Basal Body Temperature: What Does It Have to Do with Fertility?

Basal body temperature is probably not a term you’ve heard of until you start trying to conceive. Commonly abbreviated as BBT, charting this health measure can be informative about your cycle, ovulation, and fertility window. BUT, it does have some challenges. To help you better understanding BBT, this post will cover the do’s and don’ts of tracking your temperature.

DO specifically use a basal body temperature thermometer

While it may be tempting to try to use a regular body thermometer, there’s a good chance that they won’t be accurate or sensitive enough to detect the subtle but significant changes in basal body temperature. It’s recommended to use a thermometer specifically designed to measure basal body temperature.

DON’T wait to take your basal body temperature in the morning

While BBT has its perks, one of the major problems with this method is that you have to take your temperature as soon as you wake up and before you start doing anything else. The reason is because your body temperature is sensitive to any movements. Try to keep your thermometer on your bedside table so you can take a measurement as soon as you wake up, and remember take your temperature immediately—before you sit up, go to the bathroom, talk, or drink water.

Because this measure is so sensitive, know that sleeping in, drinking alcohol, and waking up to use the bathroom before checking can lead to inaccurate measurements.

DO know what a biphasic pattern is

If you track your BBT, you’ll probably see the term “biphasic pattern” come up. This refers to the fact that body temperature is lower before ovulation, and then it rises after ovulation. For example, check out the following chart:

1. The first phase is pre-ovulation aka follicular phase. During this phase, body temperature is low, typically around 34.5 – 35.9°C / 94.1 – 96.6°F
2. A few days after ovulation occurs, body temperature increases.
3. The second phase is post-ovulation aka luteal phase. During this phase, body temperature is higher, typically around 35.3 – 36.7°C / 95.5 – 98.1°F.

Together, these two phases make the biphasic pattern. When you see a biphasic pattern, it’s a good indication that an ovulatory cycle occurred.

DO know what a slow temperature rise means

For some women, it takes a few days for the temperature rise post-ovulation to become obvious while for others, the rise is sharp and clear. A slow rise in BBT does not necessarily hurt your odds of getting pregnant, but it can make it more challenging to pinpoint when exactly ovulation happened. Nonetheless, plenty of women have conceived when they’ve had a slow BBT rise.

DO know that it’s telling you when the fertile window closes

A major drawback to the temperature method is that once you see that rise in BBT, it means that ovulation happened. But, you’re fertile during the five days before ovulation and day of ovulation. So, by the time you see that BBT rise, your fertile window is closed for that cycle.

If you have regular cycles, then you could use last month’s basal body temperature data to try and predict when your fertile window would be in the following month. But, if you have irregular cycles or variation in ovulation days, then it could be tricky to use this method for your fertility calendar.

DON’T assume that basal body temperature is the only way

Basal body temperature tracking has been around for years, so it’s a pretty well-established method, but it’s definitely not the only method out there. Here are some other methods of tracking your fertility:

1. Cervical mucus changes can be helpful detecting your fertile window. It’s messy to track vaginal discharge changes and can be tricky to tell the difference, but it’s also the cheapest method because you don’t need to buy anything at all!
2. Urine tests for hormones like luteinizing hormone can help detect ovulation. Depending on how long your LH surge lasts, these tests can help you predict your fertile window.
3. The Ava bracelet measures seven key health parameters, like resting pulse rate and skin temperature, to detect your personal fertility window in real-time.

DO know the perks and disadvantages

Aarthi Gobinath, PhD

Aarthi Gobinath earned her PhD in neuroscience from the University of British Columbia. Her research covers the ways that stress affects the male and female brain differently.

She tackled the issue of sex bias in research by looking at why standard treatments for depression don't always work in the case of postpartum depression. Her work has been covered by Vice and Massive Science.