The British Medical Journal, in its Dec. 17 Christmas issue, publishes a study titled "Like a virgin (mother): analysis of data from a longitudinal, US population representative sample survey." The authors looked at 7,840 women over the period from 1996 to 2009 and found that 45 of them reported a pregnancy despite being a virgin. Of the 5,340 pregnancies reported by study participants, 0.8% were virgin pregnancies, and 36 of these women gave birth.
These virgin pregnancies were not associated with non-traditional forms of insemination. Chastity pledges appear to have a correlation with the stated virgin pregnancy. Among study participants who became pregnant, 15 percent of non-virgins reported making such a pledge. At least 15 of the 45 virgins who became pregnant, 30.8 percent, had taken a chastity pledge. Among virgins who did not become pregnant, the rate was 21.2 percent.
The study also notes:
While more virgins gave birth to boys (59.8%) or may have learnt they were pregnant during Advent, these trends did not reach statistical significance... Virgins were younger on average at the time of the birth (median age 19.3 years) than non-virgins (21.7 years).
The study authors point to data suggesting that the parents of pregnant virgins appeared to have less knowledge about sex, and expressed difficulty in talking about the subject with their daughters. The authors also point to a study from Canada that revealed that 80 percent of surveyed college students believed that "having sex" meant intercourse with female orgasm. This study used unambiguous language to reduce such misunderstandings. Still, the study points out that self-reporting of sensitive items such as sexual history carries the potential for introducing a statistical bias.