Siblings can have different fathers; can twins born of a woman have two fathers? The straight answer is yes. Twins could be “bipaternal.” This phenomenon occurs quite rarely (1 in a billion to be exact), but several pairs of non-identical twins have been born, tested and found to be the products of one womb, one pregnancy, but two fathers.

It is often assumed that for twins, both eggs were fertilized during a single act of intercourse. However it is quite possible for one egg to be fertilized during one act of intercourse, and the other during another if the woman has intercourse with two men within hours.

Normally, women ovulate only one healthy egg per cycle. Fraternal twins are born when women ovulate two healthy eggs and both get fertilised. But it is also possible for two eggs to be ovulated during the same cycle and fertilised at different points within the five-day fertility window resulting in twins. This process is known as “superfecundation”.

What this means is that each egg can be fertilised by different sperms. If a woman ovulates two or more eggs and she has sex with more than one man while she’s fertile, “heteropaternal superfecundation” can occur, if the eggs get fertilised by sperm from the two different fathers within the same ovulation period. Medical research shows that when a woman has s*x with two men within the same ovulation window, it can result in bi-paternal twins, that come when a woman releases two eggs during ovulation instead of one — and both eggs are then fertilized. With bi-paternal twins, each egg is fertilized by a different man’s sperm.

A woman’s eggs can live up to 24 hours in her reproductive system. Sperm, on the other hand, can live up to five days in the woman’s body. So it is possible that a fertile woman could have s*x with one man and have sex with a different man another time within the 24 hour period — and then when her eggs are released, the two men’s sperm living inside her could each fertilize an egg.

Marcus and Lucas ­ are twins with two ­ different fathers. They are one of only three known sets of twins in the world who are genetic ­ half-brothers. Their mother Charlotte became ­ pregnant when she slept with ex-husband Michael and new boyfriend Tommy within 48 hours. Nine months later she gave birth to twins Marcus and Lucas, who are genetic ­ half-brothers because they were fathered by both men.

Marcus was fathered by Michael and Lucas by Tommy. Charlotte, who already had two sons by Michael Nielsen, divorced him in August 2004. She began dating Tommy ­ even as Michael still hoped to get back together with her and in the heat of the moment they ended up in bed together. Charlotte kept quiet about the incident with Michael which was a one-off and went to bed with Tommy.

She later discovered she was pregnant with twins and decided ­ to inform both men either could be father of the twins. Marcus and Lucas were just a ­ couple of weeks old when it was discovered they didn’t look alike. Tommy and Michael took a DNA test to find out for certain that the boys had different fathers. The test proved the twins did indeed have different fathers. It stated: “The results do not imply that the children Lucas Hilbrandt Nielsen and Marcus Hilbrandt Nielsen have the same biological father. The probability of this preclusion exceeds 99.99 per cent.”

Similar cases of bi-paternal twins have popped up over time. In May 2015, a case in New Jersey made news when a court ruled that a father did not have to pay child support for a second twin because DNA evidence showed that he was a father to only one. The mother gave birth to twin girls and named a romantic partner, as the father of both children when applying for public assistance. But after she admitted that she had sex with another unidentified man within a week of having had sex with the bonafide partner, a DNA test was ordered.

The test results confirmed both men fathered each of the girls. CNN also reported fraternal twins in Vietnam found to have different fathers in an “extremely rare” case that came to light through a DNA test. They had their DNA tested at the Centre for Genetic Analysis and Technology in Hanoi because family members had been vocal about notable differences in the children’s appearance. There are rare cases in which a woman may ovulate more than once during her cycle, meaning that two eggs would be released at different times.

There are less than 10 known cases of twins with different fathers in the world. Fraternal twins make up about 2 percent of pregnancies and bi-paternal twins are a fraction of them. But twins, who don’t look alike, do not necessarily have separate fathers. Twins who have the same father can look completely different and may even appear to be of different races. Less than one (0.001) per cent of fraternal twins are from different fathers.

According to Dr. Virginia Beckett of the Royal College of Obstetricians and Gynaecologists, if a woman is going to sleep with more than one partner at the same time it is perfectly feasible that if she has twins they could have different fathers.

“In this case these twins were conceived at the same time even though the mother did not have sex with the fathers at the same time. The sperm from the first man would have been in her fallopian tube, where it is effective for three to four days. Then she had sex with the second man and released two eggs.

The eggs, which live for 24 hours, were fertilised by different sperm from the different men. The science is easily explained, but it may have been less easy for the mother to explain.