As early as 1904 van de Velde pointed out the existence of variations in body temperature during phases of the menstrual cycle. A number of clinical investigations since then have confirmed his observation. Rubenstein1 correlated a study of vaginal smears with a study of basal temperatures and found that during the phase of follicle development the basal rectal temperature tends to drop progressively. The low point in the temperature curve is reached when the follicle matures. Coitus at this time is therefore most likely to result in conception. The beginning progesterone production which occurs before ovulation suffices to counteract in part the temperature depressing action of estrone and therefore to cause an initial temperature rise beginning a few hours before ovulation. The temperature rise continues after ovulation and should exceed 0.5 degree F. in the first twenty-four hours after ovulation and 1 degree F. the first week after ovulation.
BASAL TEMPERATURE AND DATE OF OVULATION. JAMA. 1944;124(11):707–708. doi:10.1001/jama.1944.02850110031012
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