How can you have twins?

Twin pregnancy: how likely is it to have twins?

On the internet, we can no longer count the number of forum pages and questionable articles that give grandmothers’ tips for having twins. But are they based on scientific evidence? What factors really influence the risk (or chance, depending on your point of view) of having twins? We asked the question to two gynecologists specializing in twin pregnancy, Professors Yves Ville and Jean-Claude Pons, members of the association’s scientific committee. Twins and more.

Sweet potato for having twins: a simple correlation link

On some websites, drinking milk or enriching your diet with dairy products is cited as one of the ways toincrease her chances of having twins. These foods increase the level of FSH (follicle stimulating hormone, which is involved in ovulation), which would promote multiple ovulations. In fact, to date, there is no solid evidence to support such a thing. The only dietary factor accepted by gynecologists is the heavy consumption of sweet potato or yam. These two plants indeed have a molecule close to estrogen, which would influence the female menstrual cycle.

As the largest consumer of yams, Nigeria thus records a very high number of twin pregnancies, almost four times higher than the rate observed in France. However, it is not easy to say that it is the high consumption of sweet potato and / or yam that leads to these twin pregnancies. Other factors would also be involved, according to Prof. Yves Ville, obstetrician-gynecologist and head of department at Necker Hospital in Paris. If nothing prevents you from putting the sweet potato on your menus when you want have twins, not sure that this is enough.

Brightness and twinning: a statistical finding

Another contributing factor: the photoperiod. Behind this barbaric name hides in fact the relationship between the length of day and night. According to specialists, the more day length is important, the greater the risk of twin pregnancy. Purely statistical, this correlation would explain, among other things, why Scandinavian women have more twins conceived in summer (where the sun does not fully set) than in winter, where the sunshine duration is only a few hours. But again, this is only an observation, which deserves to be explained and refined by further studies. Be aware in any case that this factor could explain why there is more twin conceptions in summer than in winter in France, even if this difference is small.

Heredity, multiparity, ethnicity: other influential factors but difficult to quantify

The first thing we think of when we talk about probability of having twins, it is of course theheredity. In the common imagination, having twins in your family is undeniably a factor that increases the chances of having them yourself. But the reality is more complex. It seems that there is indeed un link between heredity and twinning in families where there are many dizygotic twins, fraternal twins. This could be explained by the presence of several genes playing on the ovulation of women and causing more polyovulations. For monozygotic twins, or identical twins, heredity is not really established. It is not yet clear whether genes are involved in the separation of an egg, once the egg-sperm encounter has taken place. 

Other statistical studies have also shown that the more children a woman has, the more her chances of having twins increase. But again, this is only a statistical observation, which can be explained by the risk of polyovulation which increases.

THEethnic group would also play a significant role. All specialists make the same observation: black women are more at risk of twin pregnancy than white women, themselves more at risk than women of Asian descent. There are thus a maximum of 6 twin births per thousand in Asia, against 17 per thousand in France and up to 45 twin births per 1000 in Nigeria. Note that these data can also be attributed to other factors, such as the daily sunshine duration or even diet.

Age and medically assisted reproduction: the two main factors of proven twinning

While all of the factors below must be taken with a grain of salt, there are two others that are no longer to be proven: age and medically assisted reproduction (ART) treatments. The older a woman is, the more she has a risk of twin pregnancy. In question: a greater risk of polyovulation, that is to say of multiple ovulations where several eggs are expelled into the uterus, instead of just one per cycle. If they are each fertilized by a sperm, these eggs will give rise to two embryos and two fraternal twins, or dizygotic twins.

The use of medically assisted procreation (ART) is also a major risk factor for twin pregnancy. According to Prof. Yves Ville, the use of ART increases the risk of having twins by 5, or even 6. In the assisted reproduction pathway, two methods explain this increased risk: ovarian stimulation and transfer of several embryos. The first is to boost the ovaries to force them to ovulate, which often leads to multiple ovulations. The second consists, during IVF, in selecting at least two embryos conceived in vitro from the couple’s gametes, and inserting them into the uterus. This approach generates a 20% risk of twin pregnancy, since the two embryos each have a chance to implant. Note that it also happens that only one embryo is implanted, but that this nevertheless splits in two and leads to a multiple pregnancy.