The first thing I have to tell you is that twins does not exist as a medical term. What exists is the twin pregnancy and this can be monovitelline or bivitelline. Monovitelino means that an egg and a sperm have been fertilized and split in half and therefore they will be identical twins. They will be the same at birth, with the same sex, the same eye color, the same hair color, etc … While the bivitelline twins come from two ovules fertilized by two different sperm and look as much or as little as any other pair of brothers, only that in this case they are born at the same time and this is what the people in the street call twins. These bivitelline twins can be a boy and a girl, one blond and one brown, one tall and the other low … and they could even be from a different father. If a woman has a double ovulation and has two intercourse with two different males in the fertile period and each of them fertilizes an egg, they could be of two different fathers.
To answer your question, the frequency at which twin pregnancies occur is around 1%. That is the figure of the natural frequency, which occurs spontaneously. And that is what we had always believed but with the advance of the ultrasound we have discovered that it is not exact. With the popularization of imaging tests in pregnancies we have seen that twins really are much more frequent, what happens is that on many occasions one of the twins does not go forward and the pregnancy ends up becoming a single pregnancy. So it could be even more frequent than that 1% in nature. But it must also be borne in mind that the figure is even higher today due to the use of assisted reproduction techniques.
We go by parties. In that 1% or one of ninety that is the most accurate proportion, we include the univitelinos and the bivitelinos, that is to say what in the street is known as twins and what is known as twins. It is estimated that one third of them are identical, that is, identical twins and the rest, the other two thirds, are bivitelline twins or twins. The third of the univitelinos has remained unchanged, which has increased in recent years is the number of biviteline embazaros. The first cause of this increase is the use of assisted reproduction techniques because either we cause a double ovulation or when transferring after in vitro fertilization, two embryos are transferred. This is done to try ensure that at least one of them go ahead but sometimes it happens that they both continue and the woman has twins. That is the reason that twin births have increased tremendously in recent years.
In addition there are a number of factors that also increase the number of twin pregnancies spontaneously. For example, maternal age. The older the mother is, the more possibility of twins, and the women are getting older when we look for pregnancy. It is also more frequent when in the family there is a history of twins, especially in the family of women; It is also more common in black women and when you have been taking the pill, you stop taking it and you get pregnant in the following month. All these are the reasons for the increase in twin births.
Maribel Acién Sánchez is responsible for the Assisted Reproduction Unit of the University Hospital of San Juan and professor at the Miguel Hernández University in Alicante.
Question sent via email by Elizabeth Pérez.
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