In a first for Vietnam, a paternity test has shown that a set of twins have the same mother, but were conceived by separate fathers. Known as bi-paternal twins, this occurrence is exceptionally rare, with only 10 such cases in the world ever having been confirmed using DNA testing.

Viet Nam News reports that the presumed father ordered a paternity test after receiving pressure from a family member who noted that the twins looked very different - for example, one had straight, fine hair, while the other had thick and wavy hair. The test showed that the wife was the mother of both children, but her husband was only the father of one.

To explain how this can happen, you first have to understand how twins are formed. They're actually pretty rare as it is - according to the CDC, in 2009, only 3.3 percent of births in the US resulted in twins.

There are two different types of twins: identical or fraternal. Identical twins are monozygotic, meaning the embryo forms with one sperm and egg and then splits into two. But fraternal twins are dizygotic, meaning there are two separate eggs in the uterus and are fertilised by two separate sperm. Basically, fraternal twins are ordinary siblings that just happen to be born at the same time, whereas identical twins are formed from the same egg.

Now here's where it gets interesting - seeing as fraternal twins involve two separate eggs, they don't have to be fertilised by the same father. But how does that work?

We know that sperm can survive in the uterus for up to five days after sex, and there's a 24-hour window in which an egg can be fertilised, so that means that if a woman has sex with multiple partners in the space of a few days, and has released two eggs during ovulation, she could end up with bi-paternal twins.

Unfortunately, we don't have solid statistics on the likelihood of bi-paternal twins occurring, as there have only been a handful of confirmed paternity cases relating to twins from separate fathers worldwide.

But there was another case of bi-paternal twins recorded last year in the US. As we reported at the time, in that instance, their assumed father was ordered to pay child support for only one of the twins, as the paternity test showed he was the biological father to only one of them. The mother testified that within a week she'd had sex with two men, one of which was the presumed father, and another unidentified by the court.

Interestingly, even when twins don't have different fathers, they can still look up ending totally different - as these twin sisters from the UK prove.

TMZ/YouTube

In this case, the sisters had the same father, but seeing as their mother is half-Jamaican, she carried both the genes for black and white skin, and one sister ended up with each.

It's pretty amazing what the human body is capable of.