Once often enough to get pregnant

BBC NEWS | Health: "During the six most fertile days of their cycle the rate of intercourse averaged out at 0.34 times a day, compared to 0.27 times a day for the rest of the month - an increase of 24%.
Professor Wilcox said there were at least three possible explanations:
An increase in the woman's sex drive at ovulation
An increase in the woman's sexual attractiveness
Intercourse accelerating ovulation"