Because twinning fascinates, for some couples, with twins is a dream. But is it possible to influence nature and increase your chances of having a twin pregnancy?

We must distinguish two types of twin pregnancies, corresponding to two different biological phenomena:

• identical twins or monozygotic twins come from a single egg (mono meaning “one”, zyogote “egg”). An egg fertilized by a sperm gives birth to an egg. However, this egg will, for reasons still unknown, divide in two after fertilization. Two eggs will then develop, giving two fetuses carrying the same genetic makeup. The babies will be of the same sex and will look exactly alike, hence the term “real twins”. With actually a few small differences due to what scientists call phenotypic mismatch; itself a consequence of epigenetics, ie the way in which the environment influences the expression of genes;
• fraternal twins or dizygotic twins come from two different eggs. During the same cycle, two eggs were emitted (against one normally) and each of these eggs is fertilized simultaneously by a different sperm. Being the result of the fertilization of two different eggs and two different spermatozoa, the eggs do not have the same genetic heritage. Babies can be the same or different sex, and look alike just like children from the same siblings.

About 1% of natural pregnancies are twin pregnancies (1). Certain factors may cause this figure to vary, but again, it is necessary to distinguish between monozygous pregnancy and dizygotic pregnancy.

Monozygous pregnancy is rare: it concerns 3,5 to 4,5 per 1000 births, regardless of the mother’s age, birth order or geographic origin. At the origin of this pregnancy there is a fragility of the egg which will divide after fertilization. This phenomenon could be linked to the aging of the ovum (which, however, has no connection with maternal age). It is observed on long cycles, with late ovulation (2). It is therefore impossible to play on this factor.

Conversely, different factors affect the probability of having a dizygotic pregnancy:

• Maternal age: the proportion of dizygotic twin pregnancies increases steadily until the age of 36 or 37 when it reaches a maximum. It then decreases rapidly until menopause. This is due to the level of the hormone FSH (follicle stimulating hormone), the level of which increases steadily up to 36-37 years, increasing the probability of multiple ovulation (3);
• birth order: at the same age, the rate of fraternal twins increases with the number of previous pregnancies (4). This variation is however less important than that linked to maternal age;
• genetic predisposition: there are families where twins are more frequent, and twins have more twins than women in the general population;
• ethnicity: the dizygotic twinning rate is twice as high in Africa south of the Sahara than in Europe, and four to five times higher than in China or Japan (5).

With the rise of ART, the proportion of twin pregnancies has increased by 70% since the early 1970s. Two thirds of this increase is due to treatment against infertility and the remaining third to the decline in pregnancy. age of first maternity (6).

Among the techniques of ART, several increase the probability of obtaining a twin pregnancy via different mechanisms:

IVF Transferring multiple embryos at the same time increases the likelihood of having a multiple pregnancy. In order to reduce this risk, a decrease in the number of embryos transferred by transfer has been observed for several years. Today, the consensus is to transfer a maximum of two embryos – rarely three in the event of repeated failure. Thus, from 34% in 2012, the rate of mono-embryonic transfers after IVF or ICSI rose to 42,3% in 2015. However, the twin pregnancy rate after IVF remains higher than after pregnancy. natural: in 2015, 13,8% of pregnancies following IVF led to the birth of fraternal twins (7).

L’induction d’ovulation (which does not really fall under the AMP) The simple ovarian induction prescribed in certain ovulation disorders aims to obtain a better quality ovulation. In some women, it can lead to the release of two eggs during ovulation, and lead to a twin pregnancy if both eggs are each fertilized by one sperm.

Artificial insemination (or intrauterine insemination IUI) This technique consists of depositing the most fertile sperm (from the partner or from a donor) in the uterus at the time of ovulation. It can be done on a natural cycle or on a stimulated cycle with ovarian stimulation, which can lead to multiple ovulation. In 2015, 10% of pregnancies following UTI led to the birth of fraternal twins (8).

Frozen embryo transfer (TEC) As with IVF, a decrease in the number of embryos transferred has been observed for several years. In 2015, 63,6% of TECs were performed with a single embryo, 35,2% with two embryos and only 1% with 3. 8,4% of pregnancies following TEC led to the birth of twins (9 ).

Twins resulting from pregnancies following ART techniques are fraternal twins. However, there are cases of identical twins resulting from the division of an egg. In the case of IVF-ICSI, it even seems that the rate of monozygous pregnancy is higher than in spontaneous reproduction. Alterations due to ovarian stimulation, in vitro culture conditions and handling of the zona pellucida could explain this phenomenon. A study also found that in IVF-ICSI, the monozygous pregnancy rate was higher with embryos transferred to the blastocyst stage, after prolonged culture (10).

• Eat dairy products An American study on the likelihood of twin pregnancies in vegan women showed that women consuming dairy products, more specifically cows that received growth hormone injections, were 5 times more likely to have twins than women. vegetarian women (11). The consumption of dairy products would increase the secretion of IGF (Insulin-Like Growyh Factor) which would promote multiple ovulations. Yam and sweet potato would also have this effect, which could partly explain the higher proportion of twin pregnancies among African women.
• Take vitamin B9 supplementation (or folic acid) This vitamin recommended in pre-conception and early pregnancy to prevent spina bifida could also increase the chances of having twins. This is suggested by an Australian study that observed a 4,6% increase in twin pregnancy rates in women who took vitamin B9 supplementation (12).