What is a twin pregnancy?
A twin pregnancy is a pregnancy in which two children mature simultaneously in the uterus instead of just one. Twins can share an amniotic sac and a placenta or both can develop on their own. This depends on whether the children are monozygotic or dizygotic, i.e. whether they have developed from the same egg.
Twin pregnancies occur much less frequently than pregnancies with only one baby. Some risks are somewhat increased during a twin pregnancy, which is why they are more often high-risk pregnancies. You can read more about high-risk pregnancies at High-risk pregnancy
When does a twin pregnancy occur?
There are several different causes of twin pregnancy. The first cause is that one egg happens to mature in both ovaries, instead of on one side only, as is usually the case. Both eggs can then be fertilised and mature into one child on their own.
The second reason is that in the early stages of division the cells move apart and two children develop from one egg. Another possible cause of twin or multiple pregnancies is in vitro fertilization, as several eggs are always used to increase the chances of a successful pregnancy. A very rare case is when a woman has two uteruses and two children can grow up at the same time.
Probability of a twin pregnancy
Nature actually foresees pregnancies with single children in humans and therefore twins are relatively rare. About every 80th pregnancy is a twin pregnancy and about one third of these twin pregnancies produce identical children. The number of undetected cases of twin pregnancies is relatively high, as many embryos die in the first weeks of pregnancy and only one child matures. The frequency of twin births is increasing because women are becoming more and more likely to become pregnant late and are also taking hormonal support more often.
Identical twins
At the beginning of a pregnancy, the fertilized egg cell divides several times, thus doubling the number of cells. During this division, the cells are all still very variable and could theoretically each grow into a child on its own. If the cell clusters drift too far apart in these early stages, two children are born.
Depending on the time of this division, the children can divide an amniotic sac or placenta or form one themselves. These twins consist of the same egg and sperm and therefore have identical genetic information. Identical twins always have the same sex and look very similar later.
About 30 percent of all pairs of twins are identical twins. During pregnancy, the differentiation can be examined particularly well between the eighth and the 12th week of pregnancy. Identical twins need to be observed more closely during pregnancy because the frequency of complications is higher.
However, the genetic information also codes for hereditary diseases. So if one child is affected, the sibling will also have the respective disease. In very rare cases, the twins separate so late that they do not separate completely, but remain connected somewhere as Siamese twins.
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