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Why basal body temperature (BBT) isn’t the most accurate ovulation predictor

Why basal body temperature (BBT) isn’t the most accurate ovulation predictor

5 min read

We talk about ovulation a ton here at Modern Fertility: We’ve debunked common ovulation myths, reviewed the best types of ovulation tests, shared our favorite features of ovulation tracker apps, and discussed why tracking ovulation is worth it even if you’re not trying for kids. (We even developed our own Ovulation Test and free app!)

One question that comes up a lot in the Modern Community: Can you measure basal body temperature (BBT) to help you understand your ovulation patterns?

Our takeaway? While BBT rises during the second half of your menstrual cycle after ovulation occurs (called the luteal phase), it’s a better predictor of whether you’ve already ovulated. Overall, it’s an unreliable way to get a read on your ovulation patterns due to how many factors influence your body’s temperature.

That said, when paired with ovulation tests and other fertility awareness methods (FAMS) like cervical mucus monitoring, it can be a useful way to collect more data about your cycle. Here, we’re breaking it all down for you.

What is basal body temperature?

BBT is your body’s temperature when you’re at rest. Hormonal fluctuations throughout your cycle can influence your BBT — for example, around the time of ovulation your BBT may increase slightly (we’re talking 0.5-1 degrees Fahrenheit). Your BBT may also remain high for the duration of your cycle after ovulation.

There are, however, many factors that influence your body temperature beyond your cycle, including fever, illness, stress, alcohol, hormonal contraception, lack of sleep, and jet lag.

Does charting BBT help you increase your chances for pregnancy?

A quick biology refresher before we answer that question: Ovulation is the part of your cycle that comes after your follicular phase (the first half), when hormones relay chemical signals between your brain and your ovaries to let your body know when to release an egg. Estrogen rises in your body, which triggers a rise in luteinizing hormone (LH). This cues a time of *peak* fertility (aka your “fertile window”) — the five days leading up to and the day of ovulation.

According to Dr. Eva Luo, MD, MBA, an OB-GYN at Beth Israel Deaconess Medical Center in Boston and an instructor at Harvard Medical School, “While measuring LH can tell you when you’re about to ovulate, BBT can tell you when you’ve already ovulated — marking the end, rather than the beginning, of the fertile window.” By the time you notice a spike in your body’s temperature (if you notice one at all!), you’ll have likely missed those five days of peak fertility. So, to sum it up, BBT charting is unlikely to have a significant impact on helping you time sex or insemination around your most fertile days.

Over the years, many studies (including this one and this one) have shown that charting BBT is an unreliable method of predicting ovulation. In fact, one study estimated that BBT only rose within one day of an LH surge (which comes ~24-48 hours before ovulation) 22% of the time, while another found that only around 11% of people with ovaries had a BBT that increased within a day of ovulating — and the BBT of many people studied didn’t increase until two days (or more!) after ovulation.

Beyond the list of factors we mentioned earlier (like sickness, stress, birth control) that influence your body’s temperature, another reason BBT charting is unreliable is because measurements taken at slightly different times of the day won’t give you consistent readings.

Why ovulation tests are more accurate predictors of ovulation

Ovulation predictor kits (like our Ovulation Test!) are our favorite way to track ovulation because they measure levels of LH, the hormone that triggers the ovary’s release of an egg during ovulation, in your urine. After an ultrasound (where doctors can actually visualize whether there’s an egg that’s been released from the ovaries), the second most direct measure of ovulation tracking is looking at the hormone fluctuations that are biologically implicated in ovulation itself.

But not all ovulation tests are made equal. While some (threshold tests) give you a positive or negative readout, semi-quantitative tests like ours give you a numerical value of your LH, telling you where it falls within a range. We might be biased, but we love semi-quantitative tests because they can give you a better idea of how your LH is changing over time, and help you understand LH patterns and pinpoint your most fertile days.

If you’re interested in BBT, a good option is to track body temperature along with other methods like ovulation tests or cervical mucus monitoring to get a more accurate prediction. More data can give you a clearer picture of your cycle.

Still interested in charting your basal body temperature? Here’s how to do it

Charting your body temperature is a simple form of natural family planning, but doing it consistently requires a minor time commitment. All you really need to start charting your basal body temperature is a thermometer that tells degrees to one decimal point and a way to record your efforts.

To effectively track your BBT, it’s best to track it right when you wake up and before you do anything else — that includes drinking water. (Even checking emails on your phone first could have an effect on your temperature!) You should record these temperatures daily using a BBT chart or app.

You want to have a good idea of what your typical body temperature is before you start so you have a base to gauge from. The average person has a typical body temperature (before ovulation) between 96-98 degrees Fahrenheit. When you begin to ovulate, your temperature rises a teensy bit to 97-99 degrees Fahrenheit. Because the temperature change is so slight, it may be beneficial to use a BBT thermometer (or basal thermometer), which measures the tiny increments in Fahrenheit you need). Some are used in your mouth, while others are used in the butt (the rectum, specifically). Planned Parenthood also urges you to gather three months of data before relying on it for natural family planning.

Ultimately, however you decide to track ovulation is totally up to you. What’s especially important, explains Dr. Luo, is choosing a method that “fits easily into your lifestyle.” “Any form of tracking requires commitment to some level and, for some, may be more stressful than helpful,” she says. She goes on to explain that the goal is to find the method that adds the least amount of stress to what may already feel like a stressful process. Talking to your healthcare provider can help you do that.

If you’re thinking about tracking ovulation and LH tests feel more your speed, the Modern Fertility Ovulation Test uses your actual hormone levels to predict ovulation and your most fertile days. Like we mentioned earlier, measuring LH is the most accurate way to predict ovulation before it happens (aside from an ultrasound) — and you can do it at any time each day as long as you keep it consistent. We also have a free app to log and track your levels, making predicting ovulation and your 2 most fertile days even easier.

This article was medically reviewed by Dr. Eva Marie Luo, an OB-GYN at Beth Israel Deaconess Medical Center and a Health Policy and Management Fellow at Harvard Medical Faculty Physicians, the physicians organization affiliated with the Beth Israel-Lahey Health System.

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Alex Shea

Alex is a sexual wellness and relationship writer who has a love for sparking deep conversations about the awkward and uncomfortable topics in life.

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