The relationship between basal body temperature and fertility

When trying to conceive or avoid pregnancy, it may be helpful to understand the hormonal cycles of the female reproductive system and how they affect your body. Hormones play an important role by helping the brain, ovaries and uterus communicate. Base Body temperature (BBT) is an external signal that predicts when your body is most fertile.

This article discusses the basics of a woman’s menstrual cycle, the fertile window, and how examining BBT can provide useful information for women trying to conceive or avoid pregnancy.

The terms “female” and “woman” are used throughout this document to refer to a person who identifies as female and has the reproductive organs typical of a cisgender female. We recognize that the anatomy of some people who call themselves female differs from that described in this article.

Female Reproductive System and Fertility Fundamentals

Every month, hormones cause changes in the ovaries and uterus to prepare the body for a potential pregnancy. The cycle begins with menstruation, the stage in which the uterus sheds its lining.

On the first day of menstruation, the ovaries are busy responding to a hormone released by the brain called follicle stimulating hormone (FSH). FSH tells the ovaries to make immature eggs, called follicle. These follicles secrete hormones Estrogenhas an effect on the whole body.

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

During ovulation, a mature egg is released from the primary follicle and travels through the fallopian tube to the uterus.The remaining follicle becomes a cluster of hormone-secreting cells called corpus luteumwhich means “yellow body” in Latin.

The corpus luteum secretes several hormones, including Progesterone. Progesterone causes the lining of the uterus to thicken in preparation for receiving 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 drop in progesterone and estrogen levels leads to menstruation.If fertilization occurs, human chorionic gonadotropin (hCG), secreted by the embryo, causing the corpus luteum to remain and continue to secrete the hormone. Continued progesterone secretion maintains the lining of the uterus for pregnancy.

fertile windows

The conception window is the period during which intercourse can lead to pregnancy. After ovulation, the egg can live for up to 24 hours. Sperm can survive in the female reproductive tract for up to five days.

You’re most likely to get pregnant by intercourse two days before ovulation, but you can get pregnant any time up to five days before ovulation and on the day of ovulation.

What is basal body temperature?

BBT is the temperature of the body at 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 to ovulation, body temperature is relatively low. Once ovulation occurs and progesterone levels rise, the body temperature can rise by as much as 0.5 degrees. The temperature remains relatively high until progesterone levels drop and menstruation occurs.

This information is useful for those trying to conceive and 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 for natural family planning for women who wish to use abstinence or barrier birth control methods during the fertile window to reduce their chances of conceiving.

How to measure basal body temperature

After ovulation, elevated progesterone levels during the luteal phase cause basal body temperature to rise by 0.5-1 degrees. If fertilization does not occur, the drop in progesterone causes the temperature to drop to lower levels again.

Basal body temperature changes can be as small as 0.5 degrees, so it is important to use a thermometer accurate to within 0.1 degrees. Anything less accurate can miss the small changes needed for fertility forecasts.

Some thermometers are specially designed for BBT and can provide some added value.

Risks of BBT

  • Inaccurate in women with irregular menstrual cycles

  • Elevated temperature during or after conception, so you may miss conception

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

Benefits of BBT

  • Cheap

  • no side effects

  • No risks involved with hormonal contraception

normal basal body temperature

Normal body temperature varies from person to person, but is usually between 96 and 99 degrees Fahrenheit. Temperatures vary slightly, depending on how they are measured, and may change at certain times of the day as well as changes in activity, stress, sleep patterns, or illness.

How to graph temperature changes

Because the increase in BBT occurs at or after ovulation, conception begins a few days before you notice a drop in body temperature. In other words, you can get pregnant within a week of the resulting increase in body temperature. However, graphing your body temperature over time can provide information about the timing of your cycle to predict fertility in the months ahead.

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

Body temperature should be taken at about the same time each day, and the same thermometer should be used in the same location. Oral body temperature is the most common method of measuring basal body temperature, but vaginal and rectal temperatures can also be used.

After graphing your basal body temperature for a few months, you’ll notice a pattern of cooler temperatures over a few weeks, then a slight rise in temperature, and it stays that way until your period begins. Use this information to predict your ovulation time and your fertile window in the coming months.

Tools for tracking BBT

In addition to a thermometer, all you need is a pen and paper to plot your temperature on a chart or table every day. In addition, several mobile apps have been specially designed to chart the BBT and provide insight into your cycle and fertile windows.

other signs of ovulation

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

Hormones cause several other changes in the body. For those trying to conceive, fertility awareness methods can be combined with timed intercourse, or combined with barrier contraception or abstinence during the reproductive period to avoid pregnancy. Methods include:

  • Calendar method: Many women have predictable menstrual cycle lengths that can be used to predict the window of conception. The average cycle is 28 days, and ovulation usually occurs halfway through the cycle, around day 14. This information can be used to estimate when the fertility window begins to have sexual intercourse and increase or decrease the chance of pregnancy.
  • Cervical mucus method: Cervical mucus changes predictably during the menstrual cycle. During the most fertile period, cervical secretions are clear and stretchy, which aids sperm motility.
  • central schmerz: Mid-cycle pain and cramping that some women experience during ovulation can help predict when it will happen.
  • Hormone measurements: LH can be measured at home using a urine test. Elevated LH (LH surge) triggers ovulation, so timed intercourse at this time can increase the chances of a woman trying to conceive.

Basal body temperature during pregnancy

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

A very early potential sign of pregnancy is a drop in temperature throughout the day, called a drop in implantation. If fertilization occurs and lasts only one day, this may happen a week or so after ovulation. Compare this to a typical cycle, which dips right before your period starts and stays low until your next ovulation.

Implantation dip is not the most reliable sign of pregnancy because it may not be there. If it’s there, it doesn’t always mean pregnancy.

A more predictable and pronounced change in BBT that can indicate pregnancy is the prolongation of temperature. During pregnancy, progesterone levels remain high and lead to a continuous increase in BBT.

Look for other signs of pregnancy, such as missed periods and symptoms like fatigue and nausea.


Basal body temperature is a non-invasive, inexpensive and simple measurement that can be used to track the fertility period of those trying to conceive or avoid pregnancy. BBT relies on charting daily temperatures and noting small changes over time to predict future fertility windows.

VigorTip words

Trying to conceive or avoid pregnancy using hormone-free methods can be challenging. While the BBT method is 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 can reliably predict ovulation, especially if they have irregular menstrual cycles. Talk to your healthcare provider if you have any concerns about infertility or choosing the right form of birth control.

Frequently Asked Questions

  • What is the basal body temperature during menstruation?

    As your progesterone levels drop, your basal body temperature drops when your period begins. If your basal body temperature is still elevated and you have missed your period or have only mild spots, take a pregnancy test.

  • How long is your conception window each month?

    The conception window depends on how long the sperm and released eggs can survive in the female reproductive tract. Sperm can live for up to five days. After ovulation, the released egg can live for up to 24 hours. Adding these together gives you a fertile window of about six days.

  • Can you tell if you’re pregnant just by basal body temperature?

    A persistently elevated basal body temperature due to higher progesterone levels may indicate pregnancy, but it shouldn’t be the only sign you’re dependent. It will coincide with a missed period (or a very slight spot of implantation bleeding). Always confirm a suspected pregnancy with a pregnancy test.