Failure to fertilize the egg may not necessarily mean that the man is infertile. It is quite possible that healthy semen is rejected by the women's system because of sheer incompatibility, researchers say.
This is more likely to happen if a woman has not previously been exposed to his sperm over a period of time.
"We used to think that if a couple couldn't get pregnant, and the man's semen test was normal, the problem lay with the woman. But it appears this is not always the case," says Professor Sarah Robertson from the Adelaide University's Robinson Institute.
She said, "We have discovered that sperm doesn't just fertilise an egg. It actually contains signalling molecules that are responsible for activating immune changes in women so they can accept a foreign substance in the body - in this case sperm - leading to conception and a healthy pregnancy.
"It's rather like a two-way dance. The male provides information that increases the chances of conception and progression to pregnancy, but the female body has a quality control system which needs convincing that his sperm is compatible, and also judges whether the conditions are right for reproducing. That's where the dance can go wrong with some couples - if the male signals are not strong enough, or if the female system is too `choosy'.
"If we can understand the cascade of events which come into play when the sperm enters the female reproductive tract, we may be able to mimic or assist this with new therapies, encouraging tolerance of her partner's semen, for those couples who are experiencing difficulties becoming pregnant."
Professor Robertson is seeking female participants for her ongoing research, which is funded by the National Health and Medical Research Council.
Women aged 18-40 who have had a tubal ligation but are sexually active and in a stable relationship are needed for the study. They cannot be using any other contraceptives and their partners must not have had a vasectomy. Participants will receive an honorarium to cover costs of participation.
Professor Robertson says this is the first study of its kind in Australia and will increase our knowledge of the importance of semen for reproductive health, hopefully leading to improved treatments for infertility and miscarriage.