Couples planning to have a baby should practise sex for three to six months before conception to increase their chances of a successful pregnancy, researchers say.

Prof Sarah Robertson, from the University of Adelaide, said exposure to a man's semen helps a woman's body prepare and build up tolerance to the foreign fluid, better supporting pregnancy.

"We now know that an average of at least three to six months coitus with their partner is necessary to get their immune system to respond correctly and 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."

Couples can still get pregnant with just one act of intercourse, but their chances of rejection, miscarriage and other complications are greater, Prof Robertson said.


"It's not so much about the likelihood of getting pregnant, it's more about healthy progression of pregnancy,'' she said.

"You're more likely to have a healthy pregnancy if you've had some practice beforehand.''