Rarely, twins can have totally different fathers…
There are less than 10 identified cases worldwide where twins have different fathers – one case in New Jersey in 2015 a DNA test resulted in an identified father only having to pay maintenance for one of the children.
Normally at ovulation a woman produces only one egg, but occasionally there are two and if both get fertilised, then fraternal twins are created. Sperm is tough stuff. It can stay alive for several days inside the body and although there are mechanisms to fight off the sperm of an intruder, it is possible for two men to fertilise different eggs in the same 5 to 7 day fertile part of the female cycle. One expert calculated in 1997 that this may occur in 1 in 13,000 cases, but nevertheless, in the majority of cases it probably goes unnoticed, unless of course that the babies are obviously of two different racial origins!
This of course has a complex scientific term associated with it…. hetero-paternal super-fecundation, believe it or not. The hetero-paternal means that there is a fertilisation event form more than one father and super-fecundation is the fertilisation of more than one egg from separate intercourse events. It means that different eggs are fertilised by different men with a few days. The event could be due to multiple egg re-lease during one ovulation or a second ovulation event with a short period of time.
Although this is rare, if DNA testing is required, we would always recommend that both of a set of twins are tested. You can read our explanation of twinning (click here), suffice to say here that hetero-paternal super-fecundation differed from fraternal or two cell (dizygotic) twins in that instead of being full siblings they will be half siblings, as a result of having a different father. Fraternal twins share half of their DNA with each other and occur in about two out of three twinning events. There is no particular sex bias and fraternal twins can be either both male, both female or one of each.