Lesson 7 - How to Chart Basal Body Temperature (BBT) to Detect Ovulation


  • Understand how BBT changes throughout your cycle.
  • Learn how to use BBT to detect how close you are to ovulation.

Major Points:

  • Your basal body temperature (BBT) is your body temperature while at rest. Aim to take your temperature soon after waking up and before getting out of bed.
  • After ovulation, progesterone is released from the corpus luteum which causes a rise in BBT.
  • Your fertility chart will show ovulation as the day before your temperature rises.
  • On a fertility chart, ovulation is seen as a biphasic pattern, meaning lower BBTs followed by higher BBTs.
  • Your BBT will remain high post-ovulation and throughout the luteal phase until your next cycle begins.
  • If you become pregnant, your BBT will stay high.

Basal body temperature (BBT) can reveal so much about fertility. After ovulation, the hormone progesterone is produced by the corpus luteum, the remains of the follicle which is released from an ovum during ovulation. During the luteal phase, progesterone prepares the lining of the uterus for the fertilized ovum if implantation should occur.

However, progesterone also causes your BBT to rise following ovulation. As progesterone is secreted in high levels after ovulation, you can identify your ovulation day as the day before your temperature rises. Therefore, you’ll notice low temperatures on your chart before ovulation and higher temperatures after ovulation. 

Your BBT is described as your body at rest. It’s best to take your temperature in the morning before getting out of bed and after at least three hours of sleep. Use a special BBT thermometer which is available online and at most drugstores.

Fertility charts that track ovulation using BBT data will usually feature biphasic patterns. Biphasic patterns show lower temperatures before ovulation, followed by a rise caused by a thermal shift, then higher temperatures after ovulation. On average, ovulation will occur on the last day of the lower temperatures.

Unlike other fertility signs that can only tell you if ovulation is approaching, BBT corresponds with progesterone which can help you confirm that ovulation actually occurred. As progesterone causes your BBT to significantly increase just after ovulation, you’re better equipped to pinpoint the exact day of ovulation.

Before ovulation, there is progesterone in your body but only in small amounts. During this time, your BBT is in the lower range. After ovulation has occurred, there is an increase in progesterone from the corpus luteum. This rise in temperature can be seen using a BBT thermometer.

On average, the increase in BBT is typically about 0.4 degrees Fahrenheit (0.2 degrees Celsius). However, this increase may be as small as 0.2 degrees Fahrenheit (0.1 degrees Celsius). In some cases, the increase is even less. What’s more important than the temperature change itself is the biphasic pattern that shows two temperature levels.

If you do not become pregnant during your cycle, your BBT will remain elevated for about 14 to 16 days before the corpus luteum disintegrates. Progesterone levels will also drop dramatically which will trigger your period. Your BBT will also drop around this time. However, it’s common to see erratic high temperatures during your period.

Charting your BBT is an effective way to target your ovulation day. You can increase your odds of conception by also observing other fertility signs in addition to your BBT, such as your cervical fluid. This is important as your BBT will not tell you if you have ovulated until ovulation has already occurred and it’s too late to conceive that cycle. However, if your cycles are consistent you can use this data during your next cycle.

If you’re trying to conceive, you may be worried about your timing of intercourse. Charting your BBT and other fertility signs can help give you peace of mind that you are on track with your fertility goals and will allow you relaxation time after ovulation without pressure. You can also avoid stress at the end of your cycle because you’ll know when you can accurately test for pregnancy.