Do Ovulation Calculators Actually Work? Get The Facts on Fertility

In some ways, ovulation seems pretty straightforward: One of the ovaries releases an egg each month, and that egg either gets fertilized (starting a pregnancy) or doesn’t (triggering menstruation). But that simplified explanation is based on a textbook 28-day cycle that operates like clockwork and real life is rarely that mechanical.

“Normally, if you’re like clockwork, we know that a period means you’re well away from ovulation,” says Kate White, MD, MPH, assistant professor of obstetrics and gynecology at Boston University. “But because you can’t predict the future, you don’t know what your next cycle is going to look like. If for some reason your cycle is short and there’s still sperm hanging around from sex you had on your period, that sperm could fertilize the next egg.”

While you can’t foresee the future, you can become more aware of your body’s unique rhythms and start to learn the signs and signals it’s sending you. It’s always best to work with your doctor, but whether you’re trying to conceive or trying not to, tracking your cycles and symptoms over time may help give you more insight into your body’s natural patterns and help support your goals.

When Am I Fertile?

Here’s a basic breakdown of what happens during ovulation: Each month or so, a chemical in your brain called follicle stimulating hormone (FSH) rises, causing your ovaries to produce follicles. One of those follicles grows bigger and faster than the others and causes estrogen levels to rise, which inhibits FSH production. When estrogen peaks, it triggers a surge of LH that causes the dominant follicle to release an egg that then travels through a fallopian tube and is either fertilized by a sperm cell to start a pregnancy or shed with the rest of your blood and tissue during menstruation.

“Any variability in a cycle tends to happen in the first half [the follicular phase],” says White. “The second half [the luteal phase] is pretty set at around 14 days.”

Once you ovulate, the egg that’s released from your ovary has a 12- to 24-hour lifespan. But get this: Your chance of becoming pregnant each cycle isn’t limited to that short window of time. Sperm can live in the uterus and fallopian tubes for up to five days after sex. Five! And because an egg can live for 12 to 24 hours after ovulation, that means most women are fertile for about six (possibly seven) days per cycle: five days before ovulation, the day of ovulation, and maybe a day after, though it’s less likely.

Can I Calculate When I’m Fertile?

If you’re trying to get pregnant or avoid a pregnancy, it’s best to consult with your doctor for the most accurate results. But understanding your body’s natural patterns can give you (and should you choose to share it, your doctor) super helpful insight that may help guide your choices.

Tracking your cycle is a great place to start. Fertility awareness (sometimes called “natural family planning”) involves monitoring signs like the days of your period, changes in your cervical mucus, and your resting (or “basal”) body temperature.

If you want to start practicing fertility awareness, it’s important to remember that the more data you have, the more accurate your predictions are likely to be. “More is better with regards to natural family planning,” says White. “It’s important to be really clear and have more detail and not just a quick snapshot of data.” And remember: Even with meticulous notes and charts, there are no guarantees when it comes to conceiving or not conceiving; always consult with your doctor if you’re trying to prevent pregnancy.

If you do want to start practicing fertility awareness as a supplement to a doctor’s guidance, here are some tips:

Take your basal body temperature at the same time every day. Your basal body temperature is your temperature first thing in the morning, right when you wake up, before your feet hit the floor to get out of bed. When you’re ovulating, this temperature will rise very slightly, between 0.4 to 0.8 degrees Fahrenheit above your average. If you’re recording this number every day, you may start to see some patterns.

“Most women ovulate the day before their temperature reaches its highest point,” says White. “So you’re most likely to get pregnant before your temperature spikes, since sperm can live up to five days after sex. But if you’re trying to conceive, you should have sex the day of the temperature spike as well.”

Record the days of your period for several months. If you’re using the information to help protect against pregnancy, the more data you have, the better—start with a minimum of six months’ worth). Always use protection while you’re collecting information, but as time goes on, you may be able to make more accurate predictions about your fertile windows. There are a couple of ways this information can be helpful:

• 1) Subtract 18 days from the total days of your shortest cycle. Using that number, count that many days from the predicted first day of your next period. The date you land on is the first day in that cycle you’re most likely to get pregnant if you have sex without birth control.
• 2) Subtract 11 days from the total days of your longest cycle. Using that number, count that many days from the predicted first day of your next period. The date you land on is the last day in that cycle you’re most likely to get pregnant if you have sex without birth control.

“Let’s say your shortest cycle is 26 days,” says White. “Twenty-six minus 18 is 8. And let’s say your longest cycle is 32 days. Thirty-two minus 11 is 21. That means the possible range of days when you might be able to get pregnant during your next cycle is from day 8 to day 21. The more variability there is between your longest and shortest cycles, the larger the window of possibility.”  

Track changes in your cervical mucus. Hormonal fluctuations cause the color and thickness of your cervical mucus to change throughout your cycle. For example:

• In the days after your period, you may not have any mucus at all and you may be less likely to get pregnant if you have sex without birth control.
• As you approach ovulation, your body will likely produce more mucus. This mucus will typically look white or cloudy and feel sticky. You’re more likely to get pregnant on these days if you have sex without birth control.
• Just before ovulation, mucus is usually slippery and clear, resembling raw egg whites. You’re most likely to get pregnant when you have this type of mucus if you have sex without birth control.

Tracking your basal body temperature, period, and cervical mucus can help you predict your fertile days, but it’s not a foolproof method for preventing or planning pregnancy. Remember, the timing of your fertile window can be unpredictable, even if your cycles appear to be regular.