Twins with different fathers: Case in Portugal highlights rare copulation event

A 19-year-old girl in Portugal gave birth to twins. A DNA test showed that the man she presumed to be their father was biologically related to only one of the children, UNILAD reported.

Twin pregnancies in humans are a rather event. According to the National Health Service (NHS), only one in 250 pregnancies is a case of twins. Although we do not completely understand what may have led to a twin pregnancy, factors such as genetics, body mass index, and nutritional status have been found to play a role.

All this is, of course, assuming that the father is the same. However, there have been more than one instance when the assumed father of the twins turns out to be the father of only one child. The scientific terminology for such an event is heteroparental superfecundation (HS).

How rare is a pregnancy?

Before we get into how HS occurs, a quick biology class.

During sexual intercourse, the human male deposits millions of sperm in the female reproductive tract. Inside the harsh environment of the female vagina, most sperm perish while whipping their tails to move up the female reproductive tract.

With the energy supplied by the mitochondria inside them, the sperm navigate the arduous path of the cervix and uterus and then up one of the fallopian tubes with the hope that they will meet the egg. Only a few hundred or perhaps even lesser reach this point.

To fertilize the egg, sperm needs to beat the odds of being in the correct fallopian tube, where the ovulated egg will be available. Further, it must defeat other sperm who have also beaten the previous odds and are now fighting to fertilize the egg. An egg is usually available for a brief period of 12-24 hours, so the sperm must be present at the correct time for pregnancy.

How rare is heteroparental superfecundation?

In a twin pregnancy, two eggs might be available during this window. When they are fertilized by different sperm, the result is fraternal twins. Here the two babies share genetic material from the same father and mother, but it isn't exactly the same. So, they do not look identical.

For heteroparental superfecundation to occur, sperm from two different males must be present when the eggs present themselves and must also succeed in fertilizing them amidst the competition from other sperm. This is an extremely rare event.

In the case of the 19-year-old girl from Portugal, UNILAD reported that she recounted having sex with two different men on the same day. The pregnancy went smoothly without any complications. As per the DNA reports, each sperm succeeded in fertilizing one egg, which explains the HS.

A 2014 study found three instances of HS in the parentage test database consisting of 39,000 records. Not surprisingly, when looking at paternity suits, the number of HS cases rises dramatically to 2.4 percent.