Basal body temperature (BBT) is the lowest temperature a person reaches during rest, typically measured immediately upon waking before any activity. Tracking BBT daily is a fundamental practice in fertility awareness, offering insights into the body’s hormonal state throughout the menstrual cycle. Charting these subtle fluctuations helps identify predictable patterns linked to hormonal shifts, aiding in understanding the timing of ovulation and the approach of menstruation. This method relies on the principle that reproductive hormones influence the body’s internal thermostat.
The Standard BBT Cycle Pattern
A typical BBT chart displays a distinct biphasic pattern, characterized by two temperature ranges. During the first half, the follicular phase, temperatures remain in a lower range until the body prepares for the release of an egg. Following ovulation, a temperature shift occurs, marking the beginning of the luteal phase. This shift involves an increase in the daily BBT reading of approximately 0.5 to 1.0 degrees Fahrenheit. The temperature remains sustained at this higher level throughout the luteal phase, typically lasting between 12 and 16 days, indicating that ovulation has successfully taken place.
The Hormonal Mechanism of the Temperature Drop
The sustained elevation in basal body temperature during the latter half of the cycle is caused by progesterone. After the egg is released, the ovarian follicle transforms into the corpus luteum, which produces large amounts of this hormone. Progesterone has a thermogenic effect, acting on the hypothalamus, the brain’s temperature-regulating center, to raise the body’s resting temperature set point. If fertilization does not occur, the corpus luteum begins to degrade. As the corpus luteum breaks down, progesterone production plummets rapidly, causing the basal body temperature to drop back toward the lower, pre-ovulatory range.
Timing the Drop Relative to Menstruation
The basal body temperature drop serves as a reliable signal that menstruation is imminent. In a non-pregnant cycle, this temperature decrease most commonly occurs in the day or two immediately preceding the onset of menstrual flow. Many individuals observe the drop on the morning of their period, or about 12 to 24 hours before bleeding begins. This final, sharp decline is sometimes referred to as the “terminal drop,” signifying the end of the luteal phase. A sustained high temperature for 18 or more consecutive days following ovulation is required before considering the possibility of pregnancy.
What It Means When BBT Remains Elevated
The absence of the expected terminal temperature drop is often the first sign that conception may have occurred. If the basal body temperature remains elevated beyond the typical 14-to-16-day lifespan of the corpus luteum, the luteal phase has been prolonged. This sustained elevation is due to continued high progesterone levels. In early pregnancy, a developing embryo secretes human chorionic gonadotropin (hCG), which rescues the corpus luteum from degradation. This hormonal signal ensures the corpus luteum remains active and continues to produce progesterone, maintaining the uterine lining and keeping the BBT high. Other factors can also cause prolonged temperature elevation, including minor illness or a non-pregnancy related ovarian cyst that secretes progesterone.