Understanding the hormonal cycles of the female reproductive system and how it affects your body can be helpful when trying to conceive or avoid pregnancy. Hormones play a major role in helping the brain, ovaries and uterus to communicate. Basaal 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 for BBT can provide useful information for women trying to conceive or avoid pregnancy.
The words “female” and “female” are used throughout this article to refer to people who identify as female and have typical reproductive organs of a cisgender woman. We recognize that some people who identify as female do not have the same anatomy as depicted in this article.
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Basic principles of the female reproductive system and fertility
Every month, hormones cause changes in the ovaries and uterus to prepare the body for possible pregnancy. The cycle begins with menstruation, the phase when the uterus loses its lining.
On the first day of menstruation, the ovaries are 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 estrogenwhich has effects throughout the body.
When estrogen rises to a certain level, it signals the brain to release luteinizing hormone (LH), which then activates ovulation.
During ovulation, the adult ovum is released from the most dominant follicle and moves through the fallopian tubes to the uterus. The remaining follicle becomes a hormone-secreting group of cells that the yellow bodywhich means “yellow body” in Latin.
The corpus luteum secretes several hormones, including progesterone. Progesterone causes the uterine lining to thicken in preparation for the acceptance of 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, man chorionic gonadotropin (hCG), which is secreted by the embryo, causes the corpus luteum to remain and hormones to continue to be secreted. The continuous progesterone secretion maintains the uterine lining for pregnancy.
The Fertile Window
The fertile window is the period during which sexual intercourse can lead to pregnancy. After ovulation, the egg can live up to 24 hours. Sperm can live for up to five days in the female reproductive tract.
Having sexual intercourse in the two days before ovulation has the greatest chance of resulting in pregnancy, but you can get pregnant at any time in the five days before ovulation and the day of ovulation.
Illustration by Katie Kerpel / Verywell
What is basal body temperature?
BBT is the body’s temperature when it is 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 to ovulation, the temperature is relatively low. Once ovulation occurs and progesterone levels rise, body temperature rises by up to 0.5 degrees. The temperature remains relatively high until progesterone levels drop and menstruation occurs.
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 to determine 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 reduce the chances of getting pregnant.
How to take basal body temperature
After ovulation, the increase 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 drop in progesterone causes the temperature to drop to the lower level again.
The changes in basal body temperature can be as small as 0.5 degrees, so it is important to use a thermometer with accuracy down to within 0.1 degrees. Anything less accurate may miss the small changes needed for fertility prediction.
Some thermometers are specifically designed for BBT that can provide some added value.
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Not accurate in women who have irregular menstrual cycles
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Increase in temperature occurs during or after the fertile period, so the fertile period can be missed
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BBT method alone is a less effective method of preventing pregnancy than other forms of birth control
Normal basal body temperature
Normal body temperature varies from person to person, but it usually varies 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 disease.
How to map temperature changes
As the increase in BBT occurs at or after ovulation, the fertile period begins a few days before you notice a drop in temperature. In other words, you may get pregnant during the week leading to an increase in temperature. However, determining your temperature over time can provide information about the timing of your cycle to predict fertility in future months.
When BBT is measured, the body must be completely at rest. Keep a thermometer within reach of your bed so you can measure your temperature immediately when you wake up, before getting out of bed or doing any activity.
Temperature should be taken at about the same time each day and with the same thermometer in the same place. Taking your temperature by mouth is the most common way to measure basal body temperature, but vaginal and rectal temperatures can also be used.
After charting basal body temperature for a few months, you will notice a pattern of lower temperatures for a few weeks, followed by a slight rise in temperature that plateaus and stays until menstruation. Use this information to predict the timing of ovulation and your fertile window in future months.
Tools to detect BBT
Besides a thermometer, all you need is a pen and paper to record your temperature every day on a graph or table. Several phone applications are also specifically designed 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 with time of intercourse, or it can 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 usually occurs in the middle of the cycle, around day 14. This information can be used to estimate when the fertile window is to determine 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 stretchable to promote sperm motility.
- medium pain: Mid-cycle pain and cramps that some women experience during ovulation can help predict when it will occur.
- Hormone measurement: LH can be measured at home using urine tests. An increase in LH (LH surge) causes ovulation, so timing intercourse during this time can increase the chances of pregnancy in women trying to conceive.
Basal body temperature during pregnancy
The hormone changes that occur with pregnancy also affect BBT. A few findings on the BBT chart may 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 implant dip. It can occur about a week after ovulation if fertilization has taken place and lasts only one day. Compare this to a typical cycle when a dip will be seen just before the start of your period and will remain low until the next time you ovulate.
An implant dip is not the most reliable sign of pregnancy, as it may not be present. If it is present, it does not always mean pregnancy.
A more predictable and noticeable change in BBT that may 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, such as a missed period and symptoms such as fatigue and nausea.
Summary
Basal body temperature is a non-invasive, inexpensive and simple measure that can be used to detect fertile periods for those trying to conceive or avoid pregnancy. BBT relies on mapping daily temperature and noticing small changes over time to predict future fertility windows.
A word from Verywell
Getting pregnant or avoiding pregnancy using hormone-free methods can be challenging. While the BBT method can be helpful in tracking your cycle and fertile window, your body temperature can be affected by many external factors and rely on tracking very small changes over time.
Not all women will be able to reliably predict ovulation, especially if they have irregular menstrual cycles. Talk to your healthcare provider about any concerns you may have about infertility or choosing the right form of birth control.
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