Basal Body Temperature is Key to Determining Ovulation

Contrary to popular belief, BBT charting is not the most effective way to time intercourse for conception. Your fertility is highest during the two days before ovulation and the day it occurs, but your basal body temperature changes 12 to 24 hours after ovulation. Since the egg only lives one day, by the time BBT indicates ovulation there’s little fertile time left to conceive. According to the book, Six Steps to Increased Fertility: An Integrated Medical and Mind/Body Approach to Promote Conception (Simon & Schuster, 2000), BBT cannot pinpoint the exact day of ovulation. According to the book’s co-author, Robert L. Barbieri, M.D., professor of obstetrics, gynecology, and reproductive biology at Harvard Medical School, couples trying to conceive should have sex at least every other day, beginning two to four days prior to the anticipated day of ovulation. Many women combine using a BBT chart with an ovulation predictor kit—the at-home test that most accurately pinpoints your fertile days.

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). According to Dr. Barbieri, charting BBT can help identify a fertility-threatening condition called luteal phase defect, in which insufficient levels of the hormone progesterone cause a temperature rise at ovulation, and then a drop at least several days before menstruation. If your temperature remains constant over the course of your cycle, you might not be ovulating at all.

Because the spike in body temperature at ovulation is so small, you need a special basal thermometer (available at drugstores for about $10 to $15) 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. Many doctors recommend mercury thermometers over digital because they’re more accurate, says Dr. Fossum. On the down side, they’re a bit more difficult to use. Mercury thermometers need to be left in for three to five minutes to get a reading, while digital ones take only 30 to 60 seconds. Talk over the pros and cons with your ob-gyn.

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. Depending on the results of the blood test, your doctor may then recommend medication to induce ovulation or correct a luteal phase defect.

Getting an Accurate BBT Reading
•  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.

This article originally appeared in the Spring 2005 issue of Conceive Magazine.

Related Topics: Basal Body Temperature; Home Fertility Testing; Ovulation Prediction; When Am I Fertile?