The findings were based on a study of 7,870 women and girls aged 15 to 28, as part of the National Longitudinal Study of Adolescent Health, which ran from 1995 to 2009.

The Christmas issue of the BMJ reports that, of the women who took part in the study, 45 (0.5%) reported at least one virgin pregnancy, "unrelated to the use of assisted reproductive technology."

In short, they claimed to have conceived - yet had not had vaginal intercourse or in-vitro fertilisation (IVF).

The BMJ article notes that virgin births, or parthenogenesis (from the Greek parthenos for virgin and genesis for birth), can occur in non-humans as a consequence of "asexual reproduction, where growth and development of the embryo occurs without fertilization".

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

They found that the girls who had become pregnant, despite claiming they had never had sex at the time of conception, shared some common characteristics.