Everything about Twins totally explained
Twins are two
offspring resulting from the same
pregnancy, either of the same or opposite
sex.
The general term for more than one offspring from the same pregnancy is
multiples, for example
triplets refers to cases of three offspring from the same pregnancy. A
fetus alone in the womb is called a
singleton.
Twins 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.
Twins can either be monozygotic (MZ, colloquially, "identical") or dizygotic (DZ, colloquially, "fraternal"). There are estimated to be approximately 125 million human twins and triplets in the world (roughly 1.9% of the
world population), and just 10 million monozygotic twins (roughly 0.2% of the world population and 8% of all twins). The current rate in the United States is 31 twin births per 1,000 women.
Types of Twins
There are 1,197 common variations of twinning. The three most common variations are all dizygotic: (1) male-female twins are the most common result, at about 40 percent of all twins born; (2) female DZ twins (sometimes called sororal twins); (3) male DZ twins. The other variations are monozygotic twins: (4) female MZ twins and (5) (least common) male MZ twins. Male singletons are slightly (about five percent) more common than female singletons. (6) There is also the mirror image variations. This is where the twins develop reverse asymmetric features. About 25% of identical twins are mirror image twins.The rates for singletons vary slightly by country as shown in the
CIA World Factbook . For example, the sex ratio at birth in the
US
is 1.05 males/female, while it's 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.
Another variety of twins, "
polar body twins," (one egg fertilized by two different sperm) is a phenomenon that was hypothesized to occur and may recently have been proven, very rarely, to exist. Polar body twinning would result in
"half-identical" 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. 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 fertilized by two different
sperm cells, DZ twins result.
Dizygotic twins, like any other siblings, have an extremely small chance of having the exact same chromosome profile. Like any other
siblings, DZ twins may look similar, particularly given that they're the same age. However, DZ twins may also look very different from each other. They may be 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 have the same age.
Studies show that there's a genetic basis for DZ twinning. However, it's only the female partner that has any influence on the chances of having DZ twins as the male can't make her release 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. Their traits and physical appearances are not exactly the same; although they've nearly identical DNA
(External Link), 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 isn't 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, regardless of ethnic background. The two embryos develop into
fetuses sharing the same womb. When one egg is fertilized by one sperm cell, and then divides and separates, two identical cells will result. If the zygote splits very early (in the first two days after fertilization), each cell may develop separately its own
placenta (
chorion) and its own sac (
amnion). These are called
dichorionic diamniotic (
di/di)
twins, which occurs 20–30% of the time. Most of the time in MZ twins the zygote will split after two days, resulting in a shared placenta, but two separate sacs. These are called
monochorionic diamniotic (
mono/di)
twins.
In about one percent of MZ twinning the splitting occurs late enough to result in both a shared placenta and a shared sac 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 50 percent, principally due to cord entanglement prior to 32 weeks gestation. If expecting parents choose hospitalization, mortality can decrease through consistent monitoring of the babies. Hospitalization can occur beginning at 24 weeks, but doctors prefer a later date to prevent any complications due to premature births. The choice is up to the parents when to start hospitalization. Many times, monoamniotic twins are delivered at 32 weeks electively for the safety of the babies. In higher order multiples, there can sometimes be a combination of DZ and MZ twins.
Mono/di twins have about a 25 percent mortality due to
twin-to-twin transfusion syndrome. Di/di twins have the lowest mortality risk at about nine percent, although that's still significantly higher than that of singletons.
Monozygotic twins are genetically identical (unless there has been a mutation in development) and they're 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 look alike, although they don't 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. If each member of one set of MZ twins reproduces with one member of another set of MZ twins then the resulting children would be genetic full siblings. 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.) 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.
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.
A recent theory posits that monozygotic twins are formed after an embryo 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 in
Lyon,
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.
Demographics
A recent study found that
vegan mothers are five times less likely to have twins than those who eat animal products.
From 1980–97, the number of twin births in the United States rose 52%. 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. The rate of
fraternal twinning varies greatly among
ethnic groups, ranging as high as about 6% for the
Yoruba or 10% for Linha Sao Pedro, a tiny Brazilian village. 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 didn't bear any association to the climate or prevalence of
malaria. 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:
