I have gotten questions from friends, acquaintances, and strangers as to whether I am pregnant with identical twins or fraternal twins. Or whether I am expect boys, girls, or both.
Can I tell this early, at 12 weeks? Has curiosity killed the cat?
From the little I understand, one cannot tell the difference between identical twins and fraternal twins in utero by the number of yolk sacs or placentas. Fraternal twins, for sure, must each have his or her own yolk sac and placenta. But, identical twins can also have their own yolk sacs and own placentas or any number of possibilities, depending on when the zygote split took place. The earlier the split took place, the more likely the twins have their own yolk sacs and placentas.
Moral of the story, if the babies are the same gender, from my understanding, there's way to determine whether one's pregnant with identical twins or fraternal twins while they are in utero. Once they are born, however, they can be genetically tested to determine what sort of twins they are.