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Does an ultrasound reveal if twins are identical?
Q: I’m 20 weeks pregnant with twins. The doctor told me that they’re identical. I am not sure how he knows so early. Can an ultrasound tell if they’re identical?
A: Ultrasound can help determine if twins are identical or fraternal. The trick is to look at the two sacs surrounding the babies. If they share the chorionic sac, they’re identical. Identical twins can also share the amniotic sac, but in this case they also share the chorion. I presume your doctor must have seen a single chorion surrounding the babies.

Just because the babies aren’t in the same sac, they can still be identical. This is possible depending upon the timing of the development of the twins. Without getting too technical, identical twins start as just one dividing embryo. Sometime early on, the embryo splits. If they split before the amnion has formed, they will have separate amniotic sacs. If they split after the amnion has formed, they will share a single amniotic sac. Likewise, the timing of the split will also dictate whether there are two chorionic sacs.

Fraternal twins develop independently and don’t share either sac.