Basal Body Temperature (BBT)

Definition - What does Basal Body Temperature (BBT) mean?

Basal body temperature (BBT) is the temperature of the body at resting. Tracking basal body temperature (with a basal body thermometer) is an effective tool for predicting when ovulation will occur, and pinpointing your most fertile window.

FertilitySmarts explains Basal Body Temperature (BBT)

Basal body temperature is determined by heat created by cells in the body. Basal body temperature can vary from 96.0°F to 99.0°F degrees in healthy people, and women may see basal body temperature change by up to 1.5°F degrees as hormone levels change throughout their cycle.

The progression of basal body temperature throughout the menstrual cycle usually looks like this:

  • When menstruation beings, basal body temperature is usually around the middle of a woman's personal normal range. For a woman who varies from 97.0°F to 98.3°F, for example, it may be around 97.6°F or 97.7°F.
  • During the follicular phase during which the egg matures, basal body temperature falls gradually. Basal body temperature will be at its lowest immediately before ovulation. The last few days of the follicular phase, when basal body temperature is lowest, are the start of a woman's mot fertile window.
  • When ovulation occurs, increases in progesterone cause a sudden rise in basal body temperature. Our example woman may see her BBT rise from 97.0°F to 97.6°F overnight, and climb to 98.0°F the next day. Because the egg is only viable for about one day ovulation, this body temperature spike marks the last day of the fertile window.
  • During the luteal phase when the body is waiting to see if a fertilized egg will implant in the uterine wall, basal body temperature remains high. It may begin to fall slowly in the few days before menstruation. After menstruation begins, body temperature falls quite rapidly back to the middle of a woman's range.

It is helpful to practice basal body temperature monitoring in conjunction with monitoring of cervical mucus. This practice, often referred to as the symptothermal method, provides two data points to know what a woman's most fertile period looks like, and provides a second data point to consult if BBT experiences fluctuations as a result of physical activity, fever, or sleep disruption.

Because the fluctuations that contain important information about the body's hormone state are small, basal body temperature should be taken first thing in the morning after waking up. This protects against temperature increases that could be caused by exercise or physical activity during the day.

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