what are the differences with identical twins?

They are said to be "false", but they are indeed from a twin pregnancy. They are simply distinguished from the "real ones" because they did not share the same egg. This fertilization means that they will not be identical and may even have different sexes. Small point on dizygotic twins!

Twin pregnancies represented 1.6% of births in France in 2019 (source: Insee). And among them, "identical twins" (or monozygotic twins) would account for 20% to a third while "fraternal twins" (dizygotic twins) and children born of a multiple twin pregnancy, account for two thirds, or even 80. %. Whether they are "true" or "false", they are all siblings but where is the difference?

Monozygotes, dizygotes … The egg that makes the difference

A twin pregnancy does not mean that the twins will be "real". This can be monozygous or dizygotic and it is this characteristic that will affect the nature of twinning.

“Identical twins” are the result of a monozygous (or homozygous) twin pregnancy. This means that they come from one and the same egg (mono = one, zygote = egg) arises from the fertilization of a single egg by a single sperm. During the first cell division, for a reason still unknown, the egg will split in two and thus create two embryos, two fetuses and two babies. They will be physically identical and will share the same DNA. And since they come from the same sperm which is either X or Y, they are bound to be the same sex.

The "fraternal twins" are the result of a dizygotic twin pregnancy (or heterozygous), that is to say they come from two separate eggs (di = two), produced by the meeting of two eggs and of two sperm. Normally, during ovulation, only one egg is expelled from the ovary, but in this case, two eggs have been emitted and each has been fertilized separately. It may even happen that the eggs are fertilized by the sperm of two different men, if the intercourse took place in a close together, but this is extremely rare.

Each egg develops and turns into an embryo, then into a fetus, and each evolves simultaneously in the womb of the mother-to-be but in its own amniotic pouch, its own placenta and chorion (the outer envelope of the placenta). This is called a bichorial and biamniotic twin pregnancy.
Unlike dizygotes, they will therefore not share the same genetic heritage. And since they come from two different sperms, carrying X or Y chromosomes, babies can be of different sexes (XX or XY). They will share the same genetic heritage and like brothers and sisters, without looking alike, they will share common traits, inherited from their parents or from previous generations. And despite a dizygosity, the resemblance can sometimes be striking and fraternal twins can almost pass for "real" as they share common traits.

If the birth is vaginally and without being provoked, babies may come out with a more or less long interval. The record is counted in months but this is obviously a very rare case. The time difference between the birth of the two children is more often of the order of a few minutes, sometimes a few hours, rarely a few days.




How and when do you know if you are pregnant with twins, true or false?

A pregnancy test won't tell you what to expect, just that you are pregnant. It is normally thanks to the ultrasound of 12 weeks (weeks of amenorrhea) that it is possible to diagnose a twin pregnancy and identify its nature (monozygous or dizygotic), depending on the presence of one or two placentas .

  • If there are two chorions (two placentas) and two amniotic sacs, they are fraternal twins in the majority of cases. But it can also be a monozygous pregnancy.
  • If the pregnancy is monochorial (1 placenta) and biamniotic (2 bags), they are monozygotic twins.
  • If it is monochorial and monoamniotic, it is also “identical twins”.

However, the placentas, when there are two, are not always obvious to distinguish and may appear fused together, giving the impression of forming only one. In addition, a certain chorionic and amniotic configuration can leave room for doubt and only a blood test or a placental DNA analysis can know if the twins are mono or dizygotic.

DNA test to verify twinning: why and when?

Dizygotic twins are always the result of a bichorial (two placentas) and biamniotic (2 amniotic bags) pregnancy. But it can happen that this configuration also concerns monozygotic twins. Thus, if the babies present the same sex, the question of the nature of twinning arises. A zygosity test can then be performed to find out whether the children share the same genetic heritage or not.

