Monitoring your basal body temperature (BBT) is probably the first way you started tracking ovulation and your fertile window. The method has been around since our moms were getting pregnant, and it’s backed by science, so we trust that it works. But is it really a good method for figuring out the best time to try to make a baby? We don’t think so. Here’s why:
It’s harder than you think to track BBT accurately.
BBT is influenced by many other factors besides ovulation. Sleep later than normal? It could affect your BBT. Have a little too much to drink last night? It could affect your BBT. Wake up to pee in the wee hours of the morning? It could affect your BBT. Heat wave? It could affect your BBT. See a pattern here?
By the time you see a BBT rise, it’s too late.
Even if you’re super diligent about taking your temperature and you manage to see a clear BBT rise on your chart, it’s already too late to conceive that month. Your BBT rises after ovulation, when you’re no longer fertile.
If you have very regular cycles, you can use last month’s BBT rise to estimate when next month’s fertile window might be, but this is only an estimate. For many women—especially those with irregular cycles—this prediction may not be accurate.
BBT doesn’t always rise on ovulation day.
Contrary to popular belief, BBT doesn’t always rise immediately after you ovulate. According to a study that compared BBT readings with ultrasound—the gold standard in confirming ovulation—only 11 percent of women have a BBT rise within one day of ovulation. For many of the women in the study, BBT did not rise until more than two days after ovulation.
Practically, this means that 89 percent of women using BBT are identifying the wrong ovulation day. If you’re using BBT to time intercourse, you may over- or undershooting your fertile window by several days and reducing your chances of conceiving.
BBT charting is a lot of work.
BBT charting is a decent way of retrospectively confirming ovulation at the end of your cycle, but it’s not very good at pinpointing exactly when it happened.
If taking your temperature every morning were effortless, that would be one thing. There’s nothing wrong with having another possible datapoint about your fertility. But the truth is, most women find it extremely burdensome. Who wants to get woken up with an alarm at 6:30am every day reminding you to stick a thermometer in your mouth (or your butt, if you want a really accurate reading?!)? Who wants to worry about waking up their partner when the thermometer beeps? Who wants to hold in your pee so you don’t mess up your temperature reading?
This gets into a larger issue, which is the expectation that women do all the work when it comes to fertility and baby-making. When it’s time to get pregnant, women are the ones peeing on sticks, asking google about the quality of their cervical mucus, taking their temperatures, and sometimes getting invasive fertility treatments.
All men have to do is ejaculate. Not a fair deal!
It’s cool that charting your BBT can give you insight into your body, but for the limited information it gives you, it just doesn’t seem worth it to us.
There are better ways to track ovulation.
If BBT charting was the only way to find your fertile window, we might not be so critical of it. But there are plenty of other options. We’re quite partial to cervical mucus monitoring. Some women like OPKs.
The Ava bracelet is pretty neat, too. Ava is a sensor bracelet, worn only at night, that detects subtle physiological changes that are associated with changing hormone levels throughout the menstrual cycle. It was shown in clinical trials to detect an average of 5 fertile days per cycle in real time—more than any other method, including BBT charting and OPKs. The reason it’s able to detect that many days during the current cycle is because it doesn’t only track temperature. It also tracks physiological changes that happen when estradiol rises, usually five days before ovulation.