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Men contribute more than sperm to pregnancy

Immune response Men may contribute far more to a successful pregnancy than just sperm, with evidence suggesting seminal fluid primes a woman's immune system to be more receptive to the foetus.

Researchers from the University of Adelaide have found a mechanism that may explain why women who have repeated exposure to their partner's seminal fluid in the three to six months before conception have a much lower likelihood of pregnancy complications such as pre-eclampsia and foetal growth restriction.

The team's findings were presented this week at the Australian Health and Medical Research Congress in Adelaide.

"The male makes a contribution that we hadn't appreciated - it's not just that one thing of sperm that's important," says Professor Sarah Robertson, co-director of the University of Adelaide's Research Centre for Reproductive Health.

A study in mice has revealed that more frequent exposure to seminal fluid increases levels of a type of immune cell called a regulatory T cell, which has already been shown to play a key role in helping the mother's immune system tolerate the foetus.

Early research in humans suggests the same mechanism may explain why women who conceive after less than three months of sexual cohabitation with the father-to-be have a significant increase in the risk of pre-eclampsia.

"We see it in pregnancies where people conceive very early in the relationship or after a one-night stand, but we also see it where people have used barrier methods of contraception and then immediately become pregnant after stopping using condoms," says Robertson.

Like an organ transplant

Robertson says that from an immunological perspective, a foetus is similar to a transplanted organ.

"It's a foreign tissue living in a female body for nine months and so you have to have transplantation tolerance or immune tolerance to allow that to occur," she says.

Regular exposure to the father-to-be's seminal fluid appears to contribute a more immune tolerant environment in two ways.

One component primes the female immune system to recognise the father's unique immunological signature, called the major histocompatability complex, while another component encourages a more tolerant immune environment.

"The signals that are present in the seminal plasma are telling the immune system to be tolerant - not get activated and become immune - but to mediate this quieter, nurturing, sustaining response instead," Robertson says.