Admittedly, “overpregnancy” is really not a nice word – but it’s still part of official medical German. According to current reports, what happened to a mother in Brazil can also be more elegantly called “overfertilization” or even (Latin) “superfecundatio”. What is meant is always the same: a multiple pregnancy with different fathers. So this is now being reported from Brazil. Similar cases have been reported from different countries in previous years, and each time the world is a little amazed. But why actually? If there are fraternal twins with the same father, why shouldn’t half-siblings, each with their own placenta, be able to grow up in their mother’s womb? One thing is certain: it works.
It’s much more common in other mammals, including humans. It is rarer, but not extremely rare – in the case of twin paternity disputes, which often arise precisely because the mother had several possible sex partners, there were sometimes more than two percent of such cases. Incidentally, there is also the case where two twins come from one and the same man, but are not exactly the same age. The principle behind this is always the same: in the same menstrual cycle, several eggs must successfully jump, in this case two, and both must meet a fit sperm in order to fuse with it. This can also happen with a small difference in time, and the lifespan of sperm also allows for a lot, because it can be up to five days. Conversely, it is clear: Identical siblings from different fathers cannot exist. Because they are always genetically identical.
So what is necessary for all this does not irritate biologists, but at most moralists: It requires sex with several partners at short intervals – at least if you exclude the criminal energy of immoral sperm bank operators. This is by no means theoretical either: in 2017 it became known that a Dutch doctor not only secretly fathered at least 49 offspring with his own sperm, but also mixed sperm donations from different men and used them for insemination. He had forged the relevant documents. He was never punished because he had previously died at the age of 89.
The usual cause of twins from different fathers is and remains sex, and the 19-year-old mother of the Brazilian babies told TV station Globo that it was, and was (like most people who are born twins) to Globo encountered a case for the first time) surprised that such a phenomenon exists.
Evolutionary scientists have a fairly sober view of all of this — especially when they focus on what, rather than whole humans, sperm actually “want.” And they just want to win in the race to the egg cell (or egg cells). Because they carry what survives us for generations and generations: our genes. And they are quite competitive, which is why there has been a research tradition on sperm competition, the so-called “sperm wars”, since the 1990s. Even if there is only one sexual partner, there are millions of sperm that enter the race for usually only one precious egg cell, so that the winning spermatozoon is comparable to a lottery winner. There is also competition when there are several partners – and the fear of this is evidently ingrained deep in the subconscious of male creatures: they guard females, veil females, seal female genitalia (e.g. among insects) with special plugs after copulation, and so on.
Thanks to the technology available today, it is at least possible to divide the maintenance obligations fairly in cases of doubt: each of the half-siblings from a pregnancy can be clearly assigned to their biological father.
