Twins with Different Fathers

Twins with different fathers (also called "bi-paternal twinning") can happen in either assisted pregnancies (when fertility treatments are used) or unassisted pregnancies (when pregnancies happen without the use of fertility drugs or treatments).  There are some self-proclaimed experts that will argue otherwise, but since fraternal twinning occurs when two different eggs are fertilized by two different sperm, fraternal twins can have two different fathers.

Fraternal twins happen when two of mom's eggs are fertilized by two different sperm.  Some people assume that fraternal twin fertilization occurs during one single act of sex.  This doesn't have to be the case at all.  One egg can be fertilized during one 'encounter', and the second egg can be fertilized at a later time as a result of a subsequent 'encounter''.  The length between fertilizing the eggs can actually involve more than one menstral cycle.  This means that if mom had sex with more than one person during the time she became pregnant with twins, the twins could have two different fathers.  Identical twins, on the other hand, are the result of one egg fertilized by one sperm that then splits in half.  Identical twins cannot have different fathers.

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'). The study suggests cultural standards, surrogacy pregnancy and fertility procedures, Mom's sexual activity, and perhaps even prostitution accounts for this frequency. The study also suggests that among married white women in the US, one pair of fraternal twin sets in 400 is bi-paternal. Documented cases of twins with two different fathers go back as far as 1810 in the United States. There is a newspaper article of white woman had both a white lover and a black lover, and she became pregnant and gave birth to twins, one white and the other of mixed race.