A: Most people are in their “fertile window” from about day 10-17 after the first day of their period, but this can vary a LOT.
In people not taking hormonal types of birth control, the uterus and ovaries tend to go through a cycle, trying about once a month to get the body pregnant. The average cycle lasts 28 days, but it can vary a lot from person to person. Day 1 is the first day of the menstrual period. Day 14 is the average day of ovulation, or release of the egg from the ovary. Because sperm can live in the uterus and fallopian tubes for several days (lying in wait!), having sex from a few days before to right after ovulation can result in pregnancy.
While the uterus is busy emptying in the beginning of the cycle, the ovaries spend the first two weeks preparing an egg for its big moment. Around Day 14, hormonal signals cause release of the egg, called ovulation. (Fun fact – the ovulation test kits from the drugstore look for this hormonal signal!) If sperm are already there and waiting or arrive within about a day, the egg can be fertilized, resulting in a pregnancy. During the second two weeks of the cycle, the uterus builds up its lining in case of a pregnancy. If none occurs, the lining will break down, and a new cycle begins. The body starts to try yet again!
Because sperm can live for several days, the time leading up to and just after ovulation (days 10-17 on average) is the time of highest likelihood of pregnancy. But pregnancy can still happen outside of that window! In one study, even in people with regular menstrual cycles, there was a 10% or higher likelihood of ovulation from days #6-21. That’s a two-week window! People with irregular menstrual cycles are even less able to predict when they might be most fertile. As much as we talk about average cycle days, most people do not follow those averages!
People trying to get pregnant can track body changes, such as body temperature and cervical mucus, and use test kits from the drugstore to get an idea of when they are most fertile. For those wishing to avoid pregnancy, it’s trickier! Most strategies tell you when ovulation occurs, but don’t give any advanced warning. So they are usually too late to prevent the scenario of sperm already being present and waiting. It’s no help to find out you’re ovulating today if you just had sex yesterday! For this reason, it’s highly recommended that people who are sexually active and do not want to be pregnant use birth control.
Bottom line: Most people are at highest probability of pregnancy between days 10 and 17, but pregnancy can still occur outside of that window, especially in people with irregular cycles. If you do not want to be pregnant, be sure you’re using a highly effective form of birth control!
Stay safe, stay well!
Love, Those Nerdy Girls
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