Basal Body Temperature (BBT)
Basal body temperature (BBT) is the body's lowest resting temperature. A woman measures it orally or vaginally first thing in the morning. She takes it after at least three hours of unbroken sleep, before she rises, eats, or drinks.
BBT rises 0.2 to 0.5 degrees C within one to three days after ovulation. Progesterone causes the rise. The corpus luteum, the structure that forms from the follicle after ovulation, releases this warming hormone. The sustained rise gives the chart a two-level (biphasic) pattern.1
A sustained rise is presumptive evidence that ovulation has occurred. It cannot predict ovulation in advance. A biphasic chart can also appear without a confirmed ovulation. One example is a cycle in which the follicle never releases its egg (luteinized unruptured follicle).23
BBT serves three purposes. It gives after-the-fact evidence of likely ovulation. It measures the length of the luteal phase. A flat (monophasic) chart can also be one sign of anovulation, the absence of ovulation. Some ovulatory cycles, however, lack a clear rise. For this reason, BBT alone is not reliable for identifying the fertile window.4567
BBT charting requires consistent timing and adequate sleep. Several factors can distort the reading, including illness, alcohol, broken sleep, and shift work.2 BBT is a core part of sympto-thermal methods. Several other FABMs use it as a second sign to confirm ovulation.89
In restorative evaluation, clinicians combine BBT data with other cycle signs, such as cervical fluid and hormone patterns. Together, these signs give a full picture of ovulation.9
Cited in this entry
- Hilgers TW. The Medical and Surgical Practice of NaProTECHNOLOGY. Pope Paul VI Institute Press; 2004. https://rrmacademy.org/library/the-medical-surgical-practice-of-naprotechnology-rectiyuppdjrktphh/
- Barron ML, Fehring RJ Basal body temperature assessment: is it useful to couples seeking pregnancy?. MCN. The American journal of maternal child nursing. 2005. https://pubmed.ncbi.nlm.nih.gov/16132004/
- Marik J, Hulka J Luteinized unruptured follicle syndrome: a subtle cause of infertility. Fertility and sterility. 1978. https://pubmed.ncbi.nlm.nih.gov/147778/
- Martinez AR, van Hooff MH, Schoute E, van der Meer M, Broekmans FJ, Hompes PG The reliability, acceptability and applications of basal body temperature (BBT) records in the diagnosis and treatment of infertility. European journal of obstetrics, gynecology, and reproductive biology. 1992. https://pubmed.ncbi.nlm.nih.gov/1459325/
- Prior JC, Vigna YM, Schulzer M, Hall JE, Bonen A Determination of luteal phase length by quantitative basal temperature methods: validation against the midcycle LH peak. Clinical and investigative medicine. Medecine clinique et experimentale. 1990. https://pubmed.ncbi.nlm.nih.gov/2364587/
- Bedford JL, Prior JC, Hitchcock CL, Barr SI Detecting evidence of luteal activity by least-squares quantitative basal temperature analysis against urinary progesterone metabolites and the effect of wake-time variability. European journal of obstetrics, gynecology, and reproductive biology. 2009. https://pubmed.ncbi.nlm.nih.gov/19552997/
- Guida M, Tommaselli GA, Palomba S, Pellicano M, Moccia G, Di Carlo C et al. Efficacy of methods for determining ovulation in a natural family planning program. Fertility and sterility. 1999. https://pubmed.ncbi.nlm.nih.gov/10560997/
- Stachowska A, Kicińska AM, Kotulak-Chrząszcz A, Babińska A Usefulness of the Sympto-Thermal Method with Standardized Cervical Mucus Assessment (InVivo Method) for Evaluating the Monthly Cycle in Women with Polycystic Ovary Syndrome (PCOS). Healthcare (Basel, Switzerland). 2024. https://pubmed.ncbi.nlm.nih.gov/38891183/
- Duane M, Stanford JB, Porucznik CA, Vigil P Fertility Awareness-Based Methods for Women's Health and Family Planning. Frontiers in medicine. 2022. https://pubmed.ncbi.nlm.nih.gov/35685421/
Authoritative references
How other clinical authorities define this term. RRM Academy curates these verbatim or under fair use so the medical consensus is visible alongside our RRM-contextualized definition above.
- Wikipedia Basal_body_temperature
Basal body temperature is the lowest body temperature attained during rest. It is usually estimated by a temperature measurement immediately after awakening and before any physical activity has been undertaken.
This content is for educational purposes only and does not constitute medical advice. Consult an RRM clinician or healthcare provider for guidance specific to your situation.