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How to chart your basal body temperature

Everything you need to know about charting your BBT—including why it’s falling out of favour with fertility specialists

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basal body temperature graphic

Illustration: Nica Patricio

If you’re trying to conceive, ovulation is one of the most important things you’ll need to pay attention to. For most women, it happens mid-cycle after a mature egg is released from the ovaries and into the Fallopian tubes for potential fertilization.

But that’s not always the case. If your periods are irregular, you ovulate late or you fail to release an egg, it can be extremely hard to pinpoint this very crucial moment. It’s one of the reasons why charting basal body temperature (BBT) was so promising when it was first connected with fertility in the early 1900s. By charting it, doctors were able to identify ovulation and, therefore, a woman’s two or three most fertile days (which precede the release of an egg). Having unprotected sex on these days can increase your chances of conceiving.

With all the advances that have been made in reproductive technology, however, BBT has fallen out of favour with many OB/GYNs. Why is that? Is it still helpful for some women? How does it work anyway? We talked to fertility specialists to get the lowdown on this low-tech approach.

What exactly is BBT?
BBT is our body’s coolest, resting temperature. “We maintain a relatively stable temperature throughout the day,” explains Alfonso Del Valle, an OB/GYN at St. Joseph’s Health Centre in Toronto and medical director for ReproMed, The Toronto Institute for Reproductive Medicine, “but it’s lowest in the morning before we even get out of bed.”

This base temperature changes throughout the menstrual cycle, alongside hormones like estrogen and progesterone, and spikes slightly right before ovulation (by about 0.4°F to 0.2°C), as the body’s production of progesterone increases. By identifying this slight rise, a woman can effectively identify the day she ovulates.

How to measure it
If you want to track your BBT, you’ll want to take it orally first thing in the morning before you get out of bed (excess movement can affect your temperature). You’ll want to use a special basal body thermometer—it’s extra-sensitive to temperature changes, so it will catch even the slightest bump or drop in a body’s heat reserves.

Once taken, you can log your temperature on a fertility tracking app (like Fertility Friend or Ovia) that will automatically plot a graph based on your daily measurements. These apps will also identify ovulation by highlighting the day when your temperature spikes enough to indicate an egg’s release. To see a consistent pattern, you should take your BBT for at least three months.

The pros
If you have polycystic ovary syndrome or suffer from irregular periods, BBT charting will help you identify ovulation, even if you don’t know when you’ll get your next period. Once you see a pattern, this will help you time sex accordingly.

The cons
Most doctors have stopped asking patients with regular cycles to chart their BBT for a few reasons, says Del Valle.

Interpreting a temperature graph is quite complicated. Anything from the weather to what we eat (think spicy food for dinner) can affect our body’s heat reserves, which can skew results.

Another issue is that BBT can only show that a person has ovulated, not when they will. The rise in temperature happens post-ovulation, when the fertility window is almost closed. (An egg lives for 12 to 24 hours in the Fallopian tubes, so once this spike happens, the fertility clock is already ticking.)

Charting can also be stressful, especially if a woman believes her graphs aren’t ideal. Since stress and negativity impact fertility, Del Valle says BBT is “really too much of an investment in time and emotional resources for most people.”

There also comes a point when charting BBT won’t help you get pregnant. If you have been charting for six months and haven’t identified a pattern yet and failed to conceive, it’s time to book an appointment with your doctor. There may be another issue that needs to be addressed.

What’s a better way to predict ovulation?
Cycle regularity is a better predictor of ovulation and fertility, says Del Valle. “If a woman is having regular cycles—with variations of up to four days a month—with symptoms that precede menses, that’s a good indicator that she is having predictable ovulation,” he explains.

Using ovulation predictor kits are also useful because they measure the luteinizing hormone (LH) that triggers an egg’s release. You can calculate your most fertile days using this predictor.

  • Click here to see our guide on how to get pregnant

Read more:
What your cervical mucus tells you about your fertility
How to get your body ready for pregnancy
Trying to get pregnant is exhausting