How do twins work

Identical twins

About a third of all twin pregnancies are identical twins. These arise when the egg cell divides into two viable cells after fertilization.

In identical (monozygous) twins, a sperm fertilizes an egg cell. This divides and leaves behind two fully viable cells, from which two children with identical genetic material develop. Depending on the exact time of the division, a distinction is made between different forms of identical twins.

Explanation: After fertilization, a double-walled fruit envelope (or egg sac) develops around an embryo. The outer skin of this covering is known as the chorion, the inner skin as the amnion or water skin, since it takes over the production of amniotic fluid, especially in early pregnancy. The embryo is supplied via the umbilical cord, which emerges from the placenta. In identical twin pregnancies, there are different ways in which the fruit cavity and plant tissue develop - this depends on the time at which the fertilized egg cell divides. A distinction is made between the following forms of identical twin pregnancies:

Monozygous twins with two fruit hollows and two placenta

In this case, the fertilized egg cell divides completely just a few hours after fertilization. Thus, two egg cells migrate through the fallopian tube and nest in different places in the uterus. Therefore, every child has its own pericarp - so twins develop with two amnias and two chorionic cavities. In this case one also speaks of dichorial-diamnial Twins. Each embryo also has its own placenta.

This is the cheapest variant of identical twin pregnancies - the probability that both children will be born healthy is highest with this form of twin pregnancy.

Monozygous twins with two fruit hollows and a placenta

In this form, the fertilized egg cell divides between the fourth and seventh day after fertilization. The system with two children thus nests in one place in the uterus. In this case, the two embryos develop in a common chorionic cavity, but each has its own amniotic sac. This twin variant is also called monochorial-diamnial. Both embryos are supplied by a common placenta.

There are often vascular connections between the two children via the placenta. In these cases, there is a risk of developing a fetofetal transfusion syndrome (TTTS), which means that one twin is supplying the other with blood. With pronounced forms of TTTS, the "blood recipient" ("recipient") becomes very large. It is bloated, has a lot of amniotic fluid and a large volume of blood that the small heart can hardly handle. The "blood donor", however, is extremely small - appears to be "parched" - has little amniotic fluid and is sometimes pressed against the wall of the fruit cavity (a so-called "stuck twin").

Therapy is difficult and should be reserved for prenatal diagnosis and therapy centers. There is the possibility of withdrawing amniotic fluid from the recipient, on the other hand one can try in special centers to interrupt the vascular connections between the twins with the help of a laser. This rare form poses a particular risk, especially for the receiving child: Cardiovascular failure and infant death are common.

Monozygous twins with a fruit cavity and a placenta

This twin constellation is very rare (<1 percent). It arises after implantation in the uterus and after the fruit cavity has formed, i.e. between the eighth and tenth day after fertilization. In this case, both embryos are located in one amniotic cavity and have a common amniotic and chorionic sac and a common placenta. The name for this shape is monochorial-monoamnial.

This constellation goes hand in hand with the risk that the two umbilical cords will intertwine, resulting in a lack of care for the embryos. Also with this form there can be vascular connections in the placenta, which can lead to the known problems of the fetofetal transfusion syndrome (TTTS).

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++ Continue to: Siamese twins ++

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Dr. Britta B├╝rger (first author 2002), Dr med. Kerstin Lehermayr (2014)
Editorial editing:
Philip Pfleger (2013)

Updated on:

Weyerstahl T., Stauber M .: Gynecology and Obstetrics; Georg Thieme Verlag KG Stuttgart; 4th edition 2013

Dudenhausen JW, Maier RF: Perinatal Problems in Multiple BirthsDtsch Arztebl Int 2010; 107 (38): 663-8; DOI: 10.3238 / arztebl.2010.0663

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