The explosion in the number of multiple pregnancies during the 90s can be explained by the couples’ infertility treatments. The rate has since been reduced due to better management of infertility treatments, in particular by limiting the number of embryos reimplanted during in vitro fertilization. In the case of Assisted Reproduction, a couple would have almost a 25% chance of having twins!

Mother’s age is also a plausible explanation for the phenomenon. Almost a third of twin pregnancies are linked to the increase in the average age of first childbearing (26 years in 1977 against 30 years in 2008 in France). Thus, the probability of naturally having twins goes from 6 ° / °° at the age of 20 to 13 ° / °° at the age of 35 years.

THEEthnicity would also play a role: women from Africa are more likely to have twins than women from Europe, who are themselves more likely than women from Asia to experience multiple pregnancies.

A high consumption of dairy products could increase the risk according to the obstetrician gynecologist Gary Steinman because the latter would promote the secretion and the blood concentration of a growth hormone, IGF (Insulin-like Growth Factor), favorable to the release of eggs by the ovary. Further studies are needed to validate this hypothesis.

We can distinguish monozygotic twins (or “identical twins”), which represent a third of twin pregnancies, from dizygotic twins (or “false twins”).

Fraternal twins. Dizygotic twins result from the simultaneous fertilization (or during the same menstrual cycle) of 2 different eggs by 2 different sperm. This situation is itself the result of double ovulation which can occur spontaneously or under the effect of certain medications. The 2 fertilized oocytes form 2 different egg cells, and the two twins have as many genetic differences as two siblings.

Identical twins. Monozygotic twins result from the fertilization of a single egg by a single sperm, but the egg-cell splits early in two, resulting in the development of 2 separate embryos. The 2 twins are therefore genetically identical, and therefore always of the same sex, of the same physiognomy at birth and during their childhood.

In 70% of cases, the separation of identical twins occurs after the constitution of the chorion: the two fetuses, sharing the same placenta, are described as ” monochorioniques. This poses problems of blood supply: the best placed takes the nutritional resources it needs and the other has only residual resources, leading to differences in height and weight at birth.

If the egg cell divides after the 8^st day after fertilization, the embryos develop without any separation and risk an incomplete split: we speak of Siamese. It is estimated that this anomaly affects only 1 in 100 pregnancies, or 000% of “identical twins”. 

Twin pregnancies are the cause of frequent complications:

• Severe prematurity (i.e. birth before 33 weeks) is multiplied by 8.
• Intrauterine growth retardation (which increases the risk of perinatal mortality).
• The transfusion-transfused syndrome (which constitutes an anomaly in the nutritional intake available to twins).

Seeing the birth of 2 children at the same time has destabilized many civilizations. Sometimes gifts from the gods, sometimes marks of a diabolical intervention, the twins have never left indifferent, the Christian religion having even wondered for centuries about their identity: do they have a soul or are they two different individuals?

Among the famous twins, there is possibly “the Iron Mask”, who died in 1703 after spending most of his life in prison wearing a mask, and who is said to have been the twin of King Louis XIV. This would have posed such a dynastic problem that the order was issued to lock him up without ever revealing his identity. We find this problem of the attribution of the birthright in the Encyclopedia of Diderot and d’Alembert: ” the birth of two twin brothers has given rise to an intractable question in civil society, which is the birthright. It may well be decided by law that the first to be born will be regarded as the eldest; but what takes place in the womb of the mother at the time of conception and at the end of childbirth is such an impenetrable secret in the eyes of men that it is impossible for them to dispel doubt by the light of physiology. »

In many of the Native American myths, twins are considered antithetical, that is, fulfilling complementary but opposite functions: one is good, the other is bad, one is strong, the other is weak, one is aggressive, the other is peaceful , etc.

Conversely, in Indo-European myths, the emphasis is on their complete homogeneity. We consider that they are one, with the same tastes, the same thoughts, the same characters, that they are in love with the same women, ill at the same times, etc.

Delay in language acquisition. Several authors, including Zazzo, have shown in twins the existence of a delay in the acquisition of language, and in particular of the first grammatical forms. We thus note a deformation of the words as well asa language specific to each pair of twins : this phenomenon is called “cryptophasia”.

Different personalities. Real twins are genetically identical but in no case psychologically identical. The genetic aspect does not determine psychic development, even if their environment seems identical. The relationship between the two twins, as well as their relationship to others, will create differences, more or less great at this level: “Same heredity, same background, and yet two different beings”.

Sociability. The pair of twins has been defined by D. Burlingham as a “miniature gang”, that is, “exhibiting a complete lack of interest in everything except the gang”. This self-sufficiency and the resulting lack of sociability would be found in some pairs of twins.