According to professor Sarah Robertson of the University of Adelaide in Australia has apparently discovered a battle between the sexes that has been unnoticed until now.
Robertson's research seems to indicate that the race of sperm to fertilize an egg is not as straightforward as we have believed. Not every healthy sperm that hits the finish line actually wins the race.
"University of Adelaide professor Sarah Robertson said her research suggested that sperm contains 'signalling molecules' that activate immunity changes in a woman so her body accepts it.
"But some apparently healthy sperm failed to activate these changes, leading to the suggestion that the female system can be 'choosy' about its biological mate, she said."
It seems that if the sperm's signals are not powerful enough, or if the female system is not persuaded, then this sperm will fail to fertilize an egg. It also seems that failure is more likely if exposure to this particular male's semen has been for a shorter period than three months.
http://news.yahoo.com/s/afp/20100623/hl_afp/healthaustraliawomenpregnancy