The “fraternal twins” are the result of a twin pregnancy called dizygous, that is to say from two eggs (“di” for “two”, “zygote” for “egg”). During ovulation, two eggs were emitted (compared to one normally) and each of her eggs was fertilized by a different sperm.

In utero, dizygotic twins each have their own placenta and amniotic pouch. It is therefore a bichorial twin pregnancy (two chorions) biamniotic (two amniotic pockets). This diagnosis of chorionicity or of the placental type (one or two placentas) is possible from the first ultrasound of the 12 weeks, and therefore makes it possible to define from the start of the pregnancy whether they are true or fraternal twins, at a exception:

• if there is a single placenta and one or two amniotic bags, they are identical twins;
• if there are two placentas and two amniotic bags and the babies are of different sex, they are fraternal twins;
• If there are two placentas and two amniotic bags and the babies are of the same sex, only a birth test will determine if they are identical or fraternal twins. For this we will look for the blood group of babies and if necessary, an analysis of the placental DNA.

Different factors can influence the probability of having fraternal twins:

• Maternal age: starting from a level close to zero at puberty, the proportion of dizygotic twin pregnancies increases regularly until 36 or 37 years of age, when it reaches a maximum and then decreases rapidly. This is due to the level of FSH hormone (follicle stimulating hormone, key hormone of the first phase of the cycle), the level of which increases regularly with age, increasing the probability of a multiple pregnancy (1);
• the birth order: at the same age, the rate of fraternal twins increases with the number of previous pregnancies (2);
• genetic predisposition: there are families where twins are more frequent, and twins have more twins than women in the general population;
• ethnicity: at the same maternal age and birth order, 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 . These variations are thought to be largely linked to hormonal differences of genetic origin (3);
• ART treatments (ovulation induction, artificial insemination, in vitro fertilization): these techniques lead to the simultaneous maturation of several eggs, thus promoting dizygotic pregnancies. AMP would thus be responsible for two thirds of the increase in twin pregnancies over the past 40 years, the remaining third being the consequence of the increase in maternal age during the first pregnancy.

Since dizygotic twins come from two different eggs and two different spermatozoa, they do not have the same genetic heritage. They do not resemble each other (except as siblings may resemble each other) and may be of the same or different sexes. Hence the term “fraternal twins”.

The “identical twins” come from a single egg. We are talking about monozygotic twins. As in a single pregnancy, an egg has been fertilized by a sperm. But at the very beginning of pregnancy, for reasons still unknown, the egg split in two, giving birth to two embryos carrying the same genetic heritage. Twins are therefore genetically identical: they are of the same sex and look exactly the same. Depending on the time of division, monozygous pregnancies can be monochorial monoamniotic, monochorial bioamniotic, bichorial biamniotic.

True and fraternal twins therefore correspond to two different biological phenomena: in the case of identical twins, it is an anomaly in embryonic development which is similar to cloning. In the case of fraternal twins, it is double ovulation and fertilization, due to the release by the ovaries of two eggs in the same cycle (4).

Monozygous pregnancies are rarer and represent about 20% of twin pregnancies.

Pregnancies of monozygotic triplets or “true triplets”, that is to say from the same egg that has divided into three, are extremely rare. According to figures in the medical literature, this would concern 1 in 70 or even 000 million births – the phenomenon is so rare that it is difficult to have precise figures.

In the majority of cases, pregnancies of triplets and more are pregnancies obtained after ART treatment.

• artificial insemination often takes place on a stimulated and unnatural cycle. In order to achieve quality ovulation, an ovarian stimulation treatment is administered before insemination. This makes it possible to obtain two follicles, or even three, which if they are all fertilized, can give rise to triplets;
• during IVF, the number of embryos transferred is defined on a case-by-case basis depending on the couple’s profile and the quality of the embryos. In France, it is generally limited to two embryos in order to obtain a maximum pregnancy rate while limiting the risk of multiple pregnancy. In the event of repeated failure, three embryos can be, exceptionally, implanted, and if they are all implanted, give triplets (5).

In recent years, thanks to better knowledge and use of these assisted reproduction techniques, triple or quadruple pregnancies have become more rare. When faced with three or more embryos, doctors may also suggest “embryo reduction” to avoid complications from a multiplying pregnancy. Today, the risk of a triple or quadruple pregnancy during IVF is less than 1% (6).