Re:Will They Be Identical or Fraternal?
You may not be able to tell until after they are born (and even then, it can take a DNA test). When I had my twins, I asked at the 20 week sonogram and was told that they were “in all likelihood” going to be fraternal. As soon as they were born, it was very clear that they were, in fact, fraternal. But some people actually can’t even tell immediately after birth! It all depends on the eggs.
Eggs that divide within a few days of ovulation are known as diamniotic-dichorionic (Di-Di). The amnion and chorion are the two layers of the sac of fluid that surrounds and protects the babies in utero. DI-DI means that each baby has its own amnion and its own chorion. But they will be identical because they split from the same egg (a.k.a. monozygotic – “mono” means one and “zygote” means egg).
Eggs that split at about three to eight days after ovulation create twins that are diamnionic (each has it's own amnion) and monochorionic ( they share a single chorion). And they, also, are identical because they started as the same egg.
Dizygotic (“di” means two) twins are fraternal twins. Fraternal twins are twins formed from two separate fertilized eggs.
If the sonographer can see that the babies share the inner sac or chorion, it will be clear that the twins are identical. If the babies are of opposite sexes, they are obviously fraternal. If they each have their own chorion, the twins can be identical or fraternal. In my case, I thought that the doctor might have suggested that the boys were fraternal because of the differences in their size. However, identical twins can be different lengths and weights depending on the distribution of nutrition in your uterus. Ultimately, the doctor explained that he saw such a distinct septum between the babies’ chorion that he was confident that they were dichorionic and that more than 2/3 of dichorionic twins are fraternal.
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