Male hormones play a crucial role in the womb and may help explain why fertility treatment often fails in older women, a study suggests.
New research shows that "androgens" such as testosterone help to prepare the womb lining to encourage a successful pregnancy.
Although androgens are male sex hormones, they are also produced in smaller amounts in women.
Their levels fall with age, which could be an additional reason why older women find it difficult to conceive, say scientists.
It might also be a key factor behind the poor record of IVF treatment in older women.
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The researchers from the University of Edinburgh studied womb tissue donated by women who were already undergoing surgery for gynaecological conditions.
They found that androgens in the womb - testosterone and its more potent cousin dihydrotestosterone (DHT) - worked with other hormones to prepare the womb lining so that it is ready to receive a fertilised egg.
Each month the womb lining, or endometrium, undergoes a process of re-organisation called decidualisation to create an environment that can support and sustain a pregnancy.
Without the right hormonal signals, the womb will not provide the conditions that make it possible for a fertilised egg to implant.
Typically a woman's levels of circulation androgens at age 40 are half what they were at 21.
Dr Douglas Gibson, who led the work published in the journal Scientific Reports, said: "The research is at a very early stage but it has already improved our understanding of fertility. In the long term we hope this will lead to more effective fertility therapies."
When the scientists used a drug to block the action of androgens in the womb, protein markers of decidualisation were suppressed.
They wrote: "We noted that blocking ... androgens both inhibited and delayed expression of proteins implicated as markers of the decidualisation response and endometrial receptivity.
"These data suggest intra-uterine androgens may be critical for decidualisation and thus impact on establishment and maintenance of pregnancy."
