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.
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.
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.
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.
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.
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.
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.
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.
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.
