Basal Body Temperature (BBT) is the lowest resting temperature reached by the body, measured immediately upon waking before any physical activity. Tracking this baseline temperature is a fundamental component of fertility awareness methods used to identify the different phases of the menstrual cycle. Hormonal shifts create a characteristic biphasic temperature pattern, allowing individuals to recognize when ovulation has occurred. The temperature is lower during the first half of the cycle when estrogen is dominant, and it shifts higher after ovulation due to progesterone.
The Temperature Drop Signaling Ovulation
Before the primary temperature rise that confirms ovulation, some individuals observe a brief, transient BBT dip, sometimes called the “ovulation dip.” This drop signals the end of the follicular phase and the imminent release of an egg. It is caused by a temporary spike in estrogen, a temperature-lowering hormone, just before the Luteinizing Hormone (LH) surge triggers ovulation.
This pre-ovulatory drop is often less pronounced and fleeting than the drop preceding menstruation. It is not universally observed, making it an unreliable predictor for future ovulation timing. When it appears, this dip is quickly followed by a sustained increase of about 0.5 to 1.0°F. This temperature rise confirms the egg has been released and the body has entered the post-ovulatory, or luteal, phase, resulting from the newly formed corpus luteum producing progesterone.
Timing the Drop Before Your Period
The most significant BBT drop occurs just before the menstrual period begins. The post-ovulatory phase, known as the luteal phase, is characterized by the sustained production of progesterone from the corpus luteum, which keeps the body’s temperature elevated. This prolonged high temperature is necessary to maintain the uterine lining, preparing the body for a potential pregnancy.
If fertilization does not occur, the corpus luteum begins to break down around 12 to 16 days after ovulation. As this temporary gland degenerates, the levels of progesterone in the bloodstream plummet dramatically. The withdrawal of this thermogenic hormone causes the Basal Body Temperature to fall back toward the lower baseline temperatures characteristic of the follicular phase.
This temperature shift signals that the period is imminent. For most people, this drop occurs approximately one to two days before the onset of menstrual bleeding, or sometimes on the morning the period begins. The reliability of this timing is directly linked to the consistent length of the luteal phase, which is generally fixed. A consistent luteal phase allows individuals to predict the start of menstruation accurately once ovulation is confirmed.
Why a Drop Might Not Occur
When the Basal Body Temperature remains elevated beyond the expected end of the luteal phase—typically for 18 days or more after ovulation—it is often interpreted as a probable indication of pregnancy. This sustained high temperature occurs because the mechanism that causes the temperature drop has been intercepted. If a fertilized egg implants, the developing embryo begins to produce the hormone Human Chorionic Gonadotropin (hCG).
The hCG hormone signals the corpus luteum to continue its function, preventing its breakdown and ensuring that progesterone production remains high. Since progesterone is still circulating at high levels, the BBT stays elevated, mimicking the temperatures seen during the luteal phase, but lasting much longer. A consistently prolonged high BBT is a strong sign, though a positive pregnancy test is the only definitive confirmation.
Other Factors Affecting BBT
Other, less common factors can also prevent the expected BBT drop or cause a temporary elevation. Illness, such as a cold or fever, can artificially raise the basal temperature, obscuring the true hormonal shift. Significant sleep disturbances, consuming alcohol late at night, or certain medications can also interfere with a consistent reading. In rare instances, a corpus luteum cyst may continue to produce progesterone, delaying the temperature drop and the start of the period without a pregnancy.