I have learned a lot about twins since finding out we have a set on the way. Some of it is so fascinating, I thought the rest of you might want to know:
- Some twins share both an amniotic sac and a placenta. These twins are always identical and this is a very rare and extremely high-risk pregnancy.
- Some twins share a placenta but have separate amniotic sacs; these are also always identical and this type of pregnancy also carries a higher risk.
- Some twins have separate amniotic sacs and separate placentas. This type of twin is often fraternal but sometimes identical. (YES -- it's possible to have identical twins with separate sacs and placentas!) If these twins are born the same gender, there is no way to tell if they are identical without a blood test.
Note: I am pregnant with this third kind of twin. And no, we are not going to find out the gender(s) in advance!
- Identical twins do not run in families. They are completely random, a "fluke" of nature, and have nothing to do with genetics.
- Fraternal twins do run in families -- but only in the mother's line. So there's no sense asking a mom of twins whether twins run in her husband's family!
- If you see a set of girl/boy twins, they are fraternal! There is no possible way identical twins can be different genders.