Menstrual cycle shortest and longest cycle lengths is considered very irregular. The average menstrual cycle lasts 28 days. The variability of menstrual cycle lengths is highest for women under 25 years of age and is lowest, that is, most regular, for ages 25 to 39. Subsequently, the variability increases slightly for women aged 40 to 44. The luteal phase of the menstrual cycle is about the same length in most individuals (mean 14.13 days, standard deviation 1.41 days) whereas the follicular phase tends to show much more variability (log-normally distributed with 95% of individuals having follicular phases between 10.3 and 16.3 days).
Pharmacokinetics of progesterone of the menstrual cycle with LC–MS/MS are 6.7 to 22.2 ng/mL. When the unreliable method of standalone RIA has been used to measure progesterone levels with oral progesterone, far higher peak levels of 6.5 to 10.2 ng/mL, 13.8 to 19.9 ng/mL, and 32.3 to 49.8 ng/mL have been erroneously observed after single 100, 200, and 300 mg doses, respectively. In addition, progesterone levels have been inaccurately measured by RIA as remaining elevated for 12 to 24 hours. Regardless of assay method, peak levels of progesterone following a dose of oral progesterone occur after about 1 to 3 hours. The elimination