In response to my article about Rielle Hunter's chances of having a baby at 43 , a reader asks how researchers determine a woman’s chances of becoming pregnant each month, especially when they don’t know when or how frequently she is having intercourse. Since a woman is only fertile about three days a month, the stat assumes she’s had sex during this window. About two weeks into a woman’s menstrual cycle, her ovaries release an egg into one of her fallopian tubes during the process of ovulation. The egg must be fertilized within 24 hours to result in a pregnancy. Here’s how the reproductive ballet becomes more complicated: Sperm can live in a woman’s reproductive tract (in the cervical mucus, specifically) for at least two days and wait for the egg to be released. Dr. Richard Paulson, a fertility specialist at the University of Southern California Medical School, advises his patients trying to conceive naturally to have sex on the day of ovulation or the two days prior.