What is the pattern of basal body temperature changes during ovulation?

During the menstrual cycle, basal body temperature (BBT) follows a characteristic biphasic pattern that reflects underlying hormonal changes. In the follicular phase—beginning with menses and ending at ovulation—estrogen predominates, and BBT typically remains relatively low, usually ranging between 97.0°F and 97.5°F (36.1°C–36.4°C). Following ovulation, the ruptured follicle transforms into the corpus luteum, which secretes progesterone. Progesterone has a thermogenic effect, causing BBT to rise by approximately 0.4°F to 1.0°F (0.2°C–0.6°C) within 24 to 48 hours after ovulation. This elevated temperature is sustained throughout the luteal phase—if pregnancy does not occur, BBT declines 1–2 days before the onset of menses due to falling progesterone levels. A sustained thermal shift lasting at least three days is considered presumptive evidence of ovulation. While BBT charting can help identify retrospectively when ovulation occurred, it cannot predict ovulation in real time and is less reliable than methods such as urinary luteinizing hormone (LH) testing or transvaginal ultrasound for fertility monitoring.