How Twins Work

Other Types of Twinning

Ever heard of twins who celebrate their birthdays months apart? What about twins who have different biological fathers? This seems impossible, but it turns out that both of these things happen. These incidents are rare, but real.

As we mentioned earlier, some women can release more than one egg during ovulation. In some exceptional instances, these eggs are released at different times. Perhaps at 12 or even 20 days apart. Now, let's say both of these eggs are fertilized shortly after they are released. The woman conceives twins, but on two separate dates. Since the dates of conception differ, so will the dates of delivery. The twins could be born a couple of weeks or a month apart. This is known as superfetation.


When a mother releases more than one egg during ovulation and has intercourse with more than one male during that time, it may result in superfecundation twinning. Two distinct sperm from two separate individuals fertilize the two eggs the woman ovulates. Obviously, the twins resulting from superfecundation are dizygotic.

Now that we know how twins happen and what kinds of twins there are, let's see who is likely to have them.