Women who are given additional doses of a sex hormone are more likely to have a bisexual baby, new research has found.

The children whose mothers were exposed to progesterone during the decades-long study were more likely to define as non-heterosexual.

In fact, researchers at the Kinsey Institute in Indiana found that the 17 children whose mothers were given the hormone were nearly five times more likely to be attracted to their own gender.

In contrast, levels of attraction to the opposite gender were the same in the two groups.

Progesterone naturally occurs in men and women, and is often used to increase fertility, maintain foetus development and prevent miscarriages or premature births.

The study, which started in 1959, tracked 34 Danish subjects – 17 men and 17 women – half of whose mothers were given the hormone and half who weren’t.

Researchers found that 21 percent of exposed people labelled themselves as something other than heterosexual, compared to zero of those who weren’t exposed.

In this vein, 29 percent of exposed people said they had been attracted to their own gender, whereas just six percent of the other group said the same.

The researchers said their results showed that attraction to men was more common in exposed participants, as “scores on attraction to females did not differ significantly by exposure.”

They reported that “regardless of sex, exposure appeared to be associated with higher rates of bisexuality”.

“The findings challenge the prevailing view of homosexual interest and behaviour as a simple reflection of feminisation/demasculinisation in males and masculinisation/defeminisation in females.”

They concluded that “prenatal progesterone has been an underappreciated factor in human psychosexual development”.

And since progesterone is a common treatment during pregnancy, the scientists urged that use of the hormone “warrants further investigation.”

The news comes after it was revealed that gay men could one day have babies together.

The prospect of babies being born with three parents is an even closer prospect, with a clinic approved last month to start treatment.