It’s a rare case of heteropaternal superfecundation. A report on bbc.com is telling the story of a family in Vietnam that has just discovered that their fraternal twin boys have two different fathers.
How does the family know for sure? The boys, who were put through genetic testing that determined their difference, were drawing more and more attention from relatives and close friends as one of the brothers was growing to look decidedly different from both his siblings and parents.
According to the BBC, there are currently less than 10 known cases of twins with different fathers in the world. The condition happens when a women’s eggs are fertilized by two different men in a short period of time, resulting in two different fathers for two children.
It’s such a rare event for a few reasons. Superfecundation most commonly happens within hours or simply days of the first instance of fertilization, when it involves two eggs released during the same cycle. The time window in which eggs are able to be fertilized in a woman’s body is small, with eggs remaining viable for just 12–48 hours before they begin to disintegrate.
Adding to this, sperm cells can only live inside a female’s body for around four to five days.
It is possible to have a second pregnancy occur in a woman, at a later stage of development, which is known as superfetation, but this is an equally unusual, if not rarer condition. This is because, normally, ovulation is suspended by the body during pregnancy, in order to prevent further fertilization from happening and to increase the chances of the first pregnancy going full term.
If a woman’s body acts irregularly and does actually ovulate after the first pregnancy has begun, it is possible that two babies with different due dates can be growing in the womb at the same time. But very rare.
Vietnamese news outlets began reporting on the above case earlier this month, and have altered the location, names and time frame of the occurrence.