About a third of fraternal twins are boy-girl pairs, and about two thirds are same-sex pairs. This makes sense if you consider that there are three possible combinations: GG, BB, and GB. read more
Identical twins are always the same sex and blood type. They almost always share the same placenta but may also have two separate placentas. Depending on when the egg splits usually determines if identical twins will share the same placenta, and/or chorions and amnions. read more