HETEROPATERNAL SUPERFECUNDATION

     A woman can conceive twins from two different biological fathers.

    Such twins are genetically half-siblings because they came from the sperm of different men who had sex with their mother around ovulation.           This is known as heteropaternal superfecundation.

For this to happen:

1. The woman must ovulate twice (dizygotic), meaning she ovulates 2 eggs in the same cycle instead of the usual 1 egg.

2. She must have sex (or In Vitro Fertilization) with different men within a couple of days to one week around her ovulation.

Heteropaternal superfecundation can occur in:

    One complex act of sex (if a woman has sex with different men at the same time)

    Separate acts of sex with different men within 5 days, 

   When sperms from 2 separate men are introduced into her via artificial insemination.

This phenomenon is 'rare'. But it is probably because people tend to believe that unidentical twins are perhaps merely from different eggs (heterozygous) not from different fathers (heteropaternal).

   Some unidentical twins may actually be heteropaternal superfecundated twins.

There are a handful of documented cases of heteropaternal superfecundation in the world. 

In all of them, the twins have different paternal DNA  because they are from two different fathers even though they were conceived in the same pregnancy and delivered together.

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