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Web Topic 11.1: Using Basal Body Temperature to Track Ovulation

[Referenced on textbook p. 321]

One way to estimate the day of ovulation is to track basal body temperature (BBT). To use this technique, the woman measures her temperature with a digital thermometer or a special-purpose BBT thermometer every morning before getting up. Typically, a woman’s basal body temperature is relatively low during the follicular phase (say, about 36.3°C or 97.4°F), dips slightly on the day of ovulation, and rises sharply to a level above the follicular phase level (say, to 36.8°C or 98.2°F) on the day after ovulation, staying high for the remainder of the luteal phase (see Figure 1). What matters is the change in temperature, not the absolute temperature, which may vary from woman to woman and with the measurement technique being used.

Figure 1  A typical basal body temperature chart for one menstrual cycle and the first day of menstruation. Ovulation (on day 14 in this example) is marked by a slight dip in temperature, followed by a rise of at least 0.2°C (0.4°F) over the following 48 hours. The higher temperature is sustained for the duration of the luteal phase. Random spikes, such as the one here on day 10, are common and should be ignored. If the cycle is longer than 28 days, ovulation is likely to occur later than day 14.

Since the main indicator is the rise on the day after ovulation, when coitus no longer stands a good chance of leading to conception, it is not helpful for getting pregnant on that particular cycle. Rather, the woman has to follow her temperature over several cycles to determine the usual interval between the onset of menstruation and ovulation. This information can then be used to time coitus during future cycles.

If the basal body temperature does not show the mid-cycle rise, it is possible that ovulation is not occurring. If the rise is not sustained for at least 10 to 12 days, the luteal phase may not be long enough for pregnancy to be established. These potential problems can be investigated and treated by fertility specialists.