At this time of year, many recount the birth of Jesus to the Virgin Mary. But reports consistent with virgin births are also a modern day phenomenon, according to a study in the Christmas edition of The BMJ.

A team of US researchers has identified a number of pregnancies reported by virgins since the mid-1990s among a large group of young US adults.

Virgin births in non-humans generally occur by asexual reproduction, and have been documented in multiple animals including pit vipers, boa constrictors, sharks, and Komodo dragons, but among humans the incidence of virgin pregnancy has received little attention.

So researchers based at The University of North Carolina at Chapel Hill examined the incidence of pregnancy before the onset of sexual intercourse (virgin pregnancy) reported by a representative group of US adolescents and young adults.

They analysed data for 7,870 women who were interviewed confidentially and multiple times over a 14-year period between adolescence and adulthood as part of the US National Longitudinal Study of Adolescent Health (Add Health). State of the art technology (audio computer assisted self interview and computer assisted self interview) was used to enhance the candor of the respondents.

At each interview, participants reported their history of vaginal intercourse, use of assisted reproductive technology, and pregnancy history. These data were then used to classify women by their virginity status at the time of reported pregnancy.

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Other information such as age, importance of religion, and presence of a chastity pledge, was recorded, and respondents also indicated their knowledge of different birth control methods. Parents of respondents reported how much they had talked with their child about sex or birth control, and school administrators reported whether sex education was offered in the respondent's school.

Of 7,870 women, 0.5% consistently affirmed their status as virgins and did not use assisted reproductive technology, yet reported a virgin birth. These women were more likely to have signed chastity pledges (31%) than the non-virgins who reported pregnancies (15%) or the other virgins (21%).

Virgins who reported pregnancy were more likely than non-virgins to have parents who indicated lower levels of communication about sex and birth control with their child.

While more virgins gave birth to boys (60%) or may have learnt they were pregnant during Advent, these trends did not reach statistical significance.

Virgins were younger on average at the time they gave birth (median age 19.3 years) than non-virgins (21.7 years). Perceived importance of religion was associated with virginity but not with virgin pregnancy.

Although the study used carefully designed questions and state of the art self interview technology, the authors point out that self reported measures of potentially sensitive topics are subject to some degree of respondent bias and misclassification. Nevertheless, they conclude that "around 0.5% of women affirmed their status as virgins and did not use assisted reproductive technology, yet reported virgin births."

They add: "Reporting dates of pregnancy and sexual initiation consistent with virgin pregnancy was associated with cultural mores highly valuing virginity, specifically signing chastity pledges, and with parental endorsement of items indicative of lower levels of communication about sex and birth control."

Editor's Note: The British Medical Journal traditionally publishes a Christmas issue containing a number of articles of a lighthearted nature. For a full list of articles in the 2013 issue, see: http://www.bmj.com/content/347/7938