Is having twins hereditary? What are the influencing factors?

When a woman is pregnant, one of the things that may be on her mind until the first ultrasound is performed, is whether it will be a multiple pregnancy (two or more embryos). In addition, this doubt will be even more common if there is a history of twins in the family, due to the possibilty of it being hereditary.

However, the probability varies depending on whether we are talking about identical or non-identical twins and the influence of certain factors.

The different sections of this article have been assembled into the following table of contents.

Identical or non-identical twins?

It is true that the terms twins is often used for both types. However, the two types of twins are not the same:

Monozygotic twins, also called identical twins, result from the fertilization of a single egg by a single sperm. However, the embryo splits in two at an early stage. Therefore, as the babies will have the same genetic material, they will be of the same sex and physically will be virtually identical.

Dizygotic twins fraternal twins, or, as they are popularly known, non-identical twins, come from two different eggs, each fertilized by a different sperm. In this case, the babies do not have to be of the same sex (although they can be) and look alike as do siblings born from different pregnancies.

Differences between identical and non-identical twins

Therefore, although they are sometimes confused, the way in which identical and non-identical twins originate is different.

What is the probability of having a multiple pregnancy?

It is not uncommon for a woman or couple to wonder about the likelihood of having twins naturally. Moreover, this question is even more frequent if there are already twins in the family, as there exist many doubts about whether it is hereditary or not.

Although many things are said (such as if there are twins, you are sure to get them; or that they skip a generation) it is important to distinguish whether we are referring to identical twins or non-identical twins.

Chances of having a multiple pregnancy

The odds of having monozygotic twins is 3-4 out of every 1000 births, which seems to hold true anywhere in the world. However, the rate of dizygotic twins has important regional variations: 6 out of 1000 births in Asia and up to 40 out of 1000 births in Africa.

Are multiple pregnancies inherited?

The reasons why an embryo divides, giving rise to monozygotic twins, are not well known. For a long time it was thought to be a random event. However, it could have a certain genetic component that would increase the possibility of several cases of this type of twins occurring in the same family.

However, it does seem clearer that a certain propensity to have fraternal, non-identical twins, i.e. dizygotic twins, is inherited. This propensity seems to be related to a predisposition to multiple ovulation.

Influencing factors for having twins

In addition to genetic inheritance, there are other factors that would increase the likelihood of having dizygotic twins. Among them:

• Maternal age: with age and decreasing ovarian reserve, FSH hormone levels increase. This would increase the possibility of multiple ovulation, especially between 35-39 years of age.
• Number of births: more previous births are related to a higher probability of twins.
• Location: as we have mentioned, probably due to genetic influence, the rate of dizygotic twins is different around the world, being higher in African populations.
• Body build: a body mass index (BMI) greater than 30, as well as greater height, has been linked to an increased likelihood of twins.
• Assisted reproductive techniques (ART): the mild ovarian stimulation of artificial insemination (AI) and the transfer of more than one embryo in in vitro fertilization (IVF) lead to an increase in the rate of dizygotic twins. Nevertheless, ART have also been associated with a slight increase in monozygotic twins.

What factors increase the probability of having fraternal twins

However, a multiple pregnancy, that is, a pregnancy with two or more babies, has more risks for the mother and the offspring. Therefore, thanks to improvements and advances, ART companies are adopting strategies, such as the transfer of a single embryo, to reduce the number of multiple pregnancies they produce.

You can read more about the probability of having a multiple pregnancy after ART in the following article: What is the probability of twins with IVF and artificial insemination?

FAQs from users

I want to become pregnant with twins, can I do this using IVF?

Yes, it is possible that a woman, or a couple, can choose to transfer two embryos instead of one. However, it is very important to follow your specialist´s recommendations and not run the risk of a multiple pregnancy. In addition, not all transferred embryos are cabaple of implanting into the uterus.

Is it possible to have twins more than once?

If a woman already has twins, it is possible that factors are in play that increase the chances of having twins (amongst others, a hereditary disposition) meaning the woman could have twins again.

If the pregnancy is achieved using assisted reproduction techniques, the possilbility of having twins in also increased if various follicules have matured for AI or if more than one embryo is transferred in IVF.

Is having twins hereditary or chance?

If we are referring to dizygotic twins (fraternal, non-identical), then family genetic inheritance may indeed be a factor to predispose a woman to have twins.

Less is known in the case of non-identical, monozygotic twins. It was thought it was due to a random, chance happening, but it could also have a genetic basis in that within some families there are various cases of identical twins.

Recommended reading

If you want to read more information about multiple pregnancy, you can visit the following link: What is the difference between identical twin pregnancy and non-identical twin pregnancy?

On the contrary, if you want to know more about the risks of multiple pregnancy, we recommend you to read this article: Risks of multiple pregnancy for mother and babies.

References