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'''Twins''' in animal biology is a form of [[multiple birth]] in which the mother gives birth to two offspring from the same pregnancy, some of the same gender, others of opposite. |
'''Twins''' in animal biology is a form of [[multiple birth]] in which the mother gives birth to two offspring from the same pregnancy, some of the same gender, others of opposite. |
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Revision as of 19:38, 17 March 2007
Twins in animal biology is a form of multiple birth in which the mother gives birth to two offspring from the same pregnancy, some of the same gender, others of opposite.
There are estimated to be approximately 125 million twins and triplets in the world, and just 10 million identical twins.[citation needed]
Triplets refers to three offspring from the same pregnancy. The general term for more than one offspring from the same pregnancy is multiples. A fetus alone in the womb is called a singleton.
Human twins are two individuals who have shared the uterus during a single pregnancy and are usually, but not necessarily, born in close succession. Due to the limited size of the mother's womb, multiple pregnancies are much less likely to carry to full term than singleton births, with twin pregnancies lasting only 37 weeks on average, 3 weeks less than full term. Since premature births can have health consequences for the babies, twin births are often handled with special precautions.
Types of twins
There are five variations of twinning that commonly occur. The three most common variations are all fraternal: (1) male-female twins are the most common result, at about 40% of all twins born; (2) female fraternal twins (sometimes called sororal twins); (3) male fraternal twins. The last two are identical: (4) female identical twins and (5) (least common) male identical twins.
Another variety of twins, "polar body twins", is a phenomemon that is hypothesised to occur but has not been proven. Polar body twinning would result in "half-identical" twins.[1]
Fraternal twins
Fraternal twins (commonly known as "non-identical twins") usually occur when two fertilised eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins. When two eggs are independently fertilised by two different sperm cells, fraternal twins result.
Dizygotic twins, like any other siblings, have an extremely small chance of having the exact same chromosome profile. Like any other siblings, fraternal twins may look very similar, particularly given that they are the same age. However, fraternal twins may also look very different from each other. They may be a different sex or the same sex. The same holds true for brothers and sisters from the same parents, meaning that fraternal twins are simply brothers and/or sisters who happen to have the same age.
Studies show that there is a genetic basis for fraternal twinning. However, it is only the female partner that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum. Fraternal twinning ranges from 1 or 2 per thousand births in Japan (similar to the rate of identical twins) to 14 and more per thousand in some African states.[citation needed]
Fraternals are also more common for older mothers, with twinning rates doubling in mothers over the age of 35.[citation needed] 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's Upper East Side 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.
Identical twins
Identical twins occur when a single egg is fertilised to form one zygote (monozygotic) which then divides into two separate embryos. Although their traits and physical appearances are not exactly the same due to environmental conditions in both the womb and outside the womb, they do have the same DNA. This is not considered to be a hereditary trait, but rather an anomaly that occurs in birthing at a rate of about 3 in every 1000 deliveries worldwide,[2] regardless of ethnic background. The two embryos develop into fetuses sharing the same womb. When one egg is fertilised by one sperm cell, and then divides and separates, two identical cells will result. If the zygote splits very early (in the first 2 days after fertilization) they may develop separate placentas (chorion) and separate sacs (amnion). These are called dichorionic, diamniotic (or "di/di") twins. While all fraternal twins are "di/di," this occurs 20-30% of the time in identical twins. Most of the time in identical twins the zygote will split after 2 days, resulting in a shared placenta, but two separate sacs. These are called monochorionic, diamniotic ("mono/di") twins. In about 1% of identical twins the splitting occurs late enough to result in both a shared placenta and a shared sac. These are called monochorionic, monoamniotic ("mono/mono") twins. Finally, the zygote may split extremely late, resulting in conjoined twins. Mortality is highest for conjoined twins due to the many complications resulting from shared organs. Mono/mono twins have an overall in-utero mortality of about 60%, principally due to cord entanglement prior to 32 weeks gestation. Mono/di twins have about a 25% mortality due to twin-twin transfusion. Di/di twins have the lowest mortality risk at about 9%, although that is still significantly higher than that of singletons.[3].
Monozygotic twins are genetically identical (unless there has been a mutation in development) and they are the same gender. (On extremely rare occasions, an original XXY zygote may form monozygotic boy/girl twins by dropping the Y chromosome for one twin and the extra X chromosome for the other.) Monozygotic twins generally look alike. Fine physical details such as fingerprints will differ. As they mature, identical twins often become less alike because of lifestyle choices or external influences. Genetically speaking, the children of identical twins are half-siblings rather than cousins. It is estimated that there are around 125 million identical twins and triplets in the world.
