Twin pregnancy: how it happens

Twin Pregnancy: Why More Common Today?

In the last 20 years, the number of multiple births in Germany has increased significantly. For 2009, the Federal Statistical Office recorded 22,847 multiple children, of which 22,052 were twins. In the last year alone, the number of multiple births increased by around two percent compared to the previous year. But why is it that more and more twins are being born?

• Higher average age: The average age of women who become pregnant for the first time today is around thirty years. As you age, your body releases more ovulation-triggering hormones. As a result, the ovaries are more likely to release several egg cells for fertilization and thus several fertilized egg cells can implant in the uterus. In this way, multiple births occur naturally.
• Fertility treatments: At the same time, women’s fertility decreases with age or is reduced by other factors such as smoking, stress or obesity. Many then opt for hormone treatment or artificial insemination. During hormone treatment, ovulation is strongly stimulated and several egg cells quickly mature. This also increases the likelihood of a multiple pregnancy. In artificial insemination via IVF, two or, in some countries, more fertilized egg cells are placed in the woman’s body. This also increases the likelihood of a multiple pregnancy.

Twin pregnancy: predisposition on the mother’s side is decisive!

Irrespective of age or fertility medicine, the probability of a twin pregnancy is primarily determined by your own predisposition. In particular, the hereditary factors on the mother’s side are decisive. If the mother or grandmother has already had twins, the probability of a twin pregnancy increases fivefold.

Incidentally, the rule that the hereditary factors for twin pregnancies always skip a generation has not yet been proven.

How are twins formed?

When they hear the word “twins,” many people think of identical-looking siblings. Identical or monozygotic twins have the same genetic material, are therefore always of the same sex and often look amazingly alike. They arise when the fertilized egg cell divides into two detached cells with identical genetic material and both cells continue to develop independently of one another.

Due to mutations after division or different positions in the uterus, some characteristics can also appear different in identical twins. For example, “mirrored” traits are on the left side of one twin and on the right side of the other twin.

How are fraternal twins formed?

Fraternal or dizygotic twins are twice as common in a twin pregnancy as compared to identical twins. Here, two egg cells mature at the same time and are fertilized by two sperm cells. Since both cells have different genetic material, the twins are similar to normal siblings, except that they were born on the same day.

Are you having identical or fraternal twins?

It is not always easy to determine in advance whether you will have identical or fraternal twins. The number of amniotic sacs and placentas visible on the ultrasound only gives a direction, but does not offer absolute certainty.

In fraternal twins , each fetus has its own amniotic sac, which consists of an outer and inner membrane, and its own placenta. If the placentas are close together, they may fuse together and appear as one large placenta at birth. The twins are then often wrongly mistaken for identical.

In the case of identical twins , the decisive factor is the day on which the egg cell divided. Depending on whether it is earlier or later, the number of amniotic sacs and placentas changes.

• If the division occurs in the first three days , each twin has its own amniotic sac and placenta. Here, too, it can happen that both placentas grow together.
• In most identical twin pregnancies, the division occurs between the third and eighth day . Then the placenta and outer amniotic sac have already formed, in which both babies can find space, but each has its own amniotic sac. Here there is a risk of twin transfusion syndrome, in which the blood circulation of the twins is disturbed. The syndrome occurs in about 15 to 20 percent of all cases, but it can be treated.
• At a division between the eighth and twelfth days , the babies share the amniotic sac and placenta. In addition to transfusion syndrome, there is a risk that the umbilical cords may become entangled and one or both babies may die. Fortunately, the risk is very low, with a probability of only three percent.

You can find more about a twin pregnancy, the pregnancy problems you will encounter, as well as tips for the first time after the birth of twins in our Twins & Co section.