The study was based on interviews with 7870 women and girls aged 15 to 28. Forty-five of the 5340 pregnancies in this group - 0.8 per cent - occurred in women who reported that they conceived independently of men. The figure does not include pregnancies that resulted from in-vitro fertilisation or other assisted reproductive technology.

Each year, the BMJ Christmas edition publishes untraditional science papers. In addition to the report on virgin pregnancies, the latest BMJ includes papers on whether there is a local baby boom nine months after home sports teams triumph (only a small one, but statistically significant) and whether an apple a day would keep the British doctor away (yes, saving about 8500 lives in Britain each year, about as many as would expanding the use of cholesterol-lowering drugs to everyone over 50).

For the study of putative virgin pregnancies, researchers at the University of North Carolina at Chapel Hill analysed data from the thousands of teenage girls and young women who took part in the long-running National Longitudinal Study of Adolescent Health.

The girls were 12 to 18 years old when they entered the study in the 1994-95 school year and were interviewed periodically about their health and behaviour over 14 years, including via computer as a way to encourage them to be candid when answering questions about their sexual history.

The 45 women and girls who became pregnant despite, according to what they told interviewers, being virgins at the time of conception differed in several ways from peers who acknowledged that men had had a role in their procreation.