New research suggests that exposure to progesterone in the womb is linked to bisexuality.

“These findings reveal that prenatal progesterone has been an underappreciated factor in human psychosexual development (as are the actions of fetal testosterone or externally introduced endocrine disruptors),” the researchers wrote in their study. “Our findings suggest that natural perturbations in endogenous progesterone during gestation may affect individual differences in the expression of adult sexual orientation.”

The study, published in the Archives of Sexual Behavior, tracked the sexual development of 34 Danes whose mothers were treated with progesterone to prevent miscarriage. “This hormonal medication was, and continues to be, administered by physicians to treat symptoms of staining, bleeding, to prevent prematurity in twins, and to prevent preterm birth and for imminent spontaneous abortion,” the researchers explained.

These 17 men and 17 women were compared to a carefully selected control group whose mothers had not been administered progesterone during pregnancy.

The researchers found that prenatal exposure to progesterone was linked to a decreased likelihood of self-identification as heterosexual, an increased likelihood of having engaged in same-sex sexual behaviors, and an increased likelihood of reporting attraction to the same or both sexes. Prenatal exposure to progesterone was not, however, linked to a decrease in heterosexual behavior.

“The findings highlight the likelihood that prenatal exposure to progesterone may have a long-term influence on behavior related to sexuality in humans,” the study’s lead author, June Reinisch of The Kinsey Institute, said in a news release.