Basal Body Temperature (BBT) is the body’s lowest resting temperature, measured immediately upon waking. For fertility tracking, BBT charting is used to pinpoint ovulation. Before ovulation, BBT is in a lower range, rising slightly after an egg is released. If conception does not occur, the temperature drops before menstruation. If pregnancy is achieved, the temperature elevation is sustained, offering an early physiological sign of conception.
The Hormonal Mechanism Behind Temperature Elevation
The sustained elevation in Basal Body Temperature after conception is directly caused by continued high levels of progesterone. Progesterone is thermogenic, meaning it has heat-inducing effects on the body’s thermoregulatory center in the hypothalamus. This hormone is responsible for the slight temperature rise of about 0.5 to 1.0 degrees Fahrenheit (0.3 to 0.5 degrees Celsius) that occurs after ovulation.
In a non-pregnant cycle, progesterone production stops near the end of the luteal phase, causing the temperature to drop and menstruation to begin. When pregnancy occurs, the corpus luteum, a temporary gland formed after ovulation, remains functional and continues to secrete progesterone. This continued production prevents the uterine lining from shedding and keeps the BBT elevated above the pre-ovulatory baseline.
Around the tenth to twelfth week of gestation, the placenta takes over the primary role of progesterone production from the corpus luteum. This transition, referred to as the luteal-placental shift, ensures hormone levels remain high to support the developing pregnancy. High progesterone levels maintain the higher resting body temperature throughout the first part of the pregnancy.
How Long the Basal Temperature Remains Elevated
The basal body temperature remains elevated throughout the entire first trimester of pregnancy. This period involves dramatic hormonal fluctuations as the body relies heavily on progesterone to establish and maintain the pregnancy. A sustained BBT elevation lasting longer than 18 days past ovulation is a strong indicator of conception.
As the pregnancy progresses beyond the first 12 to 14 weeks, the placenta is fully established and the hormonal environment stabilizes. The body’s overall temperature regulation changes due to an increased basal metabolic rate. While the temperature remains slightly higher than the pre-ovulatory norm, the reliability of BBT tracking diminishes significantly after the first trimester.
Most healthcare providers advise discontinuing BBT tracking once pregnancy is confirmed, usually around the six to eight-week mark. Continued charting becomes less informative because the temperature may fluctuate or drop without signifying an issue, potentially causing unnecessary anxiety. The initial purpose of BBT—to confirm ovulation and detect early conception—is fulfilled once the pregnancy is established.
Interpreting Temperature Spikes: Normal Elevation Versus Fever
The normal temperature elevation in early pregnancy is subtle, a sustained increase of only 0.4 to 1.0 degrees Fahrenheit above the pre-ovulatory baseline. This slight rise is the normal physiological response to progesterone and should not be mistaken for a fever. The normal elevated BBT range in early pregnancy is generally between 97.4°F and 99.0°F (36.3°C and 37.2°C), though individual baselines vary.
A true fever, signaling a potential infection or illness, is defined by a reading of 100.4°F (38°C) or higher. It is important to distinguish this pathological spike from the normal, hormonally induced warmth, as high fevers are a concern during the first trimester. If a pregnant person’s temperature reaches this threshold, they should contact a healthcare provider.
Fevers during the first trimester are concerning because sustained high temperatures may interfere with fetal development. Immediate medical attention is necessary if the temperature exceeds 102°F (38.9°C), or if a lower fever is accompanied by symptoms like chills or a stiff neck. The distinction between a normal, sustained basal temperature and a pathological fever is based on the significant difference in the magnitude of the temperature increase.