A basal body temperature (BBT) is the lowest temperature attained by the body during a twenty-four hour period, which is usually attained during a period of rest. For most women, the rest necessary for a BBT occurs after three or more hours of uninterrupted sleep, which usually coincides with nighttime sleep. In women, basal body temperature is also affected by hormones, most notably estrogen and progesterone. Prior to ovulation when the body is dominated by estrogen, basal body temperatures remain low.
However, after ovulation when progesterone dominates, the BBT rises an average of approximately half a degree. (See “Classic Rise” chart.) For many women, this noticeable temperature spike occurs immediately after ovulation due to the increase in progesterone and decrease in estrogen. However, not all women with ovulatory cycles experience an ideal thermal shift. The following sections describe the most common variations—and perfectly normal—in basal body temperature chart patterns.
Variations in Basal Body Temperature
The first most common variation in the BBT chart pattern is the sloping rise. In a sloping rise pattern, a thermal shift is obvious, but the individual temperatures rise more gently in a curved pattern over a few days rather than abruptly. (See “Sloping Rise” chart.) Eventually, the post-ovulation temperatures plateau at least half of a degree above the pre-ovulation temperatures.
The second variation in the basal body chart pattern is the slow rise, which is extremely similar to the sloping rise. In a slow rise pattern, a thermal shift is not apparent until a few days after ovulation. (See “Slow Rise” chart.) Like the sloping rise, temperatures in a slow rise increase gradually but in even smaller increments and over a longer period of time. The temperatures in a slow rise pattern also eventually reach the highest post-ovulation temperatures after four to six days.
The third common variation in the BBT chart pattern is the fallback rise, which is also sometimes referred to as a fallback thermal shift. In a fallback rise, an obvious thermal shift occurs immediately after ovulation. Following the obvious shift, the temperature drops the next day, sometimes under the coverline. However, on the third day after ovulation, the temperatures return to high post-ovulation temperatures and continue to remain high. (See “Fallback Rise” chart.)
The fourth variation in the basal body temperature chart pattern is the staircase rise. In a staircase rise, a thermal shift is not always obvious. Temperatures generally alternatively increase and decrease over a period of a few days until reaching the highest post-ovulation elevated temperatures. Sometimes, the first decrease falls below the coverline. However, after the period of a few days, the post-ovulation temperatures plateau at approximately half a degree above the pre-ovulation temperatures. (See “Staircase Rise” chart.)
Other Irregular Basal Body Temperatures
In addition to the four variations in basal body temperature, other irregular temperatures sometimes happen that make reading the BBT fertility chart more difficult. The first irregular temperature is a significant temperature drop immediately before the thermal shift. Such a decrease is often referred to as an ovulation dip and is the result of an increase in estrogen immediately before ovulation. Not all women experience an ovulation dip.
The second irregular temperature is a significant temperature drop about a week after ovulation. Such a decrease is often referred to as an implantation dip and is the result of a fertilized egg implanting in the uterine wall. An implantation dip may be accompanied by implantation bleeding. Not all women who have conceived experience an implantation dip, nor does a dip after ovulation always indicate pregnancy.
The third irregular temperature is a temperature that seems out of place either before ovulation or during the luteal phase after ovulation. In general, out-of-place temperatures may be explained by an insufficient period of rest before the taking of the basal body temperature. By following the rule of thumb, a woman may ignore one or two unusual temperatures before or after ovulation when reading her chart.
Although some women experience a clear shift in their basal body temperature immediately following ovulation, not all women can identify such obvious patterns in their BBT fertility charts. Fortunately, some variation and irregularity is perfect normal. Ovulation is possible without an obvious temperature shift as clearly demonstrated by the four variations in basal body temperature.
Disclaimer: The information contained in this article is for educational purposes only and should not be used for diagnosis or to guide treatment without the opinion of a health professional. Any reader who is concerned about his or her health should contact a doctor for advice.
Weschler, Toni. 2006. Taking charge of your fertility, 10th anniversary edition: The definitive guide to natural birth control, pregnancy achievement, and reproductive health. New York: Collins.
Classic Rise © 2011 Heather Johnson
Sloping Rise © 2011 Heather Johnson
Slow Rise © 2011 Heather Johnson
Fallback Rise © 2011 Heather Johnson
Staircase Rise © 2011 Heather Johnson