The funny thing about fertility is that when you're trying to avoid getting pregnant, it can feel like you're fertile all of the time. And if you are trying to get pregnant, then your fertile window — the time that intercourse could lead to pregnancy — seems nearly impossible to pin down.
Those of us who are actively trying to avoid having a kid may have wondered: If you can calculate when you're most fertile, can you also tell the days in your cycle are you least likely to get pregnant? It's complicated, because everyone is different. But you can sort of guesstimate.
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The fertile window falls around ovulation, which is when the egg is released from the ovaries, explains Kate O'Connell White, MD, MPH, director, Fellowship in Family Planning, department of Ob/Gyn at Boston University, Boston Medical Center. For the average person, ovulation occurs around day 14 of the menstrual cycle, but "the day of ovulation is flexible and not fixed in time," she says.
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.
Ultimately, you shouldn't rely on timing or tracking your period for your only method of birth control, because it's too risky, Dr. White says. "Using your cycle as a cheap birth control will get you pregnant," she says. "It's not an something to take lightly." Of course, lots of people use apps to track their periods to get as much information as possible about their cycle — but that should be a complementary method of birth control along with condoms or spermicide, she says. Identifying and understanding your fertile window is just one step, she says. "Making sure you don’t have unprotected sex during the window is the second step."