The 'easiest' time to get pregnant is about 12 hours after you ovulate. If you don't know when that is... well, that's a challenge.
Some women ovulate once every 5 years, usually because they're surrounded by 'drivers' who excrete airborne pheromones that dampen everyone else's fertility (teacher and nurses are particularly prone to this, but just being around other women can nail it)... but sometimes because they have thyroid disorders, PCOS or ovarian cancer... others ovulate multiple times in a month for a variety of reasons: different thyroid abnormalities, sensitivity to estrogens in food, vast fluctuations in food intake or exercise habits, sleep problems, recent pregnancy or recently stopping birth control pills.
There is some evidence that people can become pregnant weeks after sexual activity --that sperm can live a lot longer than previously thought... but.. yeah... it's a crapshoot. Try having sex every two days.
at 1:06 AM on Sep. 18, 2010