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How to Track Basal Body Temperature (BBT): A Complete Guide

BBT rises 0.2–0.5°C after ovulation due to progesterone. This guide covers thermometer selection, protocol, chart interpretation, and why consistent technique

BBT charting is the oldest and most evidence based method of retrospective ovulation confirmation available without blood tests. It works well when executed with care and is nearly useless when done casually. The technique is demanding in one specific way: it requires consistent, precise measurements taken under controlled conditions. The temperature difference between follicular and luteal phases is 0.2–0.5°C — well within the noise of a thermometer that's "good enough" for checking a fever. What BBT Measures and Why Progesterone, produced by the corpus luteum after ovulation, has a thermogenic effect — it raises resting body temperature by 0.2–0.5°C. This temperature elevation persists throughout the luteal phase and drops as progesterone falls before menstruation (or stays elevated if pregnancy occurs, since hCG maintains the corpus luteum). Basal body temperature is your temperature at complete rest — specifically, after a minimum of 3 consecutive hours of sleep, measured before any physical activity. This is the lowest temperature state of the day and the only state where the progesterone driven difference is measurable through a standard thermometer. What this means for timin