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What Is Basal Body Temperature?

A.E. Freeman
Updated Mar 03, 2024
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Basal body temperature, or BBT, is the lowest temperature a person's body will be throughout the day. Typically, BBT is taken first thing in the morning, before a woman becomes active and her temperature increases. Women who wish to become pregnant can determine when the best time to attempt pregnancy is by shifts in their body temperature.

The best way to get an accurate basal body temperature reading is to take the temperature before getting out of bed in the morning. Ideally, a woman will take her temperature at the same time each morning after getting a decent amount of sleep. It doesn't matter whether she takes her temperature orally or rectally, as long as she is consistent in her method. She shouldn't take her temperature orally one week and rectally another, for example. As an accurate temperature reading is only possible if a woman hasn't been active yet, she should plan ahead and keep a BBT chart and thermometer by the bed so that she doesn't have to get up and look for it in the morning.

Typically, a woman's basal body temperature is around 97.2 degrees Fahrenheit (36 degrees Celsius). At the start of ovulation, it will rise 0.4 degrees. Usually, a woman's basal body temperature increases by up to 1 degree Fahrenheit a few days after she ovulates. Some women may notice a drop in temperature just before ovulation begins, but this doesn't happen to every woman. If she tracks her BBT for a few months before attempting to become pregnant, she can begin to recognize a pattern in her temperature and use the pattern to determine when she will ovulate.

In most cases, the BBT stays high for up to 12 days after a woman ovulates. If her temperature remains elevated, it can be a sign that a woman is pregnant, especially if her temperature remains high for more than 18 days. Some women will notice an additional increase in temperature. This increase doesn't necessarily indicate pregnancy, though. In some cases, a woman's basal body temperature can move through three phases instead of the usual two.

If a woman's BBT doesn't remain elevated for 12 days after ovulation, it is usually a sign that a woman has luteal phase defect, which can cause infertility. The luteal phase is the timing between ovulation and the start of a new cycle. A luteal phase defect usually indicates that a woman isn't producing enough progesterone, which can interfere with the production of the endometrium lining, potentially making conception difficult.

The Health Board is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
A.E. Freeman
By A.E. Freeman
Amy Freeman, a freelance copywriter and content creator, makes engaging copy that drives customer acquisition and retention. With a background in the arts, she combines her writing prowess with best practices to deliver compelling content across various domains and effectively connect with target audiences.
Discussion Comments
By Bertie68 — On Sep 11, 2011

This was an interesting article. I didn't have any problems getting pregnant, but I really feel for those women who for whatever reason have trouble getting pregnant. It must be a frustrating time waiting and hoping.

A good way to try some self-diagnosis is to take the basal temperature consistently and write it down. If she can determine when she ovulates and everything else is okay, she will hopefully become pregnant.

But if the time between ovulation and when the next cycle starts is off, she may be infertile. Maybe a lack of progesterone is causing the problem. I don't know if giving extra progesterone to women will possibly solve this problem.

By manykitties2 — On Sep 11, 2011

My husband and I have been trying to get pregnant for quite a few months and just recently learned about basal body temperature and the role it plays in signaling that you are ready for pregnancy. We really wanted to know how to get pregnant and ended up purchasing a basal body temperature thermometer.

If you go online and search you can also find a free basal body temperature chart which is a great way to track your fertility cycles one you learn how to read the temperatures. If you are really serious about getting pregnant you can purchase smartphone apps that help you track everything in one simple place. They really vary in price so make sure to shop around.

By ElizaBennett — On Sep 10, 2011

@MrsWinslow - I agree with you that every woman trying to get pregnant should chart. I would go further and say that every woman who's not on hormonal birth control should chart!

When I was trying to get pregnant, I used my basal body temperature and cervical mucus together to know what my best days were. (Cervical mucus increases in the few days before ovulation, being a really helpful sign that it's coming and now is the time to do the deed!)

Now I am "between babies"; not ready for another one just yet. I still take my temperature. On the evening of the third day after the temperature increase, I know I done for the month and we don't need condoms any more. And if I did get pregnant by accident, I would know right away.

(Disclaimer: obviously, condom-less sex is only for committed, monogamous couples. And if you are interested in trying the Fertility Awareness method of birth control, you need to read a book about it or take a class.)

By MrsWinslow — On Sep 10, 2011

Something to be aware of with your basal body temperature chart is that illness and other things can affect it.

We think of getting flu and getting "a fever," but I have found that my temperature goes up at least a half a degree if I have even a bit of a cold. When I had strep throat, I never got a fever but my basal body temp went up quite a bit, to maybe 99.2.

If you happen to ovulate while you are ill, you might not be able to tell what the exact day was, or having a cold can make you think you've ovulated when really you haven't.

But I think *every* woman who is trying to conceive should take her temperature, even if she's just getting started and there's no sign of infertility. That way, she'll know when the show's over for the month, and if she does get pregnant, she'll have a very accurate idea of her due date. (I never ovulate before day 20 of my cycle - not 14 - so when I do finally get pregnant, my due date based on last menstrual period will be off by a week!)

A.E. Freeman
A.E. Freeman
Amy Freeman, a freelance copywriter and content creator, makes engaging copy that drives customer acquisition and...
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