The likelihood of a single fertilisation resulting in identical twins appears to be a random event, not a hereditary trait, and is uniformly distributed in all populations around the world.[citation needed] This is in marked contrast to fraternal twinning which ranges from about 6 per thousand births in Japan (almost similar to the rate of identical twins, which is around 4-5) to 15 and more per thousand in some parts of India[4] (and up to 24 in the US, which might mainly be due to IVF, in vitro fertilisation). The exact cause for the splitting of a zygote or embryo is unknown.
Studies have shown that identical twins reared in different environments share similar personality traits, mannerisms, job choices, attitudes, and interests. These findings add to the belief that many behaviors are derived from genes.
Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigenetic modification. A study of 80 pairs of human twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences between identical twins increases with age. 50-year-old twins had over three times the epigenetic difference of 3-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. [5]. However, certain characteristics become more alike as twins age, such as IQ and personality [6] [7]. This phenomenon illustrates that genetics play a dominant role in many aspects of human characteristics and behaviour.
Mirror twins
Some monozygotic twins are known as "mirror twins" or mirror image twins. These are identical twins with opposite features, that is, one may be right-handed and the other left, mirrored hair curls, etc. This condition is comparatively rare in humans: about 25% of all identical twins are mirrored to some degree, but in the vast majority of these cases it is a very minor feature - such as mirrored fingerprint sworls.[4] Mirroring results from a late split of the fertilised egg at around 9-12 days. One mirror may have situs inversus, where some or all of the organs are on the opposite side of the body, such as the left ventricle of the heart on the right (dextrocardia). Such conditions are usually associated with a higher incidence of other birth defects, and are very rare.
Polar twins
Polar twins are monozygotic but develop differently, due to different genes being activated.[8] These "half-identical twins" may occur when one egg is split into two, and then the two eggs are fertilised by different sperm. This would result in a set of twins with identical genes from the mother's side, but different ones from the father's side. Scientists have still not shown that polar twins do indeed exist.
Demographics
A recent study found that vegan mothers are five times less likely to have twins than those who eat animal products.[9]
From 1980–97, the number of twin births in the United States rose 52%.[10] This rise can at least partly be attributed to the increasing popularity of fertility drugs like Clomid and procedures like in vitro fertilisation, which result in multiple births more frequently than unassisted fertilisations do. It may also be linked to the increase of growth hormones in food.[9]
Ethnicity
Historically, about 1 in 80 human births (1.25%) results from a twin pregnancy. The rate of fraternal twinning varies greatly among ethnic groups, ranging as high as about 6% for the Yoruba or 10% for a tiny Brazilian village.[11] 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. Thus, approximately 5.8% of children born in the US in 2001 were twins. Among Hausa of Nigeria and Niger the incidence of multiple births was studied using the maternity records of 5750 Hausa women living in the savannah zone of Nigeria. There were 40 twins and 2 triplets/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 country. The incidence of multiple births was related to maternal age but did not bear any association to the climate or prevalence of malaria.[12]Nevertheless, the rate of identical twins remains at about 1 in 333 across the globe, further suggesting that pregnancies resulting in identical twins occur randomly...
Predisposing factors
The cause of monozygotic twinning is unknown..
Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:
- She is of African descent (especially Yoruba, or Hausa)
- She is between the age of 30 and 40 years
- She is greater than average height and weight
- She has had several previous pregnancies.
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. Many fertility treatments have no effect on the likelihood of multiple births.
There is also speculation that the West African predisposition to twinning is due to the large amount of yams in their diet because yams contain phytoestrogen, which may stimulate the ovaries. Most likely this is nothing but a myth — phytoestrogens would tend to suppress twinning by reducing gonadotropin levels.
Complications of twin pregnancy
Vanishing twins
Researchers suspect[citation needed] 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 not been detected or recorded. 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.
Conjoined twins
Conjoined twins (or "Siamese twins") are monozygotic twins whose bodies are joined together at birth. This occurs where the single zygote of identical twins fails to separate completely, and the zygote starts to split after day 13 following fertilisation. 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 brain, heart or liver.
Chimerism
A chimera is an ordinary person or animal except that some of his or her parts actually came from his or her twin or from the mother. A chimera may arise either from identical 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 colouration in human chimeras. A chimera may be a hermaphrodite, composed of cells from a male twin and a female twin.
