Gemini - Fertilization and birth Gemini

Generality

Twins are individuals born from a single pregnancy. This event, although common in many mammals - such as dogs or cats - is relatively rare in humans, so much so that the incidence of twin pregnancies is around 1, 5% of all gestations.

In any case, the probability that the mother will give birth to two or more twins seems to be linked - among other things - to heredity (on a maternal basis) and to the age of the mother.

In recent years the cases of twin or multi-twin pregnancies have increased, both due to the increase in the average age of pregnant women, and to the increasingly frequent use of pharmacological therapies against hypofertility (which stimulate ovulation) and fertilization techniques assisted.

There are two categories of twins:

HOMOZYGOT GEMINI: also called monoovular or monochorionic;

HETEROZYGOT GEMINI: also called biovoular, bicorical or bizygotic.

Biovular twins are born in about 65-75% of multiple pregnancies, while the remaining 25-35% of cases are covered by monoovular individuals.

Homozygous twins

Homozygous twins derive from the same zygote, which at an early stage of its development divides into two or more parts (the zygote is the cell that is formed by the union of the spermatozoon with the egg cell during fertilization).

In this case the individuals have the same genetic patrimony, the same sex and are almost identical (also in physiognomy); any differences, as regards physical appearance and incidence of certain diseases, are an expression of the different environmental conditions in which they grew up (diet, stress, physical activity, pollution, etc.). When the division of the zygote does not take place in a complete way, fortunately rare event, Siamese twins are born, which have one or more organs in common.

Heterozygous twins

Heterozygous twins (see figure) derive from two or more distinct egg cells, fertilized - almost simultaneously - by two or more spermatozoa; as such, they are distinguished by appearance (their degree of similarity is similar to that found between siblings) and genetic heritage (different is the blood group, the same in homozygotes, and sometimes the sex). In theory, it is possible that the father of the twins is different, as happens when distinct oocytes are fertilized within a short time but in two different coitus.

Risks for the Mother

Multiple pregnancy involves a greater risk of gestosis, abortion and premature birth (on average, gestation ends about three weeks before the expected date); this requires a particularly scrupulous antenatal care, which has proved capable of significantly reducing the incidence of such complications.