Paternity of Twins-Not Always the Same Father
Most of us would suspect that seeing a couple with fraternal twins means that two eggs were released at the same time or close to the same time and one act of intercourse produced two babies. However, this is not always the case due to two phenomena called heteropaternal superfecundation and superfetation.
Heteropaternal superfecundation occurs when two eggs are released during the same cycle, but the fertilized at different times by different men. This is not as rare as it sounds, with one study estimating that 1 in 400 sets of fraternal twins has different fathers or what experts term as bipaternal.
More rare though is superfetation, which occurs when an already pregnant woman releases an egg a few weeks after she has already conceived. Experts believe this can occur when implantation is delayed which prevents the initial spike in pregnancy hormones. Without that spike, the ovaries do not get the signal to go into pregnancy mode, which may lead to a pseudo-cycle that can produce another healthy egg. Again, if the woman has multiple sexual partners, then it is quite possible that these children could have different fathers as well.