For more than 30 years, the basal body temperature (BBT, the temperature of the body upon wakening) has often been used as a guide for timing sexual intercourse to achieve or avoid pregnancy. However, according to a recent article by two nurse researchers who reviewed more than 20 studies conducted over the past 27 years, there is scant evidence supporting the use of the BBT to predict ovulation.

The study authors explain that the BBT is, at best, an indicator that ovulation has already occurred, not that it's about to occur. Ovulation is often accompanied by a slight rise in body temperature, as measured first thing in the morning. The optimal time for intercourse to result in pregnancy, however, is three to five days before ovulation, before the BBT begins to rise. Once the BBT has begun to rise, there's only about a day left for the sperm to reach the ovum-and if illness, shift work, or other factors have thrown the woman's temperature cycle off track, the temperature cannot reliably predict conception.

These factors render the BBT less useful than other methods of predicting ovulation, including observing estrogenic cervical mucus (reported to resemble egg white, when at its optimal consistency for conception); using a kit or electronic device to detect urinary luteinizing hormone (which is released just before ovulation); and ultrasonography, which is considered the gold standard in ovulation detection.

The authors also caution against using the BBT to avoid or prevent pregnancy. They recommend that women wishing to use natural methods of birth control seek a qualified teacher to learn how to use more effective ones.

Barron ML, Fehring RJ. MCN Am J Matern Child Nurs 2005;30(5):290-6.