Parasitic twins
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.
Partial Molar twins
A very rare type of parasitic twinning is where a single viable twin is endangered when the other zygote become 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 female 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.
Human twin studies
Twin studies are studies that assess identical (monozygotic) twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from 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 invaluable in the exploration of human nature.
Unusual twinnings
There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most obstetricians or midwives may go their entire careers without encountering a single case.
Among fraternal twins, in rare cases, the eggs are fertilised 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.
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[13]) that multiples having different genders 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 genders 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.
Twins and health
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
Identical 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.
Twins and society
Identical twins can cause some observers to question their own beliefs about identity and individuality, and results in twins being treated differently or regarded as special. In some earlier cultures, twins were either elevated to mystical status and worshipped (for a recent case see Johnny and Luther Htoo), and sometimes sacrificed in others. Parents frequently give twins names that rhyme or are alliterative, further blurring the distinctions between them. Twins often claim to be psychically or telepathically connected to their twin. Twins tend to socialise with other twins, forming clubs and marrying other twins more often than would be expected by chance.[citation needed] When twins have children with other twins, their offspring are double, triple or quadruple cousins.
Animal twins
Twins are common in many animal species, such as cats, sheep, and ferrets. The incidence of twinning among cattle is about 1-4%, and research is underway to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed. A species of armadillo (Dasypus novemcinctus) has identical twins (usually four babies) as its regular reproduction and not as exceptional cases.[5]
See also
- List of twins
- List of twins in fiction
- Multiple birth
- List of multiple births
- NICU (Neonatal Intensive Care Unit)
Notes and references
- ^ [1]
- ^ [[2]]
- ^ Benirschike K, "Multiple Gestation" in Creasy RK, Resnik R, (eds.) Maternal-Fetal Medicine: Principles and Practice; 5th ed. (2004):55-62. Philedelphia: Saunders.
- ^ Oleszczuk JJ, Keith DM et al. Projection of Population-based twinning rates through the year 2100. J Reprod Med. 44(11), 1999. 913–921.
- ^ Fraga, M., Ballestar, E., Paz, M., Ropero, S., Setien, F., Ballestar, M., Heine-Suner, D., Cigudosa, J., Urioste, M., Benitez, J., Boix-Chornet, M., Sanchez-Aguilera, A., Ling, C., Carlsson, E., Poulsen, P., Vaag, A., Stephan, Z., Spector, T., Wu, Y., Plass, C., Esteller, M. | title=Epigenetic differences arise during the lifetime of monozygotic twins | journal=Proceedings of the National Academy of Sciences | volume=102 | issue=30 | year=2005 | pages=10413–10414
- ^ Segal, K. Entwined Lives: Twins and What They Tell Us About Human Behavior, Plume Publishing, 2002
- ^ DeFries, J. McGuffin, P., McClearn, G., Plomin, R., Behavioural Genetics, Worth Publishers, 4th Edition, 2000
- ^ Non-identical Monozygotic Twins
- ^ a b Vegan moms less likely to have twins
- ^ National Vital Statistics Reports Vol. 47, No. 24: "Trends in Twin and Triplet Births: 1980-1997" (PDF)
- ^ U. Matte et al. Study on possible increase in twinning rate at a small village in south Brazil. Acta Genet Med Gemellol (Roma). 45(4), 1996. 431–437.
- ^ ""MULTIPLE BIRTHS IN HAUSA WOMEN"". "An International Journal of Obstetrics and Gynaecology".
{{cite web}}: Cite has empty unknown parameter:|1=(help) - ^ [3]
- Nieuwint A, Van Zalen-Sprock R, Hummel P, Pals G, Van Vugt J, Van Der Harten H, Heins Y, Madan K. (1999). "'Identical' twins with discordant karyotypes". Prenatal Diagnosis. 19 (1): 72–6. PMID 10073913.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - Wenk RE, Houtz T, Brooks M, Chiafari FA (1992). "How frequent is heteropaternal superfecundation?". Acta geneticae medicae et gemellologiae. 41 (1): 43–7. PMID 1488855.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - Girela E, Lorente JA, Alvarez JC, Rodrigo MD, Lorent M, Villaneuva E (1997). "Indisputable double paternity in dizygous twins". Fertility and Sterility. 67 (6): 1159–61. PMID 9176461.
{{cite journal}}: CS1 maint: multiple names: authors list (link)
External links
