Basal Body Temperature and Fertility (2023)

Understanding the hormonal cycles of the female reproductive system and how they affect your body can be helpful when trying to become pregnant or avoid pregnancy. Hormones play a big role by helping the brain, ovaries, and uterus communicate. Basal body temperature (BBT) is one outward sign to predict when your body is most fertile.

This article discusses the basics of the female menstrual cycle, the fertile window, and how checking BBT can provide useful information for women trying to conceive or avoid pregnancy.

The words “female” and "woman" are used throughout this article to refer to people who identify as female and have typical reproductive organs of a cisgender female. We recognize that some people who identify as female do not have the same anatomy as that depicted in this article.

Basal Body Temperature and Fertility (1)

Basics of the Female Reproductive System and Fertility

Each month, hormones cause changes in the ovaries and uterus to prepare the body for potential pregnancy. The cycle begins with menstruation, the phase when the uterus sheds its lining.

On the first day of menstruation, the ovaries are busy responding to a hormone released from the brain, called follicle-stimulating hormone (FSH). FSH tells the ovaries to make immature eggs, called follicles. These follicles secrete the hormone estrogen, which has effects throughout the body.

When estrogen rises to a certain level, it signals the brain to release luteinizing hormone (LH), which then triggers ovulation.

During ovulation the mature egg is released from the most dominant follicle and travels through the fallopian tubes to the uterus. The leftover follicle becomes a hormone-secreting cluster of cells called the corpus luteum, meaning “yellow body” in Latin.

The corpus luteum secretes several hormones, including progesterone. Progesterone causes the uterine lining to thicken in preparation for accepting a fertilized egg. Progesterone also causes changes in cervical secretions that promote sperm motility.

If fertilization does not occur, the corpus luteum breaks down and the subsequent decreased levels of progesterone and estrogen cause menstruation. If fertilization occurs, human chorionic gonadotropin (hCG), secreted by the embryo, causes the corpus luteum to remain and keep secreting hormones. The ongoing progesterone secretion maintains the uterine lining for pregnancy.

The Fertile Window

The fertile window is the period in which sexual intercourse can result in pregnancy. After ovulation, the egg can live for up to 24 hours. Sperm can live in the female reproductive tract for up to five days.

Having sexual intercourse in the two days leading up to ovulation has the highest chance of resulting in pregnancy, but you can become pregnant any time in the five days before ovulation and the day of ovulation.

Basal Body Temperature and Fertility (2)

What Is Basal Body Temperature?

BBT is the body’s temperature when in a state of rest. In general, the body’s resting temperature is stable from day to day. In women, predictable changes in body temperature occur at specific points in the menstrual cycle.

During the first half of the menstrual cycle, from menstruation until ovulation, the temperature is relatively low. Once ovulation occurs and progesterone levels rise, the body temperature increases by up to 0.5 degrees. The temperature remains relatively high until progesterone levels drop and menses occurs.

(Video) How to Measure Basal Body Temperature | Pregnancy Questions | Parents

This information can be useful for people trying to conceive and for those trying to avoid pregnancy. Women trying to conceive can use BBT to predict ovulation in order to time intercourse during the fertility window.

BBT is also a method used in natural family planning for women who want to use abstinence or barrier contraception methods during the fertile window to decrease the chances of becoming pregnant.

How to Take Basal Body Temperature

After ovulation, the rise in progesterone levels in the luteal phase causes an increase in basal body temperature of 0.5–1 degree. If fertilization does not occur, the fall in progesterone causes the temperature to again drop to the lower level.

The changes in basal body temperature may be as small as 0.5 degrees, so it's important to use a thermometer with accuracy to within 0.1 degrees. Anything less accurate can miss the small changes needed for fertility prediction.

Some thermometers are designed specifically for BBT that may provide some added value.

Risks of BBT

  • Not accurate in women who have irregular menstrual cycles

  • Increase in temperature occurs during or after the fertile period, so the fertile period could be missed

  • BBT method alone is a less effective method for preventing pregnancy than other forms of birth control

Benefits of BBT

  • Inexpensive

  • No side effects

  • Does not involve risks of hormonal contraception

Normal Basal Body Temperature

Normal body temperature varies from person to person, but it generally ranges from 96 to 99 degrees F. Temperature varies slightly depending on how it is measured, and it can change during certain times of the day and with activity, stress, changes in sleep patterns, or illness.

