Treating a woman with progesterone during pregnancy appears to be linked to the child's sexuality in later life. In a small-scale study, researchers found the children of these mothers were less likely to describe themselves as heterosexual by their mid-20s, compared to those whose mothers had not been treated with the sex hormone.

Progesterone is produced by both men and women. In the latter, it is involved in the menstrual cycle and helps foetus development – specifically neural development and the production of sex and steroid hormones. It also helps maintain pregnancies. For this reason, when women are at risk of miscarriage they are often treated with progesterone.

In a study published in the Springer journal Archives of Sexual Behaviour, a team led by June Reinisch from the Kinsey Institute in Indiana, followed 34 Danish people whose mothers were treated with progesterone during pregnancy.

The participants came from the Copenhagen Perinatal Cohort, a database that was collected on children born in Denmark between 1959 and 1961. They included 17 men and 17 women whose mothers had been treated with progesterone, all of whom are now in their mid-20s.

They were asked questions on their sexual orientation, attraction to each sex, sexual history and self-identification. Responses were then compared to a comparative control group, whose mothers had not received progesterone treatment.

Findings showed that 20.6% of progesterone-treated participants described themselves as bisexual, homosexual or that they didn't know. "Among the exposed men, one identified as homosexual, two as bisexual, and two said don't know. Among exposed women, two identified as bisexual. All other subjects, exposed and unexposed, self-identified as heterosexual," they wrote.

Researchers also asked participants if they had ever been attracted to their own sex. Of the progesterone group, 29.4% said yes, while 5.9% of the controls had.

Reinisch said: "Progesterone exposure was found to be related to increased non-heterosexual self-identification, attraction to the same or both sexes and same-sex sexual behaviour. The findings highlight the likelihood that prenatal exposure to progesterone may have a long-term influence on behaviour related to sexuality in humans."

Researchers note that many factors are involved in the development of sexual orientation, with progesterone potentially being one of these. Other limitations include the self-reporting aspect: "the non-normative status of same-sex attraction and behaviour tends to produce a social desirability effect toward underreporting these behaviours," the team wrote.

Furthermore, they note the study was only carried out on 34 people whose mothers had been treated with progesterone – and details of the treatment parameters were not given.

"In conclusion, these findings reveal that prenatal progesterone has been an underappreciated factor in human psychosexual development... Our findings suggest that natural perturbations in endogenous progesterone during gestation may affect individual differences in the expression of adult sexual orientation. Specifically, progesterone exposure was found to be related to increased non-heterosexual self-identification, attraction to the same or both sexes and same-sex sexual behaviour.

"Further studies of offspring of progesterone-treated pregnancies are warranted and may provide important insights into the role of this hormone in human behavioural development."