Basal Body Temperature (BBT) is your body’s lowest resting temperature, typically measured first thing in the morning before any activity. It is a fertility awareness tool used to track menstrual cycle patterns, gain insights into hormonal fluctuations, and understand ovulation timing. After ovulation, a physiological change occurs: BBT rises. This temperature shift is primarily driven by the hormone progesterone, which is released by the corpus luteum. Progesterone acts on the hypothalamus, leading to a slight temperature increase that signifies ovulation has likely happened.
Typically, BBT will increase by about 0.5 to 1.0 degree Fahrenheit (0.3 to 0.6 degrees Celsius) and remain elevated throughout the luteal phase of the menstrual cycle. Before ovulation, temperatures usually range between 97.0 and 98.0 F (36.1 and 36.6 C). After ovulation, they generally rise to between 97 and 99 F (36.1 and 37.2 C). This sustained higher temperature confirms that ovulation has occurred, establishing a biphasic pattern in the BBT chart.
Sustained Temperature Elevation and Pregnancy
If pregnancy occurs, the basal body temperature typically remains elevated beyond the usual length of the luteal phase. This sustained elevation results from continued progesterone production. After conception, the corpus luteum receives a signal from human chorionic gonadotropin (hCG), a hormone produced by the developing embryo. This hCG signal prevents the corpus luteum from breaking down, ensuring continued progesterone secretion.
Progesterone is essential for maintaining the uterine lining, providing a nourishing environment for the implanted embryo, and preventing uterine contractions. The corpus luteum continues this progesterone production for approximately the first 10 to 12 weeks of pregnancy. After this period, the placenta takes over the primary role of progesterone production, ensuring the hormone levels remain high to support the ongoing pregnancy. This continuous progesterone supply keeps the body temperature elevated, differentiating a pregnant cycle from a non-pregnant one where temperature would drop before menstruation.
Interpreting Your BBT Chart for Pregnancy Signs
When interpreting a BBT chart for potential pregnancy, a key indicator is a sustained temperature elevation lasting for 18 days or more after ovulation. This prolonged rise suggests sustained high progesterone, common in early pregnancy. While a sustained elevation is a promising sign, it is not a definitive diagnosis of pregnancy.
Some individuals may observe a “triphasic shift” on their BBT chart, characterized by a secondary rise in temperature approximately 7 to 10 days after the initial post-ovulation temperature increase. This pattern can indicate early pregnancy, possibly linked to increased progesterone after implantation. However, only about 12% of pregnancy charts exhibit a triphasic pattern, and it can also occur in non-pregnant cycles due to other factors.
BBT charting has limitations. Various factors unrelated to pregnancy, such as illness, sleep disturbances, stress, alcohol consumption, and even changes in bedroom temperature, can affect BBT readings. Therefore, while BBT charting can provide valuable clues and help identify patterns, it is not a diagnostic tool for pregnancy. Any suspected pregnancy should always be confirmed with a home pregnancy test and a visit to a healthcare provider.