Number of previous pregnancies — the greater the number of pregnancies a woman has already had, the higher her odds of conceiving twins. Heredity — a woman is more likely to conceive fraternal twins if she is a fraternal twin, has already had fraternal twins, or has siblings who are fraternal twins.
Race — Black African women have the highest incidence of twins, while Asian women have the lowest. Assisted reproductive techniques — many procedures rely on stimulating the ovaries with fertility drugs to produce eggs and, often, several eggs are released per ovulation.
Fertilisation Hormones secreted by the ovaries, and a small gland in the brain called the pituitary, control the menstrual cycle. The average cycle is around 28 days. After a menstrual period, rising levels of the hormone oestrogen help to thicken the lining of the womb the endometrium and release an egg from one of the ovaries ovulation.
If the egg is fertilised on its journey down the fallopian tube, it lodges in the thickened womb lining, starts dividing and evolves into an embryo. This occurs because the fertilised egg divides in two while it is still a tiny collection of cells.
The self-contained halves then develop into two babies, with exactly the same genetic information. The biological mechanisms that prompt the single fertilised egg to split in two remain a mystery. Approximately one quarter of identical twins are mirror images of each other, which means the right side of one child matches the left side of their twin.
These babies will be no more alike than siblings born at separate times. The babies can be either the same sex or different sexes, with the odds roughly equal for each. It is proposed that the egg splits in two, and each half is then fertilised by a different sperm. This theory is an attempt to explain why some fraternal twins look identical.
Gestation for twins The normal length of gestation for a single baby is around 40 weeks. Researchers are still trying to understand the origins of this type of twin birth. This happens when the egg divides 12 or more days after conception. Another theory is that the fertilized egg divides completely, but it later fuses back together.
A conjoined twin pregnancy is high-risk, but breakthroughs in surgery have been able to improve outcomes. Surgery can even allow conjoined twins to live independently. Although a small study found that just 7. Sometimes, as twins develop in utero, one twin becomes larger and more dominant. The other twin stops developing and begins to depend on their sibling. Known as parasitic twins , these twins are physically conjoined. This is because the smaller twin is usually missing vital organs or a fully developed brain or heart.
In fact, you might not even recognize the smaller twin as a separate individual. A milder version of this is called twin to twin transfusion syndrome TTTS. Acardiac twins experience a more extreme form of TTTS that may result in developmental issues for the fetuses. Treatments like surgery can improve outcomes, according to research.
In semi-identical twins, two separate sperm fertilize one egg. The fertilized egg then splits in two. So, semi-identical twins share all the same chromosomes from their birthing parent, but only about 50 percent from their non-birthing parent. Sometimes identical twins can be assigned the sex of male and female at birth. These twins start off as identical males with XY sex chromosomes.
But shortly after the egg divides, a genetic mutation called Turner syndrome occurs, leaving one twin with the chromosomes X0. This twin will be assigned female at birth but may have developmental issues, as well as difficulties with fertility later in life, according to the National Health Service. Once a person becomes pregnant, their body stops releasing new eggs for potential fertilization — except in some rare cases.
A phenomenon known as superfetation can occur when a second egg gets released and fertilized after a person is already pregnant. In this case, both fertilized eggs will develop, but one twin will be slightly older than the other. Because these twins come from one zygote, they are also known as monozygotic. Identical twins are the same sex. About one-third of identical twins split soon after fertilisation and form completely separate twins. Like fraternal twins, these twins have separate placentas.
The other two-thirds split after they attach to the wall of the womb. As a result, they share a placenta. The technical name for this is monochorionic. In a very small number of identical twins, splitting might happen even later. In this case, both twins share an inner sac , called the amnion, in addition to sharing a placenta. The technical name for this is monoamniotic twins. For example, a twin who receives less blood from a shared placenta might weigh less at birth.
Sharing a placenta means that twins share a blood supply during pregnancy. Sometimes the blood supply is shared unequally, which can cause health problems for both twins. Women who are pregnant with twins sharing a placenta need to be checked more often than women with twins with separate placentas. This type of twin formation begins when one sperm fertilizes one egg oocyte.
In the case of monozygotic twins, the blastocyst then splits and develops into two embryos. Simply put, monozygotic twins happen when a single fertilized egg divides into two. From there, two embryos grow into two babies. Monozygotic "mono" means one and "zygote" means fertilized egg is the term used to describe this process. In contrast, dizygotic fraternal twins form when two separate eggs are released and fertilized by two sperm.
Then those resulting embryos develop at the same time, but individually. Dizygotic twins are common with fertility treatments that cause multiple eggs to be released or more than one embryo to be transferred into the uterus. Overall twinning rates monozygotic plus dizygotic vary globally. However, most of the variation is due to differences in dizygotic twin rates.
The rate of monozygotic twins is very consistent around the world: about 3 to 5 in every 1, twin births. In some tribal and isolated populations, monozygotic twins seem to run in the family or in the greater tribe.
It remains unknown if genetics are at play or the environment, but it's likely that something in the environment triggers the split, or it occurs randomly. Contrary to popular belief, the chances of having monozygotic twins are most often not related to your family history. If there is more than one set of identical twins in a family, it is probably due to luck or external environmental factors. Some genetic mutations have been identified that increase the chance of monozygotic twins, but these are very rare.
However, dizygotic twins do run in families. This is mainly thought to be due to genes that increase the number of eggs released.
Interestingly, in vitro fertilization IVF appears to increase the likelihood of having monozygotic twins. IVF research has given us some insight into how identical twins form.
IVF embryos are more likely than naturally conceived embryos to split into identical twins. In normal embryo development, a fluid-filled cavity grows inside the embryo. This is known as the blastocoel. During IVF, the blastocoel may collapse on itself. While this typically destroys the embryo, sometimes the embryo survives, causing the inner cell mass to split into two.
The two inner cell masses lead to the development of twins. The embryo is initially kept in an artificial solution in the lab.
Perhaps this solution increases the risk of collapse. It seems that transferring the embryo later may slightly increase the odds of identical twinning.
It was once thought that monozygotic twins shared the exact same DNA. This is one reason why identical twins are at an increased risk of congenital diseases. After one split, individual cell masses continue splitting on their own.
With every split, there is another chance of mutation. Usually, by birth, the genes of twins are extremely similar but not identical. Over time, genetic dissimilarities continue to increase. This is due to epigenetics—the way the environment changes how our DNA is expressed. Some studies have found that slight DNA differences are more common in older identical twins than in very young identical twins.
Even though monozygotic twins look alike and share similar characteristics, they are not clones. The two embryos have the same genetic components to start, but slight differences develop over time. When twins are born with genetically different sexes one twin is genetically male and the other is genetically female they are usually dizygotic.
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