Originally posted by hikix

Twins with different fathers was once thought to be very rare. However, one recent study estimates that, worldwide, one in 12 fraternal twin sets are bi-paternal (they even have a special name for it: 'Hetero-paternal Superfecundation’).

www.multiplemom.com...

I can't believe the number is actually that high!

In the case of fraternal twins, a woman has ovulated more than once in a given month, thus releasing more than one egg. If this occurs and each egg is fertilized by sperm cells from the same man, fraternal twins are formed that have the same father.

While extremely rare, it is possible that each egg is fertilized by sperm cells from two different men, thus forming twins with two different fathers, called bi-paternal twins. According to recent studies published by the National Institutes of Health, it is estimated that approximately 1 in 12 fraternal twin sets are bi-paternal, meaning that each twin has a different father (Note that fraternal twinning is a relatively rare event in itself—fraternal twins make up about 2% of the world's population).

Superfecundation most commonly happens within hours or days of the first instance of fertilization with ova released during the same cycle. There is a small time window when eggs are able to be fertilized. Sperm cells can live inside a woman’s body for 4-5 days. Once ovulation occurs, the egg remains viable for 12-48 hours before it begins to disintegrate. Thus, the fertile period can span 5-7 days. Ovulation is usually suspended during pregnancy to prevent further ova becoming fertilized and to help increase the chances of a full term pregnancy. However, if an ovum is released after the female was already impregnated when previously ovulating, there is a chance of a second pregnancy -- albeit at a different stage of development. This is known as superfetation.