Monozygotes or Dizygotes: what is the difference between true and fraternal twins?

As in any conception, twin pregnancy begins with a meeting. This is of course a meeting between the two partners, but above all between their gametes, an egg for the woman and a sperm for the man. This is precisely where the difference between true and fraternal twins, monozygotes and dizygotes lies.

Monozygotic twins or identical twins: only one egg at the start

In the case of monozygotes (also called homozygotes), a single egg meets a single sperm. Together they form an egg. But instead of giving a single embryo, this egg will split in two during the first cell division, and result, from cell division to cell division, in the formation of two embryos. These will give two fetuses then two babies with strictly identical genetic makeup, since it comes from the same sperm and the same egg. They are monozygotic twins, which are called identical twins in everyday language. Moreover, 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 or fraternal twins: two eggs at the start

During ovulation, an ovary may expel two oocytes at the same time. This is called a polyovulation, or double ovulation, a phenomenon which is more and more frequent with age. And then it happens that these two eggs are fertilized by two sperm during 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 intercourse takes place close together and around ovulation. It is a very rare phenomenon but it can still exist. It is explained by the lifespan of the gametes: an egg has a lifespan of 12 to 24 hours while the spermatozoa survive from 3 to 5 days in the uterus.

At the dizygotic twins, unlike monozygotes, there is no separation of the egg into two embryos since there is two different eggs from the start. It is then a question of two individuals as distinct as siblings, with the difference that they evolved at the same time in their mother’s womb. The two embryos then evolve in two amniotic sacs and with two different placentas and two chorions (outer layer of the placenta). We are talking about biochorionic and biamniotic dizygotic twins, and a bichorial and biamniotic pregnancy. Here, there is no risk of cord blood transfusion from one fetus to another, since each has its own placenta for blood exchange with the mother.

The 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 sex. On the other hand, if it is a biochorial and biamniotic pregnancy but the fetuses are of the same sex, they can be monozygotic twins (identical twins) as they can be dizygotic twins (fraternal twins). . Only a thorough examination at birth will allow us to know whether they are real or fraternal twins, given their blood group, their rh or their genetic heritage. Because two dizygotic twins do not have the same genetic heritage, unlike monozygotic twins, or identical twins.

Sex and resemblance of fraternal twins

Derived from two different spermatozoa, these embryos are therefore not necessarily of the same sex. The egg being X, it is indeed the sperm (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.

Real twins: what about sharing the placenta and amniotic sac?

There are different types of monozygotic twins, depending on when the egg has split into two embryos.

• When the initial egg divides into two embryos upon fertilization (up to about 3 days later), the two embryos will implant themselves separately in the endometrium, the mucous membrane that lines the uterus. As a result, we speak of bichorionic monozygotes and biamniotics (or dichorionic and diamniotic), 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 evolve with its own chorion and in its own amnion.

• When the separation takes place a little later in the cell division (4-8 days after fertilization), the implantation is done differently so that there is a single placenta, a single chorion, but two amniotic sacs. We are talking about monochorionic and biamniotic monozygotes. In this configuration, the two identical twins are separated by a partition delimiting the amniotic pockets.

• Finally, if the separation of the embryos takes place 8 to 13 days after fertilization, we will have twins monochorionic and monoamniotic monozygotes. The latter share the same chorion, the same placenta and the same amniotic sac, or amnion. This is the rarest configuration among these three. Note also that sometimes the separation of the two embryos takes place incompletely, giving birth to Siamese twins, linked together by a part of the body.

Note that 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.

Monochorial pregnancy: pooling of blood that can be a problem

Note that in the case of a twin pregnancy monochoriale (a single chorion, a single 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 transfusion-transfusion syndrome, a rare case which involves one of the two babies ending up with all or almost all the blood (the transfused), while his twin is anemic and weakened (transfusionist). This syndrome requires very special care, in an attempt to limit the sequelae of the twins.

Multiple pregnancy: what about triplets, quadruplets etc. ?

Note that these names of monozygotes and dizygotes apply to twin pregnancy but also to multiple pregnancies beyond twins. Triplets can thus be monozygotes, resulting from a single ovum and a single sperm, or trizygotes, resulting from three ova and three spermatozoa. And the same goes for quadruplets, quintuplets, etc. On the other hand, if it is quite common for a woman to produce two eggs during an 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 the result of the same egg which split to give several embryos, giving rise to real triplets, real quadruplets etc.