How to use a basal thermometer

If you’re trying to get pregnant, you’ve probably heard about tracking your basal body temperature (BBT). This refers to your body’s lowest temperature when you’re at rest. When you ovulate, your body temperature goes up just a tiny bit. Using a basal thermometer allows you to take your temperature and keep track of your readings. This lets you find out when you ovulate and what your cycle typically looks like over the course of a few months so that you can plan to have sex during your most fertile time.

How does a basal thermometer work?

A basal thermometer measures your temperature, just like a regular digital thermometer. The difference is that it measures very small increments: 1/10th or even 1/100th of a degree.

How do you use a basal thermometer?

Most basal thermometers are used orally, like regular thermometers. But you may buy one with instructions that recommend taking your temperature vaginally or rectally (obviously, you’re going to pick one method and stick with it!). You should take your temperature at the same time every day, as soon as you wake up and before you get out of bed in the morning. Record your reading on a paper graph (it usually comes with the thermometer or you can download one and print it off) or digitally with an app (choose one that you like or is associated with the thermometer brand) every day and look for a pattern—your BBT will increase by about 0.3C a few hours after you ovulate and stay elevated for two or three days. You’re most fertile about two days before your temperature rises.

How do you choose a basal thermometer?

It all comes down to personal preference. You can buy a basic basal thermometer and track your temperature on an old-school paper graph. There are also a variety of basal thermometers associated with free or paid apps on your smartphone to store and track your information digitally. This can be helpful but also subject to technical glitches and wrong information (a 2019 study published in the Journal of Obstetrics and Gynaecology Canada evaluated 140 menstrual- and fertility-tracking iPhone apps and found that 31 of them had serious inaccuracies).

Because you have to take your temperature first thing in the morning before you get out of bed, a built-in light and an easy-to-read screen are handy features. A timer that lets you know when your reading is complete is also a good idea.

How much is a basal thermometer?

You can buy a basal thermometer in a pharmacy or grocery store or online. Prices start at around $10, depending on how many features it has, and go up to $50.

What are the pros and cons of using a basal thermometer?

On the plus side, a basal thermometer is inexpensive and easy to use. It’s a low-intervention way to get a general idea of how your cycle operates. BBT thermometers can also be used to detect pregnancy—a rise in BBT for 18 days after ovulation can be a sign that you’ve conceived. It’s not a reliable method of birth control, though: About 24 in 100 women who use fertility-awareness-based methods like BBT to prevent pregnancy will still get pregnant over 12 months of use. If you’re seeing a naturopathic doctor about your fertility, they may ask you to track your BBT to help assess hormone levels and ovulation.

When it comes to cons, your BBT reading isn’t always reliable. Some sources put its accuracy at 76 to 88 percent because other factors, such as feeling ill, getting a poor night’s sleep, being in a different time zone or having a few drinks the night before, can affect your temperature, too. It only tells you when you’ve already ovulated, which isn’t helpful information for planning baby-making sex that month. “Many of my patients track their temperatures,” says Rhonda Zwingerman, an OB-GYN and a fertility specialist at Mount Sinai Fertility in Toronto. “My universal recommendation is to tell them to stop. The problem with BBT tracking is that it only gives you retrospective information, and most people want to know when to time sex that month rather than establish a pattern and apply it to future months.” For fertility patients, there can be an emotional toll to taking your temperature on a daily basis. “It’s a very strong daily reminder that you’re not pregnant,” she says. She thinks over-the-counter ovulation kits (where you pee on a tester to detect increases in your luteinizing hormone levels, which trigger the release of an egg) are a better low-intervention option, although they are pricier than most basal thermometers. You can use the kits in the days leading up to your expected ovulation (not every day) and use the results to time your trying-to-conceive sex that month.

Read more: 5 signs you're ovulating 6 totally sexy date destinations for when you’re ovulating