(IN PHOTO) Forensic scientists of the Criminal Research Institute of the National Gendarmerie (IRCGN), in Pontoise, collects DNA taken from the victims of the crash of the Germanwings Airbus A320 in the French Alps, March 30, 2015. Forensic scientists are in the active phase in the identification process of the 150 victims in the air crash. Reuters

Cases of twins having different fathers are rare in medical history, so when one such case is confirmed by doctors, it become a medical phenomenon. But to a woman in New Jersey, it became an embarrassment because DNA tests that confirmed the paternity of her two babies also revealed she had sex with another man on the same week she made love to her boyfriend.

The twin girls were born in January 2013 in Passaic County. In October 2014, the county’s board of social services, on behalf of the twin’s mother, filed for child support from the boyfriend after the woman named him as the father of the two girls, reports the New York Times.

A DNA test was ordered, and the results shocked everyone. It turned out that the boyfriend was the father of only one of the twins. The other twin was sired by another man with whom the woman had sex with on the same week when she made love to her boyfriend.

At a hearing before a Superior Court presided by Judge Sohail Mohammed, the woman admitted she left out the information in her child-support petition that she had sex with another man. The court then sought to confirm the result of the DNA test from an expert, Karl-Hanz Wurzinger.

Wurzinger pointed to a 1997 study that there was a previous case of two men fathering twins. It is a rare case that happens in one of 13,000 paternity tests of twins. The phenomenon is called heteropaternal superfecundation in which two eggs were fertilised by different men during the same menstrual cycle.

American Society of Human Genetics Executive Vice President Joanne Boughman confirms that heteropaternal superfedundation does happen. While most of the time, a woman ovulates one healthy egg per menstrual cycle, if the woman ovulates two healthy eggs and both get fertilised within the five-day fertility period, fraternal twins would occur – whether it’s by one father or two fathers.

As a result, the boyfriend was ordered to support only one child at $28 a week. Mohammed said the father of the second twin is not yet identified.

To contact the writer, email: v.hernandez@ibtimes.com.au