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. 1999;23(1):55–64.

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graphic file with name arh-23-1-55f2.jpg — Blood levels of the major female steroid reproductive hormones throughout the menstrual cycle. Concentrations are expressed in the following units: for progesterone, nanograms of hormone per milliliter of blood (ng/mL); for estradiol, thousands of ng/mL. Estradiol is the primary estrogen in humans. Day 1 represents the onset of menstruation.

SOURCE: Adapted from Broverman, D.M.; Vogel, W.; Klaiber, E.L.; Majcher, D.; Shea, D.; and Paul, V. Changes in cognitive task performance across the menstrual cycle. Journal of Comparative and Physiological Psychology (now known as the Journal of Comparative Psychology) 95:646–654, 1981.