This examination can be carried out from birth, thanks to a blood test, or by a DNA analysis. The latter is usually done by collecting cells from the mouth with a cheek swab (by rubbing the inside of the cheek with a special cotton swab). The samples are then sent to the laboratory and the results are communicated within a week of receipt. This test is at the parents' expense and is therefore not reimbursed.

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What medical follow-up when expecting fraternal twins?

Whether monozygous or dizygous, a pregnancy should be particularly monitored. This is because twinning increases the risk of preterm delivery and complications, both during term and at birth. However, depending on the presence of one or two placentas, the pace of monitoring will be different in the first two trimesters. This rate will normally be determined by the doctor following the analysis of the first ultrasound. In the case of a bichorium pregnancy, clinical monitoring is monthly and becomes bimonthly in the last trimester. If the pregnancy is monochorial, monitoring is fortnightly. Regarding childbirth, in half of the cases, it is done by cesarean section and it is also very often premature, without impacting the health of the babies.

Expecting twins being particularly physically demanding, rest is recommended and prenatal leave is 12 weeks (compared to 6 for a simple pregnancy). As for postnatal leave, it goes from 10 weeks to 22 weeks.




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What are the odds of a dizygotic twin pregnancy?

In the world, and particularly in the West, we have seen a real boom in the births of dizygotic twins from the 1970s. This strong growth is explained in particular by the strong development of two factors favoring the birth of fraternal twins: the age of the mother and the treatments for medically assisted procreation (or medically assisted procreation).

Indeed, women tend to have children later. The likelihood of having fraternal twins increases with age. Why ? Because the level of follicle stimulating hormone (FSH) increases steadily until the age of 37 on average, promoting the expulsion of several eggs and therefore the chances of multiple pregnancies.

Because it consisted, until 2010, in the maturation and implementation of several embryos to increase the chances of pregnancy, Medically Assisted Reproduction, whether it is IVF, artificial insemination or even induction of ovulation, is considered to be the main cause of the increase in births of fraternal twins. Today, however, advances mean that only an embryo, cultured longer, can be implanted and sufficient, but in some cases it is still preferable to proceed with a double implantation, which may result in a twin pregnancy.

Other factors are also involved in dizygotic twinning:

  • Ethnic origin. The dizygotic twinning rate is twice as high in sub-Saharan Africa as in Europe and remains very rare in China. For example, in Nigeria, among the Yoruba people, fraternal twins represent 45 births in 1000! Fun fact, identical twins are also much more common there and no one really knows why.
  • Genetic inheritance. If a woman has a family history of a dizygotic pregnancy, she is more likely to become pregnant with fraternal twins because she may have inherited a tendency to double ovulate.
  • History of multiple pregnancy. Likewise, this tendency to multiple ovulation can mean that a woman who naturally had fraternal twins during pregnancy could once again have a twin pregnancy. This case can also occur and recur with ART and the implantation of two embryos.
  • Fertility. A study conducted on a previous dizygotic twinning boom after World War I found that hyper-fertile couples were more likely to have fraternal twins.
  • The rank of birth. A comparative study conducted in the 1960s in the United States showed that at the same age, the rate of fraternal twins increased with the number of previous pregnancies.




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Triplets, quadruplets, etc. : other true and fraternal twins

We tend to distinguish, surely to avoid confusion, twin pregnancies and multiple pregnancies to speak of triplets, quadruplets or more. However, the two designate the same thing since twinning is not limited to two children, and two children are, by definition, multiple. And just like double designs, they can give rise to real or false triplets or quadruplets.

Triplets can be monozygous and result from a single egg (a single egg fertilized by a single sperm) which, during the first cell division, would have split into three. They can also be trizygous and come from the fertilization of 3 eggs simultaneously expelled naturally by 3 different spermatozoa. The same goes for quadruplets or quintuplets. However, both cases remain extremely rare. They are so infrequent that truly reliable figures are lacking.

Most of the time, these multiple pregnancies are the result of ART treatments and the implantation of several embryos.