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).
Menstrual cycle maturity is called a tertiary or Graafian follicle, and it contains the ovum. Ovulation is the second phase of the ovarian cycle in which a mature egg is released from the ovarian follicles into the oviduct. During the follicular phase, estradiol suppresses release of luteinizing hormone (LH) from the anterior pituitary gland. When the egg has nearly matured, levels of estradiol reach a threshold above which this effect is reversed and estrogen stimulates the production of a large amount of LH. This process, known as the LH surge, starts around day 12 of the average cycle and may last 48