There are less than 10 cases in the world where twins have been found to have different dads.
In New Jersey 2015, a DNA test led to a dad only having to pay maintenance for one of the twins.
Normally when a woman ovulates she releases just one egg. But sometimes she can release two eggs. Non-identical twins are created if both of these eggs are fertilised by a one man’s sperm.
Sperm is tough stuff. Even though a woman’s body goes into a defensive mode to fight off sperm, it can stay alive for a couple of days inside her body.
Weird as it may seem, it is possible for two men to fertilise a different egg each in the same 5-7 day window when a female is ovulating (when egg/s are released during her monthly cycle). The likelihood of having twins to different men has been calculated to happen 1 in 13,000 events. In many cases it probably goes unnoticed, unless the babies are obviously of two different racial origins.
The sciency name for this phenomena is ‘hetero-paternal super-fecunation’. The hetero-paternal part means that the eggs have been fertilised by more than one man. The super-fecunation part is the fertilisation of these eggs from different intercourse events. Basically, it means that different eggs are fertilised by different men within a few days of each other. This could be down to more than one egg being released during one cycle or a second ovulation event happening within a short period of time.
These cases are very rare, so if DNA testing is required, we would always recommend that both twins in the set are tested.
Hetero-paternal super-fecundation, a set of twins with different dads, will be half siblings. Unlike fraternal twins (2 separate eggs fertilised by the same man) which are full siblings, as they share half of their DNA with each other.
Fraternal twins can come in any combination. They can either be both boys, both girls or one of each! Obviously identical twins are the same sex as one egg fertilised by the sperm of one person has split in two!