What are monozygotic twins?

In twin pregnancy, you can give birth to either monozygotic twins / twins or dizygotic twins / twins. What is the difference and how do you know it? We will explain everything to you.

Congratulations, you are pregnant … with twins! In a few months, it will not be one but two babies that you will welcome into your little family. Twice the love then, but also twice the work!

You will also find out during the ultrasound whether they are monozygotic twins / twins or dizygotic twins / twins. What are the differences between these two types of twins / twins and how are they formed? We take stock.

Everything happens during the design!

Monozygotic twins are what are more commonly referred to as identical twins / twins. They represent about 20% of twin pregnancies. Unlike dizygotic twins / twins, monozygotic twins must be of the same sex. This is because they are genetically identical!

Indeed, in the case of monozygotic twins / twins, at the time of fertilization, a single egg will meet a single sperm to form an egg. Then, at the beginning of pregnancy, during the first division of the cells, this egg will split into two to give not one but two embryos. Thus, there will be two fetuses with strictly identical genetic heritage, from the same egg.

In the case of dizygotic twins, two eggs are fertilized by two sperm at the same time. This is especially possible in the case of double ovulation, when the ovary expels two oocytes. There are therefore two different eggs from the start. Babies will be separate individuals and may be of different sex. Since there are two sperm, there are three possibilities: twins, twins, or twins with a girl and a boy. As a reminder, it is the sperm (and not the egg) that determines the sex of the baby.

Note that triplets can also be monozygous, but these are exceptional cases!

How do you know if they are monozygotic twins or dizygotic twins?

In a twin pregnancy with dizygotic twins / twins, each fetus develops in its own amniotic pouch, with a different placenta and chorion (outer layer of the placenta). This is'a bichorian and biamniotic pregnancy : there are two amniotic sacs with two placentas and two chorions.

However, there are several types of monozygotic pregnancies. It all depends on the date of separation of the egg after fertilization:

  • If it takes place within two days of fertilization, the fetuses will develop in two separate amniotic sacs and have two placentas. This will be abouta bichorial and bioamnetic pregnancy, like that of dizygotic twins.
  • If it takes place between the 3rd and 7th day after fertilization, the pregnancy will be monochorial and biamniotic. Clearly, there will be a placenta but two separate amniotic sacs.
  • If it takes place after the 8th day, the two fetuses will share an amniotic pouch with the same placenta. This is monochorial and monoamniotic pregnancy.

If the pregnancy is bichorial and biamnetic, and the gender of the children on ultrasound is different, there is no doubt that you are expecting dizygotic twins. On the other hand, if the fetuses are of the same sex, they can be dizygotic twins / twins as well as monozygotic twins / twins! A more careful medical examination will then be necessary at birth to find out if they are real or fraternal twins, depending on their blood type. Placental DNA analysis may also be performed.

If the pregnancy is monochorial and biamniotic or monochorial and monoamniotic, it is bound to be monozygotic twins.

Monozygous pregnancy: pregnancy at risk?

Twin pregnancies are often considered high-risk pregnancies. Indeed, they present a greater risk for the mother of developing high blood pressure (pre-eclampsia) and for babies, growth retardation or premature birth. Monitoring the development of these pregnancies therefore requires very special attention.

Moreover, in the case of a monozygous pregnancy, it should be noted that the more the fetuses share appendages (placenta, amniotic sac), the greater the risk of complications. Depending on the type of pregnancy, the medical follow-up will be adapted accordingly, so that you can give birth to your children in peace. The delivery can be done vaginally or by cesarean section.

How to recognize identical twins / twins?

In general, parents can tell the difference between identical twins or twins. But for those close to them, it can be more complicated! Even more so at birth, when one baby easily looks like another. Reassure them, although they share the same genetic makeup, as they get older, your children's physical traits or peculiarities may allow them to differentiate them.

Also, just because they are monozygotic twins / twins does not mean that they will have the same character. Their behavior will be a good way for those around you to distinguish who is who. Be careful, however, that they do not play tricks on you!