With Christmas all but a week away, what better way for scientists to spend the holiday season than researching modern day cases of virgin birth?

As it turns out, researchers at the University of North Carolina at Chapel Hill found the number to be slightly greater than zero. In fact, out of the 7,870 women surveyed as part of the U.S. National Longitudinal Study of Adolescent Health (Add Health), 0.5 percent of respondents confirmed their status as virgins who, without the use of assisted reproductive technology, such as in vitro fertilization, still reported having given birth.

The most glaring question, of course, is: How? How could a woman conceive a child without the necessary second set of chromosomes? How could the egg know to divide without first receiving a signal from the sperm? And what about the genes that get imprinted in the correct order once a sperm cell fertilizes an egg — how do those genes get properly sorted? These questions aren’t easily answered, but according to lead researcher of the present study, Professor Amy Herring, perhaps they don’t need to be.

Herring and her colleagues set up an experiment where subjects were able to reply candidly to both computer-generated self-interviews and audio computer-generated self-interviews when asked about their sexual history. The team didn’t explicitly ask about virgin pregnancy. Instead, they used the subjects’ replies to create a rough timeline of when they began having sexual intercourse and at which point they became pregnant. The median age at which virgins reportedly gave birth was 19.3 years.

Interestingly, Herring and her team didn’t set out to analyze virgin births. “While analyzing data for a separate project that examined correlates of virginity in adulthood, we were surprised to discover that a number of these individuals who stated they were virgins also reported pregnancies,” Herring said. “Once we confirmed these were not programming errors, we became interested in understanding factors related to this type of response pattern.”

They wanted to know why, for instance, 45 women reported birthing a child despite never having had sex. Further intriguing, those affirming virgin births were more likely to have signed chastity pledges (31 percent), compared to non-virgins reporting pregnancies (15 percent) and other non-virgins (21 percent). Such a finding is difficult to extract meaning from, though Herring acknowledges this may reflect the team’s anticipated reporting bias.

“Signing a chastity pledge has been shown to be related to religious faith and other cultural mores valuing virginity,” she said, referring to the potential for women to essentially cover up their pregnancy with their faith, in order to protect their pledge, “though we have no way of knowing for sure,” Herring said. As for the science behind such a phenomenon, she said she knew of “no medically-validated reports of virgin births in humans, or other mammals.”

Invertebrates are another story, however, as many reproduce through duplicate genes in the maternal germline. Even some worms and scorpions, along with certain species of bees, are able to produce genetically identical offspring through an asexual form of reproduction called parthenogenesis. While the research into mammalian parthenogenesis has been spotty, a great deal of mystery still surrounds the third constraint, namely, the imprinting of parental genes into the offspring.

Under normal conditions, both the sperm and egg orient the DNA in such a way that each individual protein gets produced in the correct amounts. When the two join, they work as a tag team to compromise on agreeable levels. Whether the women in Herring’s study gave birth because of such a quirk of genetics may be a mystery only the women themselves can solve.

“I just think it’s too complex and you’d need too many things to happen accidentally,” Dr. Marisa Bartolomei, molecular geneticist at the University of Pennsylvania, told Popular Science. “Is there a mutation that could eliminate all imprinting, so we would see that we didn’t need Dad or Mom in order to have normal development? This is a question that people have asked a lot, and we don’t know the answer.”

Source: Herring A, Attard S, Gordon-Larsen P, Joyner W, Halpern C. Like a virgin (mother): analysis of data from a longitudinal, US population representative sample survey. BMJ. 2013.