Identical twins: why it's never a boy and a girl
Boy and girl? Not when the twins are the same! This text explains, in an accessible way, the different types of twin pregnancies, the role of genetics and the particularities of identical twins.
Wow, twins, maybe you're going to have a guy and a girl... No!
This comment made me laugh, because sometimes it came out even when I explained that they were the same...
I understand, when we are not in it, we may have forgotten our biology classes. But right off the bat, I can confirm that identical twins are automatically... identical! A penis and a vulva are not!
There are 2 main categories of twins. Either false twins (called dizygotic) and identical twins (called monozygotic). The first type of twins is formed by the fertilization of 2 distinct eggs by 2 distinct sperm cells. In terms of genetic makeup, they are as different as a brother and a sister from 2 separate pregnancies. This type of twin is called in jargon di-di (dizygotic, di-chorial) (the chorion is one of the membranes surrounding the baby, forming the wall of the pool in which it evolves in the mother's womb). Non-identical twins represent 75% of twin couples.
The 2E twin category is formed thanks to a single egg that has been fertilized by a single sperm. The embryo thus formed split into 2 in the first days of life, making it 2 beings with the same genetic background. This type of twin represents 25% of twin couples.
How is the sex of a child determined? It is the sperm that decides the sex of the child. The egg has 2 X chromosomes (female chromosome) and the sperm cell has either an X and a Y (which gives a boy) or 2 X (which gives a girl). Since only one sperm is involved in creating identical twins, they will automatically be of the same sex.
Where it gets more complicated is that there are 3 types of identical twin pregnancies, depending on how many days the embryo had when it divided into 2 embryos.
First, there are the embryos that will have divided before their 3E day. They will each have their own amniotic pouch and each their placenta, like dizygotic twins. (In fact, this type of identical twin is often mistaken for false twins, and only DNA analysis can prove that they have the same genome.)
Second, there are the embryos that have divided in 2 between the 3E And the 7E fertilization day. They will each have their own amniotic pouch, but will only have one placenta. So they share their pantry, but they cannot touch each other directly, because they each have their own pool, a membrane separates them. This type of twin is called mono-di (monozygotic, di-chorial) in the jargon. This type of identical twin is the most common, accounting for 70% of cases.
And finally, there are the embryos that divided into 2 after the 8E fertilization day. These will be in the same amniotic pouch and will have the same placenta. They can therefore directly touch each other in the mother's womb. (This is the type of twin that Siamese twins come from, when cell separation occurs after 13E fertilization day and that it is not complete.) This type of twins is called, in jargon, mono-mono (monozygotic, mono-chorial) .This type of twin pregnancy is rare, it represents 1.2% of identical twins (the number of miscarriages associated with this type of pregnancy is high).
In-vitro fertilization, it seems, slightly increases the chances of having identical twins.
In my case, as the majority of the embryos fragmented in the test tubes, I like to think that they all wanted to be twins! In addition, I wonder what would have happened to my babies' embryo, which was transferred to me on day 3, if I had decided to let it evolve in the test tube until day 5, as I originally planned? Probably they would never have sent it to me, but I will never get an answer!
One thing is certain, I will have either 2 boys or 2 girls! And no, it is not a disappointment for us not to have “the little couple”, but rather a pride and the infinite feeling of being privileged to have identical twins.
The information I have given you here is taken from the book “The Twin Guide” written by Dr. Jean-Claude Pons, Christiane Charlemaine and Professor Émile Papiernik.
But for visuals, you can find more information here https://www.passeportsante.net/fr/grossesse/Fiche.aspx?doc=grossesse-gemellaire
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