Identical twins share the same spirit

Body & Mind - Identical twins - different forms of origin

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The same nose, eye color and an identical smile: at first glance, identical twins look the same - after all, they share the same genetic make-up. And yet the formation of identical twins is different.

A third of all naturally formed twins are identical. This means that they arise from one and the same cell that divides after fertilization and therefore have the same genetic make-up and gender.

Even 50 years ago, the case was clear: if the twins have a mother cake at birth that supplies the children via the umbilical cords, the twins are identical. If not, they are dizygoti. However, today's medicine has shown that this is not the case.

In the case of identical twins, a distinction is also made on the basis of their developmental form. This is less important for the future twin parents than for the attending physician: Because the time at which the fertilized egg cell (zygote) divides determines whether there is a risk in pregnancy or not. There are these possibilities:

  • The egg divides for the first time shortly after fertilization. If the separation takes place in this two-cell stage, the twins each develop in their own amniotic sac and each with their own placenta, and sometimes both placenta can grow together to form one large placenta. However, this early point in time of division rarely occurs.
  • If it splits between the third and eighth day of development, the egg has already divided several times and is ready to implant itself in the uterine lining. In this case, identical twins develop that grow up with only one placenta, but with a common outer membrane and two separate inner membranes. This type of cell division is the most common.
  • However, some twins arise later, when the fertilized egg has already implanted in the uterus (8th to 10th day). They then develop in a common amniotic sac with a placenta. Particular attention must be paid to the development of this particular type of identical twins, as complications can easily arise. For reassurance: the frequency is less than one percent.
  • If the fertilized egg cell only divides at a very late point in time, i.e. around 14 to 15 days after fertilization, the result is an incomplete division of the germ cell. The embryos remain connected to each other and may have common organs (Siamese twins). However, this development is extremely rare: it affects about one in 900 twin births. Today, however, most Siamese twins can also be successfully separated through surgery.

More and more children see the world in a double pack

Dizygoti twins, on the other hand, arise from two egg cells, each of which is fertilized by a sperm. In a woman's cycle it happens that two egg cells mature and migrate from the ovary into the fallopian tube. Both ovulations occur within 24 hours. The egg cells in the fallopian tube can then be fertilized for 12 to 18 hours.

In addition, sperm can survive for up to five days after intercourse. This means that the woman is ready to conceive for five to six days - enough time to father a child twice. Both times the cards are reshuffled. One of the fertilized egg cells has a different genetic makeup than the other. Dizygoti twins are genetically neither stranger nor more similar than two "normal" siblings who were conceived a few years apart. Two girls, two boys or a girl and a boy - all these combinations are possible.

Family tree analyzes have shown that women who have dizygoti twin partners give birth to twins somewhat more often than other women. And women who have identical twin sisters are more likely to have twin daughters. Studies show that the most important factor for dizygoti twins is the mother's age. According to this, the number of twin pregnancies increases continuously up to the age of 38 and then decreases again. This is ensured by a hormone that is released more strongly over the years: the follicle-stimulating hormone, or FSH for short. It allows the follicles, the follicles in the ovary, to mature. More FSH means more double ovulations. Some women therefore try to help their desire for children with hormone injections on the jumps.

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