Know your cycle
General wisdom has it that your most fertile time is the fourteenth day after you get your period. But in fact, ovulation occurs on the fourteenth day before your period begins, if you're not pregnant, says Toni Weschler in Taking Charge of Your Fertility (HarperPerennial Library, 1995). If your cycle is 28 days long, that makes day 14 (counting from the first day of your last period) your most fertile day. But if you have a 30-day cycle, day 16 is the most fertile, and 32-dayers should go for it on day 18.