What is a momo twin pregnancy ?
Monozygotic or dizygotic: what is the difference between identical and fraternal twins?
Whenever we talk about twin pregnancy, several terms come up. Identical twins, fraternal twins, but also monozygotic, dizygotic… What is behind these names? How are they formed? Here's a quick guide to never confuse the two again.
As in any conception, a twin pregnancy begins with a meeting. It is of course a meeting between the two partners, but especially between their gametes, the egg for the woman and the sperm for the man. This is precisely where the difference between real and fraternal twins, monozygotic and dizygotic twins lies !
Monozygotic (homozygous) twins or identical twins: what does it mean?
In the case of monozygotic twins (also called homozygous twins ), a single egg meets a single sperm. Together, they form an egg. But instead of giving a single embryo, this egg will split into two during the first cell division , and result, from cell division to cell division, in the formation of two embryos. The latter will give two fetuses and then two babies with strictly identical genetic heritage , since they come from the same sperm and the same egg. These are monozygotic twins, which are commonly called identical twins .
Girl and boy: can monozygotic twins be different sexes?
Since they come from the same sperm, which determines the sex (X for a girl, Y for a boy), identical twins are necessarily of the same sex .
Dizygotic twins: Are they identical - or not?
During ovulation, an ovary sometimes releases two eggs at the same time. This is called polyovulation, or double ovulation , a phenomenon that is increasingly common with age. And then it happens that these two eggs are fertilized by two spermatozoa during sexual intercourse, giving rise to two different embryos. This is the same pattern that occurs when two embryos are implanted in the uterus during in vitro fertilization , or when several follicles are fertilized and nestle in the uterine cavity during simple ovarian stimulation .
Note that the two spermatozoa can come from two different men, if the two sexual intercourses took place close together and around ovulation. This is a very rare phenomenon, but it can still exist. It is explained by the lifespan of gametes : an egg has a lifespan of 12 to 24 hours while spermatozoa survive 3 to 5 days in the uterus.
In dizygotic twins , unlike monozygotic twins, there is no separation of the egg into two embryos, since there are two different eggs from the start. These are then two individuals as distinct as brothers and sisters , with the difference that they evolved at the same time in their mother's uterus. The two embryos then evolve in two amniotic sacs and with two different placentas and two chorions (outer layer of the placenta ). We speak of biochorionic and biamniotic dizygotic twins , and of a bichorionic and biamniotic pregnancy.
Here, there is no risk of transfusion of cord blood from one fetus to another, since each has its own placenta for blood exchanges with the mother.
How to tell if twins are identical or false?
Diagnosis of fraternal twins is quite simple, but depends on the configurations. Thus, one can be sure that they are fraternal twins if there are two placentas, two chorions and two amniotic sacs, and if the babies are of different sexes.
On the other hand, if it is a biochorionic and biamniotic pregnancy but the fetuses are of the same sex, they may be monozygotic twins (real twins) as they may be dizygotic twins (fraternal twins). Only a thorough examination at birth will allow us to know whether they are real or fraternal twins, based on their blood group, their rhesus or their genetic heritage. Because dizygotic twins do not have the same genetic heritage, unlike monozygotic twins, or real twins.
Sex and resemblance of fraternal twins
Coming from two different spermatozoa, these embryos are therefore not necessarily of the same sex. The ovum being X, it is in fact the spermatozoon (X or Y) which will determine the sex of the fetus: female if the fetus is XX, or male if it is XY.
Like brothers and sisters, dizygotic twins or “fraternal twins” are not exactly alike; they each have their own genetic heritage.
Identical twins: monozygotic bichorionic or monochorionic… What about sharing the placenta and amniotic sac?
There are different types of monozygotic twins, depending on when the egg splits into two embryos.
- When the initial egg divides into two embryos upon fertilization (up to about 3 days later), the implantation of the two embryos occurs separately in the endometrium, the mucous membrane that lines the uterus. Consequently, we speak of bichorionic and biamniotic (or dichorionic and diamniotic) monozygotes, to signify the presence of two placentas (the chorion representing the outer layer of the placenta) and two pockets of amniotic fluid. Each embryo will therefore develop with its own chorion and in its own amnion.
- When separation occurs a little later during cell division (4 to 8 days after fertilization), implantation occurs differently so that there is a single placenta, a single chorion but two amniotic sacs. These are called monochorionic and biamniotic monozygotes . In this configuration, the two identical twins are separated by a partition delimiting the amniotic sacs.
- Finally, if the separation of the embryos occurs 8 to 13 days after fertilization, we will have monochorionic and monoamniotic monozygotic twins . The latter share the same chorion, the same placenta and the same amniotic sac, or amnion. This is the rarest configuration among these three. It should also be noted that it happens that the separation of the two embryos occurs incompletely, giving birth to Siamese twins, joined together by a part of the body.
What ultrasound can tell if it is a monozygotic or dizygotic pregnancy?
It is during the first ultrasound that the gynecologist or midwife can diagnose a twin pregnancy and specify whether there is one or more placentas.
Monochorionic pregnancy: pooling of blood that can cause problems
In the case of a monochorionic twin pregnancy (one chorion, one placenta, 20% of twin pregnancies), whether mono or biamniotic, the fetuses must "share" the same blood. Their umbilical cords are also connected. However, this configuration can lead to twin-to-twin transfusion syndrome , a rare case in which one of the two babies ends up with all or almost all the blood (the transfused one), while his twin is anemic and weakened (transfuser). This syndrome requires very specific management, to try to limit the after-effects of the twins.
Multiple pregnancy: what about triplet, quadruplet babies, etc.?
These terms monozygotic and dizygotic apply to twin pregnancies, but also to multiple pregnancies beyond twins. Triplets can thus be monozygotic, from a single egg and a single sperm, or trizygotic, from three eggs and three sperm. And the same goes for quadruplets, quintuplets, etc. On the other hand, while it is quite common for a woman to produce two eggs during ovulation, cases of tri-ovulation or quadri-ovulation are still very rare, apart from in vitro fertilization with implantation of three or four embryos.
Triplets, quadruplets and so on are therefore more often born from the same egg, which split to give several embryos, giving birth to true triplets, true quadruplets, etc.
