Researchers have found that men don't just contribute sperm when it comes to pregnancy. Their seminal fluid is also very important in conceiving a healthy baby. Couples that have sex for three to six months before conceiving have a better chance of a successful pregnancy.

Prof Sarah Robertson, one of the researchers, said that exposure to the partner's seminal fluid helps the woman's body prepare and build up tolerance to the foreign fluid that leads to a healthier and complication-free pregnancy.

"We now know that an average of at least 3-6 months coitus with their partner is necessary to get their immune system to respond correctly to enable a healthy pregnancy," Prof Robertson told The Australian. "In some people it does take longer ... for some people it could take 12 months or more."

Although, chances of a conceiving after first intercourse are high, these pregnancies carry a risk of miscarriage or preeclampsia, Robertson said.

The study, conducted on mice, found that frequent exposure to seminal fluid from partner increases levels of regulatory T cell. These cells are already known to play an important role in keeping the mother's immune system from killing the growing fetus during pregnancy.

Researchers say that pregnancy, from an immunological perspective, is like an organ transplant. The partner's seminal fluid builds tolerance against the fetus so that the mother's body doesn't react to it and lets the fetus develop.

"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 said.

The study was presented at the Australian Health and Medical Research Congress in Adelaide.