How to Chart Temperature Changes

Since the increase in BBT occurs at or after ovulation, the fertile period begins several days before you notice a drop in temperature. In other words, you can become pregnant in the week leading up to an increase in temperature. However, charting your temperature over time can provide information about the timing of your cycle to predict fertility in future months.

When measuring BBT, the body should be completely at rest. Keep a thermometer within reach at your bedside so you can take your temperature immediately upon waking, before getting out of bed, or doing any activity.

Temperature should be taken at approximately the same time each day and using the same thermometer in the same location. Taking your temperature by mouth is the most common way to measure basal body temperature, but vaginal and rectal temperatures may also be used.

(Video) Why don't my basal body temperature chart patterns match the my ovulation predictor kit results?

After charting basal body temperature for a few months, you will notice a pattern of lower temperatures for a couple of weeks, followed by a slight rise in temperature that plateaus and remains until menstruation. Use this information to predict the timing of ovulation and your fertile window in future months.

Tools to Track BBT

Other than a thermometer, all you need is a pen and paper to plot your temperature on a graph or table each day. Also, several phone apps have been designed specifically to plot BBT and provide insight into your cycle and fertile window.

Other Signs of Ovulation

BBT is one of several signs you can use to track your fertile window.

Hormones cause several other changes in the body. A combination of fertility awareness methods can be used for those trying to conceive to time intercourse, or they may be combined with barrier contraception or abstinence during the fertile period to avoid pregnancy. Methods include:

  • Calendar method: Many women have a predictable menstrual cycle length that can be used to predict the fertile window. The average cycle is 28 days, and ovulation typically occurs midway through the cycle, around day 14. This information can be used to estimate when the fertile window is to time intercourse and increase or decrease chances of pregnancy.
  • Cervical mucus method: Cervical mucus changes predictably during the menstrual cycle. During the most fertile period, cervical secretions are clear and stretchy to assist sperm motility.
  • Mittelschmerz: Mid-cycle pain and cramping that some women experience during ovulation can help predict when it's occurring.
  • Hormone measurement: LH can be measured at home using urine tests. A rise in LH (LH surge) triggers ovulation, so timing intercourse at this time can increase chances of pregnancy in women trying to conceive.

Basal Body Temperature When Pregnant

The hormone changes that occur with pregnancy also affect BBT. A couple of findings on the BBT chart can be early signs of pregnancy. Of course, suspected pregnancy should be confirmed with a pregnancy test and a visit to your healthcare provider.

One very early potential sign of pregnancy is a one-day drop in temperature, called the implantation dip. This can occur about a week after ovulation if fertilization has occurred and lasts for just one day. Compare this to a typical cycle when a dip will be seen just before the onset of your period and will remain low until the next time you ovulate.

An implantation dip is not the most reliable sign of pregnancy because it may not be present. If it's present, it does not always mean pregnancy.

A more predictable and noticeable change in BBT that can indicate pregnancy is a prolonged increase in temperature. During pregnancy, progesterone levels remain high and cause a sustained increase in BBT.

Look for other pregnancy signs, like a missed period and symptoms like fatigue and nausea.

Summary

Basal body temperature is a noninvasive, inexpensive, and simple measure that can be used to track fertile periods for those trying to conceive or avoid pregnancy. BBT relies on charting daily temperature and noticing small changes over time to predict future fertility windows.

A Word From Verywell

Trying to get pregnant or avoid pregnancy using hormone-free methods can be challenging. While the BBT method can be helpful for tracking your cycle and fertile window, your body temperature can be affected by many external factors and relies on tracking very small changes over time.

Not all women will be able to reliably predict ovulation, particularly if they have irregular menstrual cycles. Speak with your healthcare provider with any concerns you have about infertility or choosing the right form of birth control.

Frequently Asked Questions

  • What is the basal body temperature during your period?

    Basal body temperature drops when your period starts because of the decline in progesterone levels. If basal body temperature remains elevated and you miss your period or only have light spotting, take a pregnancy test.

  • How long is your fertile window each month?

    The fertile window is based on how long sperm and a released egg can survive in the female reproductive tract. Sperm can survive for up to five days. A released egg can survive for up to 24 hours after ovulation. Adding these together give you a fertile window of approximately six days.

    (Video) Basal Body Temperature Tracking Do's & Don'ts
  • Can you tell if you are pregnant through just your basal body temperature?

