Basal Body Temperature (BBT) Fluctuations in Early Pregnancy

You can use a thermometer to see how hormones change from day to day.

Updated October 31, 2022
Woman's test shows she's no longer pregnant

Charting your basal body temperature (BBT) may help you identify the changes that your body goes through during your menstrual cycle. It may even be able to offer signs that you have become pregnant. Learn what to look for if you plan to use basal body temperature before or during pregnancy.

Basal Body Temperature and Menstruation

Basal body temperature is a measurement of your lowest body temperature when you're at rest. Many people who are trying to conceive track their BBT throughout their menstrual cycle to help predict when they will ovulate. A rise in basal body temperature over 18 or more days is often one of the first signs of pregnancy.

How to Chart Your BBT

To track your menstrual cycle using BBT, you'll take your temperature every day and note it on a chart. Your chart will begin on day one of your period and end on the day of your next period. Here's how to chart your BBT:

  • Every morning before you get out of bed, take your basal temperature using a basal body thermometer or a digital oral thermometer.
  • Mark the temperature reading on a BBT chart and draw a line to connect the marks each day. This will show you the pattern of your daily temperature readings.
  • On your chart, you may choose to indicate the days you have your period, the days you've had intercourse, and any changes in your cervical mucus. You may want to chart additional symptoms, such as pelvic pain and breast tenderness. This can help you know what your normal is from month to month.
  • Three days of a sustained elevation in your BBT above 98 degrees Fahrenheit generally indicates ovulation. You are most fertile 3 to 5 days before ovulation and the day of ovulation.

If you have passed the day of your expected period and have a sustained rise in body temperature above 98 degrees for 18 days or more, you may be pregnant.

Typical BBT Patterns During Menstruation

During the menstrual cycle, a person's basal body temperature usually drops right before their period begins. In early pregnancy, the BBT stays high due to the rise in progesterone, a sex hormone.

If you're charting your BBT over your menstrual cycle, here's the pattern you can expect to see:

  • Before ovulation: During the follicular phase, the body prepares to ovulate and basal body temperatures average between 97 and 98 degrees Fahrenheit. You may notice a slight decrease in your BBT right before you ovulate, then a spike when you do ovulate.
  • After ovulation: Immediately after ovulation in the earlier part of your luteal phase, your BBT will slightly increase. According to the Cleveland Clinic, most will have a BBT ranging from 97.6 to 98.6 degrees Fahrenheit during this time.
  • Period: If you are not pregnant, your BBT will plateau around day 12 to 14 of your luteal phase and then drop suddenly as your progesterone levels decrease. Your period should arrive shortly after this decrease in BBT.

BBT Fluctuations in Early Pregnancy

Implantation occurs in the mid to late stages of the luteal phase, which is about 6 to 12 days after ovulation. For those with a 28-day menstrual cycle, this would be between days 20 to 26 of your cycle.

One of the earliest signs of pregnancy is an increase in BBT that occurs after implantation, as progesterone levels rise to support a pregnancy. If your BBT stays at 98 degrees or higher past the time your period is due, this may be a sign that you are pregnant.

In early pregnancy, a few factors play a role in your slightly higher BBT, including:

  • Higher metabolism: Increased metabolism during pregnancy plays a role in slight increases in BBT.
  • Increased blood volume: Your body produces extra blood during pregnancy to support your growing baby.
  • Progesterone production: In early pregnancy, your ovaries produce a consistent supply of progesterone to support the uterus and uterine lining in providing a supportive environment for a developing fetus. After 8 to 12 weeks of pregnancy, the placenta takes over progesterone production from the ovaries.

Basal Body Temperature and Miscarriage

Just as your BBT can be an early indicator of pregnancy, some limited sources have also suggested it can point to a potential problem with the pregnancy. But evidence supporting this association is lacking. The last study done on this was published in 1976, and that study only analyzed 11 women. There is no high-quality recent research that finds a link between basal body temperature and miscarriage.

Keep in mind that just because you see a change in temperature doesn't necessarily mean that something is wrong. There can be simple problems with the thermometer or other factors at play. If you are concerned, reach out to your healthcare provider. They can give you personalized guidance that will likely set your mind at ease.

Should You Track Your BBT Daily?

Tracking your BBT in the first weeks of pregnancy may offer peace of mind that everything is okay with your pregnancy until you can confirm your baby is growing through an early ultrasound. Once you've reached eight weeks of pregnancy, however, tracking your BBT isn't likely to offer much insight into how your pregnancy is going. At that point, an ultrasound is a better way to assess the health and development of your growing baby.

For some people, however, tracking BBT in early pregnancy can cause anxiety and lead to extra stress. Fluctuations in body temperatures from day to day are normal, but if you're worried about your pregnancy, even small shifts in your temperature can induce worry. According to the Cleveland Clinic, other factors can impact your BBT, including:

  • Alcohol consumption
  • Certain medications
  • Exercise
  • Having sex
  • Illness
  • Indigestion
  • Interrupted sleep
  • Stress
  • Temperature changes in your sleep environment
  • Travel

If you do choose to track your BBT in early pregnancy, remember that slight changes are often normal and expected. But if you notice changes that cause you concern, always remember that your healthcare provider is your best source for personalized guidance about what is going on with your pregnancy.

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Basal Body Temperature (BBT) Fluctuations in Early Pregnancy