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I am pregnant with twins. When will I know if they are fraternal or identical twins? The expert answers:
Congratulations on your twin pregnancy! You may be able to determine whether you have fraternal or identical twins as early as your first ultrasound at six to 10 weeks.
At that time, the membrane between the two sacs can be evaluated. If there is no membrane (a single sac) or if the membrane is very thin, it means the babies both came from a single embryo and are identical twins.
If the membrane is thick, the twins could be fraternal or identical. You will then need to wait for later ultrasounds for more information. If you have an amniocentesis or an ultrasound for anatomy at 18 to 20 weeks, you may be able to learn the sex of the babies. If they are different sexes, then they are definitely fraternal. If they are the same sex, you may need to patiently wait until they are born. Even then, it sometimes takes blood tests on the babies to be absolutely sure.