozygotic twins are born with different sexes it is because of chromosomal birth defects. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.

Semi-identical twins

See also: half twin

Monozygotic twins can develop differently, due to different genes being activated.[43] More unusual are "semi-identical twins". These "half-identical twins" are hypothesized to occur when an unfertilized egg cleaves into two identical attached ova and which are viable for fertilization. Both cloned ova are then fertilized by different sperm and the coalesced eggs undergo further cell duplications developing as a chimeric blastomere. If this blastomere then undergoes a twinning event, two embryos will be formed, each of which have different paternal genes and identical maternal genes.

This results in a set of twins with identical genes from the mother's side, but different genes from the father's side. Cells in each fetus carry genes from either sperm, resulting in chimeras. This form had been speculated until only recently being recorded in western medicine.[44][45][46]

Twin calves of the Hereford breed

Animal twins

Twins are common in many animal species, such as cats, sheep, ferrets and deer. The incidence of twinning among cattle is about 1–4%, and research is under way to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed. The nine-banded armadillo (Dasypus novemcinctus) has identical twins (usually four babies) as its regular reproduction and not as exceptional cases.[47][48]

See also

Wikimedia Commons has media related to: Twins

References

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Further reading

Twin conditions (Q89.3-Q89.4, 759.3-759.4)
Zygosity Monozygotic · Dizygotic · Half twin
Monochorionic twins Twin-to-twin transfusion syndrome · Twin reversed arterial perfusion · Monoamniotic twins
Conjoined twins Parasitic twin (Craniopagus parasiticus, Fetus in fetu)
Other Vanishing twin · Chimerism

Categories: Congenital disorders | Twins | Reproduction | Zoology

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