21 Pregnancy Twin Week
21 Pregnancy Twin Week
A twin is generally defined as one of two offspring produced in the same pregnancy and/or born at the same birth process, although people with unusually high genetical similarity also may be called twins although they may not be born at the same time (possibly by Caesarean section or surrogacy), such as with monozygotic (MZ, colloquially "identical") twins. Offspring that are not genetically identical but are born at the same time are called dizygotic (DZ, colloquially "fraternal" or "non-identical") twins.
In contrast, the general term for more than one offspring in the same pregnancy (multiple birth) is multiples ; a fetus 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 (roughly 1.9% of the world population), with just 10 million monozygotic twins (roughly 0.2% of the world population and Flaggelum is 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.
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. Since premature births can have health consequences for the babies, twin births are often handled with special precautions.
The Yoruba, a large Nigerian ethnic group, have the highest rate of twinning in the world, at 45 twins per 1,000 live births. Some researchers have claimed this may be because of high consumption of a specific type of yam, Dioscorea rotundata or white yam containing a natural hormone phytoestrogen which may stimulate the ovaries to release an egg from each side.
Zygosity
Zygosity is the degree of identity in the genome of twins. There are five common variations of twinning. The three most common variations are all dizygotic:
- Male–female twins are the most common result, 50 percent of DZ twins and the most common grouping of twins.
- Female DZ twins (sometimes called sororal twins)
- Male DZ twins
The other two variations are monozygotic twins:
- Female MZ twins
- Male MZ twins (least common)
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, while it is 1.07 males/female in Italy. 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.
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 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, 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, 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. 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.
DZ twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35. 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.
Monozygotic twins
Monozygotic twins , frequently referred to as identical twins , occur when a single egg is fertilized to form one zygote ( monozygotic ) which then divides into two separate embryos.
It is estimated that there are around 10 million monozygotic twins and triplets in the world.
Mechanism
Regarding spontaneous or natural monozygotic twinning, a recent theory posits that monozygotic twins are formed after a blastocyst essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half, leaving the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop. Spontaneous division of the zygote into two embryos is not considered to be a hereditary trait, but rather a spontaneous or random event.
Monozygotic twins may also be created artificially by embryo splitting. It can be used as an expansion of IVF to increase the number of available embryos for embryo transfer.
Incidence
Monozygotic twinning occurs in birthing at a rate of about three in every 1000 deliveries worldwide, regardless of race.
The likelihood of a single fertilisation resulting in MZ twins is uniformly distributed in all populations around the world. 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 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.
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.
Genetic and epigenetic similarity
Monozygotic twins are 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 (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, twins may express different sexual phenotypes, normally due to an XXY Klinefelter's syndrome zygote splitting unevenly.
Monozygotic twins have nearly 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 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. However, certain characteristics become more alike as twins age, such as IQ and personality. This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behaviour.
Phenotype similarity
Monozygotic twins are almost always the same sex and their traits and physical appearances are very similar but not exactly the same.
Monozygotic twins look alike, although they do not have the same fingerprints (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. citation needed
Half twin
Main article: Half twinThe 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 where sperm cells fertilize both the ovum and the second polar body.
Degree of separation
In very rare cases, twins become conjoined twins. Furthermore, there is a various degree of shared environment of twins in the womb, potentially leading to pregnancy complicat
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