It might sound like a twist from a movie, but it is possible for fraternal (not identical) twins to come from two different dads. The circumstances must have been very specific, but some experts estimate that 1 to 2 percent of fraternal twins have different fathers. This phenomenon is known as Superfecundation or heteroparental superfecundation. We all know that siblings can have different fathers, technically making them half-siblings. But what about twins?
Yup, it happens. One study estimates that as many as 1 in 400 sets of fraternal twins is ‘bi paternal’. It is more common these days thanks to Assisted Reproductive Technology, for example, a gay male couple both contribute sperm to pregnancy they can have a fraternal twin.
So what exactly is this phenomenon and how can it occur?
This is twins born from two fathers. Each twin has a separate father. For this to happen, two things must have to happen:
- The woman should have to ovulate twice (dizygotic). This is not so rare and probably happens in more than 1 in 50-100 ovulations.
- She should have to have sexual intercourse(or IVF) with different men within a couple of days to one week before ovulation. That allows two different sperms to fertilize two separate eggs.
Both of these events can happen from having intercourse with separate men on the same day or different days, as sperm can survive for up to 5 days. Multiple ovulations happen in about 1% (or more) of all ovulations, and more often when you take ovulation-inducing medications. This is only logical then, that when a woman has sex with more than one man while she’s fertile, ‘heteropaternal superfecundation’ can occur. That is, each egg can be fertilized by a different father.
A word of advice
If you know twins who look nothing alike, don’t assume that they have separate fathers. Like any siblings, twins who have the same father can look completely different from each other. They may even appear to be of different races.