When do identical twins occur

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.

Monozygotic twins are typically born as the same genetic sex. However, while it's extremely rare, it is possible for monozygotic twins to be two different sexes. This goes back to the idea that genetic mutations can occur from the very first split.

If an egg carries two X chromosomes when a normal egg should carry just one X chromosome and is fertilized with a Y sperm, you can get an XXY embryo.

This is also known as Klinefelter syndrome. However, what happens if that XXY embryo splits into monozygotic twins?

You can end up with one twin with XX expression female , and the other with XY male. This situation is so rare that it has only been described in medical literature four times. On this page. Multiple births are more common than they were in the past, due to the advancing average age of mothers and the associated rise in assisted reproductive techniques, in particular the use of fertility drugs. Twins account for over 90 per cent of multiple births. There are two types of twins — identical monozygotic and fraternal dizygotic.

To form identical twins, one fertilised egg ovum splits and develops two babies with exactly the same genetic information. This differs from fraternal twins, where two eggs ova are fertilised by two sperm and produce two genetically unique children, who are no more alike than individual siblings born at different times. Twins are more or less equally likely to be female or male. Factors that increase the odds of having twins Some women are more likely than others to give birth to twins.

The factors that increase the odds include: Advancing age of the mother — women in their 30s and 40s have higher levels of the sex hormone oestrogen than younger women, which means that their ovaries are stimulated to produce more than one egg at a time. 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.

Read more about the different types of twins on the Twins Trust website. More detailed information on DNA testing and how to find out whether twins or triplets are identical is available on the Multiple Births Foundation website. Page last reviewed: 21 May Next review due: 21 May Home Pregnancy Finding out you're pregnant Back to Finding out you're pregnant. Pregnant with twins. Different types of twins One-third of all twins will be identical and two-thirds non-identical.

Identical twins Identical monozygotic twins happen when a single egg zygote is fertilised. The egg then divides in 2, creating identical twins who share the same genes.

Early splitting also means that the twins are perhaps less identical than those who separate at the late blastocyst stage. Around a quarter of all identical twins are said to be mirror images of each other. This means that the left side of one twin exactly matches the right side of the other. How amazing is that! Although identical twins share the same DNA and genetic coding, they are still unique little individuals. Small differences separate them and although at birth they can appear like little clones of each other, they are far from it.

As identical twins grow, their unique qualities particularly in relation to their personalities become more apparent. The environment in which they are raised as well as their own personalities help to shape the adults they will become. This is exactly the same as any other baby born the world over. Because the splitting of the zygote occurs so early in pregnancy, mothers are not initially aware they have conceived with twins.

Even at birth it can be difficult to be sure if twins are identical or non-identical.