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 half-life of progesterone in the circulation is very short at a range of about 3 to 90 minutes. Previous studies using RIA have reported an overestimated half-life of oral progesterone of about 16 to 18 hours. A subsequent, reliable study using high-performance liquid chromatography–tandem mass spectrometry (HPLC–MS/MS) reported a half-life of oral progesterone of about 4.6 to 5.2 hours when it was taken with food. Due to the short half-life and duration of action of oral progesterone, it is often taken in divided doses of two or three times per day. Because of studies that used RIA, it was incorrectly