Considering how difficult it is to tell precisely when you're ovulating the fertile window ends up spanning about four to five days before ovulation, the day of ovulation, and maybe the day after, Dr. White says. Once you've ovulated, an egg only lives for 12–24 hours, she says. So, if you were a robot, and you ovulated at the exact same mathematical point in time each month, then you could assume that the days right after your period, but before your fertile window begins would be your "least-likely-to-conceive" days. "In reality, the window can shift," Dr. White says. For example, if your period lasts longer than 3-4 days , then you could be very fertile right after your period, she says. Not to mention, sperm can live in the cervix for up to five days, so you can't expect to be completely in the clear. In other words, there just are too many variables and caveats to determine when you're least-likely to get pregnant just by looking at a calendar.