The Hot and Cold of Conception: Basal Body Temperature

Basal body temperature (BBT) is your morning body temperature before you get out of bed. Charting this temperature over the course of your menstrual cycle is an inexpensive, low-tech way to help determine if you’re ovulating. Because ovulation disorders are one of the major causes of female infertility, many OB/GYNS recommend BBT charting to patients when they first start trying to conceive. This way, doctors can identify and treat any ovulation problems as early as possible.

Contrary to popular belief, BBT charting is not the most effective way to time sexual intercourse for conception. Your fertility is highest during the several days preceding ovulation and the day it occurs, but the change in BBT that indicates ovulation happens 12 to 24 hours afterwards. So BBT doesn’t predict ovulation, but tells you that it happened. If your cycle is regular, tracking your BBT for a couple of months will give you a good idea of when in your cycle you ovulate.

How does measuring BBT help detect ovulation?

A woman’s normal non-ovulating temperature is between 96 and 99 degrees Fahrenheit, depending on the individual. Following the release of the egg, BBT increases by about half a degree in almost all women. The hormone progesterone, secreted by the ovary after ovulation, heats things up; it also prepares the uterine lining for a possible pregnancy. Body temperature will remain roughly half a degree higher until right before menstruation, when it will return to normal. (If you get pregnant, your temperature will stay higher through the first trimester). If your temperature doesn’t follow this pattern, it might indicate an ovulation problem.

Detecting the change

Because the spike in body temperature at ovulation is so small, you need a special basal thermometer (available in drugstores) to measure it. A basal thermometer records temperatures in one-tenth of a degree increments instead of the two-tenth increments on fever thermometers.

Basal thermometers come in mercury and digital versions. The mercury BBT thermometers look like fever thermometers, except the divisions between degrees are large and easy to read. These thermometers can be used orally or rectally. Digital BBT thermometers also look like fever models, except they boast special features like an illuminated display (for easier reading on dark mornings). The digital thermometers are used orally. Most thermometers come with several graphs so you can chart your BBT over two to three cycles.

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. False readings can be caused by a variety of things, including waking up at different times in the morning. Here’s how to get the most accurate results:

•  Take your temperature when you first wake up and are lying or sitting quietly in bed. You need to do the reading at the same time, give or take 30 minutes, every morning.

•  Leave the thermometer on your night table before you go to bed so there’s no need to get up for it in the morning. Shake mercury thermometers down at night or dip them briefly in cool water. Doing the motions in the morning can cause a rise in temperature.

•  Don’t eat or drink anything, even water, before doing the reading.

•  Be aware of factors other than ovulation that can increase BBT: emotional disturbance, stress, a cold or infection, jet lag, drinking alcohol the night before, using an electric blanket.

•  Don’t pull all-nighters: You need to have at least three hours of uninterrupted sleep to get an accurate reading.

Please note: The Bump and the materials and information it contains are not intended to, and do not constitute, medical or other health advice or diagnosis and should not be used as such. You should always consult with a qualified physician or health professional about your specific circumstances.