How can women have twins with two different fathers?

WATCH: A New Jersey family recently made headlines after a DNA test showed a set of twins had two fathers. Alana Holland speaks to one of the fathers.

A paternity case out of New Jersey is drawing international attention: a DNA test revealed that a set of twins have different dads, letting one father off the hook for child support payments for the second baby.

Superfecundation twins are rare, and are conceived when a woman has sex with two different men within a small window of time while she’s ovulating. In this case, the mother said in court testimony that she had sex with both men in a span of about a week.

READ MORE: Twins have different fathers, judge finds in paternity case

It’s quite the anomaly – Passaic County Superior Court Judge Sohail Mohammed wrote in his opinion that he found only two other court cases nationally.

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“This is a case of first impression in New Jersey and only a handful of reported cases exist nationwide,” Mohammed said in his ruling.

The father has to pay $28 per week for his daughter while the other was conceived with another man’s sperm.

In superfecundation, the woman releases two eggs and each man fertilizes an egg, impregnating her separately. It’s happened before: U.S. outlets covering the rarity point to a 1991 study that suggest that these types of twins sparked more than two per cent of paternity lawsuits in the U.S.

READ MORE: After IVF surprise, mom gives birth to two sets of identical twins

One in every 13,000 paternity cases involving twins stem from sharing different fathers, according to Karl-Hans Wurzinger, a doctor who testified in the case. An egg has a lifespan of up to two days while sperm lives for up to about a week, leaving a short period of time for the phenomenon to occur.

“It is more common than we think,” Dr. Keith Eddleman, director of obstetrics at Mount Sinai Hospital in New York, told Fox News.

“In many situations, you would never know because there is no reason to do a paternity test on twins,” he said.

Dr. Jennifer Wu, an obstetrician-gynecologist at Lenox Hill Hospital, told the New York Times that the textbook example portrayed in medical journals are twins that are of different races.

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Superfecundation is becoming more common as families turn to reproductive technology to help with pregnancy.

“That’s why we’re seeing it more often than we were in the past when we were relying on nature and women who have more than one sexual partner in the same cycle around the time of ovulation,” Wu told the Times.

Identical twins share the same genetic makeup. They occur when a single sperm fertilizes a single egg, which then splits into two. Fraternal twins occur when two sperm from the same man fertilize two eggs. Both babies share the same parents.

– With files from the Associated Press