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Why do multiple pregnancies occur and what is the difference between twins and twins?
Medical expert of the article
Last reviewed: 08.07.2025
">Twins are two or more babies that develop together in the same womb and are born at about the same time. Twins can be identical or multi-ovular (fraternal).
Usually, multiple pregnancy is the result of abnormal division of one fertilized egg or fertilization of two or more eggs simultaneously. The birth of identical twins is a random event and can occur with equal frequency in all races and nations. The birth of fraternal twins is considered to be the result of a genetically determined trait, since there is a pattern: if a woman has twins in her family (she is a twin, her mother or grandmother is a twin or triplet), then the probability that she will have multiple pregnancy is much higher than in other women. Moreover, this applies less to men, since the trait is passed on through the female line. Apparently, it is associated with the X chromosome. And if this is so, then twins in the male line can only be passed on from a father to his daughter, since in this case he passes on the X chromosome to her. As for the frequency of twin births in different races, it is lower in whites than in women of the Negroid race. In addition, younger primiparous and older multiparous women are more likely to give birth to twins than other pregnant women.
Identical twins, as you already know, are born from one fertilized egg when it splits into two or more parts at the earliest stages of development. In this case, the children are always born very similar to each other - almost like two peas in a pod and always of the same sex. But they are not just similar - they have very similar characters, habits, hobbies and interests. They even tend to dress alike.
If development occurs from two or more eggs fertilized at the same time, then the children, although they have common features, are not very similar to each other and may be of different sexes. Then they are called twins.
Twins (or triplets) make up about 2% of all newborns. More than 15% of all babies weighing less than 2500 g are twins. Most often, multiple pregnancies lead to babies being born with low birth weight and mostly premature (born before the usual term).
Early intrauterine death of one of the fetuses is often observed. In mothers pregnant with twins, preeclampsia and eclampsia (late gestosis) occur three times more often than in women pregnant with one fetus. In multiple pregnancies, premature placental abruption also occurs more often, especially after the birth of the first twin.
Very rarely among twins there are so-called "Siamese twins". "Siamese twins" are, as a rule, identical twins, fused to each other by some part of the body.
Naturally, giving birth to twins is much more difficult than with a normal pregnancy, but we will talk about this later.