When the question, “Are they twins?” is answered with a yes, you can pretty much guarantee that the next question will be, “Fraternal or identical?” We like to place people into categories, and there is no easier way to do that than by keeping it to as few options as possible. But the real world is a much more complex place, and when it comes to twins, there are many more options than simply A or B.
The purpose of a woman’s reproductive cycle is quite simple: to get pregnant. So, when that goal has been achieved, the body stops dropping eggs and shifts its attention to nurturing the fetus. This usually happens within ten days of getting pregnant, meaning that unless multiple eggs were dropped, you are probably as pregnant as you are going to get. Probably.
Superfetation is the phenomenon of a woman getting pregnant when she is already pregnant. With only ten recorded cases of superfetation in human history, to describe it as rare would be an understatement. What makes superfetation so rare is that it requires the body to “malfunction” in two very specific ways. Firstly, the uterus has to remain open for longer than the usual ten days in order to allow a second egg in. On top of that, the second egg must drop much sooner than expected. It is usually one egg every 28 days, but this can vary. Even if both of these things do happen, the woman still has to go through all the usual motions of getting pregnant.
The biggest risk with superfetation is that the second baby will be born prematurely when the mother goes into labor with the first child, but since the children are conceived within weeks of each other, this is not a major issue. In fact, the likelihood is that this has happened more than we realize, but that the parents simply assume the twins are not identical.