    A persistent increase in basal body temperature can indicate pregnancy due to higher progesterone levels, but this should not be the only sign you rely on. It will coincide with a missed period (or very light spotting from implantation bleeding). Always confirm suspected pregnancy with a pregnancy test.

3 Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

Basal Body Temperature and Fertility (3)

(Video) Tracking Your Basal Body Temperature - BBT Basics with Monica, NFP Instructor

By Angela Ryan Lee, MD
Angela Ryan Lee, MD, is board-certified in cardiovascular diseases and internal medicine. She is a fellow of the American College of Cardiology and holds board certifications from the American Society of Nuclear Cardiology and the National Board of Echocardiography. She completed undergraduate studies at the University of Virginia with a B.S. in Biology, medical school at Jefferson Medical College, and internal medicine residency and cardiovascular diseases fellowship at the George Washington University Hospital. Her professional interests include preventive cardiology, medical journalism, and health policy.

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FAQs

Can basal body temperature affect fertility? ›

Ovulation may cause a slight increase in basal body temperature. You'll be most fertile during the two to three days before your temperature rises. By tracking your basal body temperature each day, you may be able to predict when you'll ovulate. This may help you determine when you're most likely to conceive.

What should your BBT be when trying to conceive? ›

After the female egg cell is released at ovulation, BBT increases to between 97.6°F (36.4°C) and 98.6°F (37°C). Basal body temperature will drop again if pregnancy doesn't happen. This temperature drop causes the uterus lining to shed, causing a period to start, and a new menstrual cycle to begin.

How do I know if I am fertile enough to get pregnant? ›

If your menstrual cycle lasts 28 days and your period arrives like clockwork, it's likely that you'll ovulate on day 14. That's halfway through your cycle. Your fertile window begins on day 10. You're more likely to get pregnant if you have sex at least every other day between days 10 and 14 of a 28-day cycle.

How accurate is basal body temp for ovulation? ›

[11][12] It has been predicted in some studies to be only approximately 22% accurate in detecting ovulation. [1] Although research outlets have determined the unreliability of basal body temperature measurement, the principle is still widely used and accepted by women to this day.

What is a normal basal body temperature? ›

Before ovulation, a woman's BBT averages between 36.1°C (97°F) and 36.4°C (97.5°F). After ovulation, it rises to 36.4°C (97.6°F) to 37°C (98.6°F). You can track your cycle by taking your BBT every morning. Take your temperature at the same time every day before getting out of bed.

How long is woman fertile after ovulation? ›

A person can get pregnant 12–24 hours after ovulation, as a released egg can survive up to 24 hours within the cervix.

How many hours after ovulation does BBT rise? ›

Your body temperature dips a bit just before your ovary releases an egg. Then, 24 hours after the egg's release, your temperature rises and stays up for several days. Before ovulation, a woman's BBT averages between 97°F (36.1°C) and 97.5°F (36.4°C).

How much should BBT rise after ovulation? ›

The temperature change after ovulation is slight — BBT rises by only about 0.5ºF/0.3ºC to 1.0°F/0.6ºC — and may be easily affected by factors such as illness, alcohol, and sleep changes (2,3).

How do I know if I'm ovulating? ›

your cervical mucus – you may notice wetter, clearer and more slippery mucus around the time of ovulation. your body temperature – there's a small rise in body temperature after ovulation takes place, which you may be able to detect with a thermometer.

Can you ovulate without a BBT rise? ›

If you don't detect an ovulation-indicating temperature rise after several cycles, your doctor will give you a blood test to confirm the findings. BBT thermometers are not 100 percent accurate, and some women ovulate even without an increase in temperature.

What is the best temperature for ovulation? ›

For most people, 96°– 98° Fahrenheit is their typical temperature before ovulation. After you ovulate, it goes up to 97°–99°F — about four-tenths of one degree higher than your usual temperature.

How can I increase my BBT naturally? ›

What are some other tips for increasing your body temperature?
  1. Stay close to someone else. If it's safe (and comfortable) to do so, share body heat with someone else. ...
  2. Take a warm bath. A quick way to raise your internal temperature is hydro-immersion therapy — better known as taking a bath. ...
  3. Change into warm clothes.
5 Jan 2021

What does a drop in basal temperature mean? ›

An implantation dip is a one-day drop in temperature on a basal body temperature chart. It occurs about one week after ovulation. Usually, a dropping temperature is a sign that your period is coming or has already arrived.

Videos

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