Twin trivia

While at the Perinatologist last week, I got a fast track biology lesson in twins. In case you are interested, here's what I learned:

There is 1 type of fraternal twin and 4 types of identical:

fraternal= 2 eggs, 2 sperm separately fertilized and they have their own placenta and amniotic sac with a fairly thick membrane separating them.

identical= 1 egg and 1 sperm fertilized and split during cell division. The following are the types of identical twins:

di/di= zygote split during cell division 1-4 days after fertilization, and they have their own placenta and amniotic sac.

mo/di= zygote split during cell division 4-8 days after fertilization, and they have their own amniotic sac but share a placenta. High risk of twin to twin transfusion syndrome (TTTS).

mo/mo= zygote split during cell division 8-11 days after fertilization, and they share both amniotic sac and placenta. High risk of cord entanglement.

conjoined twins= zygote attempted to split 12 days after fertilization, share amniotic sac, placenta and body part(s).

--Also interesting to note that the reason my hCG numbers didn't indicate twins from a blood test taken 14 and 16 days past ovulation may be because they split later in the process.