A woman in China recently gave birth to two twins, but found out soon after that they had two different fathers, Vice reported.

This phenomenon has happened before and is called heteropaternal superfecundation.

Heteropaternal superfecundation occurs when two sperm, each from a different man, fertilize two different eggs from the same woman.

Soon after the birth of her twin babies, a Chinese woman and her husband noticed something strange — the two babies looked pretty different, and only one resembled the husband.

Following a DNA test, the couple learned that the babies were actually from two different fathers and the mother then revealed she had a previous one-night stand, Vice reported.

Although cases like this are rare, they do happen. The phenomenon is known as heteropaternal superfecundation. Usually, these cases only become known for legal reasons, when a man is questioning whether he is the biological father.

Usually, these cases only become known for legal reasons, when a man is questioning whether he is the biological father. Wikimedia Commons

According to a 1992 study, an estimated 2.4% of paternity lawsuits are found to be cases of heteropaternal superfecundation. That number should be taken with a grain of salt, however — it only looked at cases where people sought legal counsel or went to court. Also, paternity tests aren't normally part of the birth process so it's possible that many cases are never discovered.

Heteropaternal superfecundation happens when two sperm from two different men fertilize two eggs from the same woman. This all happens in about a week's time, according to doctors, since ejaculated sperm can only survive for about five days.

Another reported case of heteropaternal superfecundation occurred in 2015, when a New Jersey woman learned that the man she was trying to get child support from was only the father of one of her twins.