Heteropaternal (meaning “from different fathers”) superfecundation and heteropaternal superfetation are both extremely rare events. Despite this, they have both been documented in medical records. Both of these processes result in fraternal, or dizygotic, twins, since identical (monozygotic) twins can only come from a single fertilized egg.
Superfecundation or superfetation can both occur naturally, but they can also be the result of certain fertility treatments. In one recorded case, there was an error in an in vitro fertilization (IVF) lab. Equipment that was contaminated with sperm from one man was used again before being cleaned.
The woman receiving the IVF treatment ended up having twins from different fathers. In theory, IVF could lead to a multiple pregnancy, even if a woman isn’t trying to get pregnant with twins!
In some cases, a woman might already have a fertilized egg or embryo implanted in her uterus when she spontaneously ovulates again. If she has sex while the second egg is viable, the phenomenon known as superfetation can occur. The resulting twins might have the same dad, or they could be twins with different fathers.
But, how do these processes really work?