At this time of year, Christians celebrate the birth of a human baby to a virgin mother, an impossible event without artificial reproduction (or, perhaps, divine intervention). However, in the modern day, too, a number of women report having given birth as virgins, researchers have found incidentally.

During a study of American teenagers' health from 1995 to 2009, the researchers were surprised to discover that a number of women who reported being virgins also reported being pregnant.

Of the nearly 8,000 women in the study who were interviewed confidentially and multiple times for 14 years, 5,340 women reported a pregnancy, and 45 (0.8 percent) of those who reported pregnancies also reported being a virgin. These women had not used assisted reproductive techniques that can artificially induce pregnancy.

"Our first thought was that we had made a programming error," said study researcher Amy Herring, a professor of biostatistics at the University of North Carolina at Chapel Hill.

But the researchers hadn't made an error. A number of participants had indicated the date of their first sexual intercourse and the date of their pregnancy in a way that it meant they had given birth before having had sex.

"We didn't ask the participants, specifically, if they gave birth as a virgin. Instead, they answered a series of questions on pregnancy history and a separate series of questions on vaginal intercourse," Herring said. "Based on [these] two sets of questions, we derived the virginity status at the time of pregnancy." [10 Surprising Sex Statistics]

The researchers then wanted to find the factors involved in this response pattern, Herring said.

The analysis showed that women who reported virgin births were more likely to have signed chastity pledges, which are often used by people who plan to remain chaste until marriage.

More than 30 percent of the women reporting apparent virgin births in this study had signed chastity pledges, compared with 15 percent of non-virgins who reported pregnancies and 20 percent of the other virgins.

The researchers also found that parents of women who reported virgin births were more likely to indicate lower levels of communication with their children about sex and birth control. The women, however, were not particularly more religious than the rest of the participants, according to the study published today (Dec. 17) in the medical journal BMJ.

Virgin births can happen in certain animals, including certain fish, snakes, lizards and birds, by asexual reproduction, which doesn't exist in humans.

In the religious context, however, virgin birth can have a slightly different meaning from asexual reproduction. Births without a human father still involve a non-mortal father; for example, the gospel of Matthew reports that Mary was found to be "with child" from the Holy Spirit, the researchers wrote in their study.

One source of inaccuracy in self-reports about virginity can be different definitions of sexual intercourse and virginity, the researchers said. To avoid any confusion, the questions in the survey were very explicit and precise regarding sexual intercourse that can lead to pregnancy, Herring said.

Generally, in studies regarding potentially sensitive topics, the participants may choose not to report certain experiences during in-person interviews. In the new study, the surveys were carried out using audio computer-assisted self-interview to enhance the candor of the respondents.

"Still, it's possible that some women may have discounted certain experiences or have chosen not to report certain experiences," Herring said.

This means that despite the enhancements and safeguards used in these surveys to optimize reporting accuracy, scientists may still face challenges when collecting self-reported data on sensitive topics, the researchers said. Such self-reports are still subject to some degree of respondent bias and misclassification, they said.

"This type of reporting wasn't limited to women in the study. In fact, there were a few virgin fathers lurking around in the data as well," Herring said. "That's a little harder to wrap my head around."

Email Bahar Gholipour. Follow us @livescience, Facebook & Google+. Original article on Live Science.