The menstrual cycle is the interval from first day of bleeding (menses) to beginning of next bleeding, and it includes two phases, the first is called the follicular phase, and the second the luteal phase. The follicular phase begins at the onset of menses and during this phase there is thickening of the endometrium (lining of the uterus) and recruitment and maturation of an egg. It ends at ovulation, at which the mature egg leaves the ovaries, whereupon the luteal phase begins. During the luteal phase the cell accompanying the egg become a corpus luteum (yellow body, important for maintaining pregnancy). If conception does not occur, the corpus luteum disintegrates and menstruation occurs.
The ‘textbook’ menstrual cycle in young healthy women is 28 days. Most women aged 19–42 years have follicular phases of 14.6-day durations and luteal phases of 13.6-day durations; thus ovulation occurs at day 14-15. Highest fertility is associated with a 30 or 31 menstrual cycle in which there are 5 day-bleeding, and ovulation at day 14-16.
The accurate prediction of ovulation, and thereby the fertility window, is very difficult in individual women, because:
Menstrual cycle length is highly variable, even between similarly aged young women, ranging from 25 to 34 days. The major contributor to this variation is the length of the follicular phase;
Length changes, particularly 5 years after the first period, and after the age of 40-45 (2-5 years before menopause);
Wide ranges in the follicular (10–23 days) and the luteal phase (7–19 days) are reported, and only 10% of women with a 28-day cycle show a 14-day follicular and luteal phase. However, once the cycle is set in maturity, luteal phase length remains relatively constant through to menopause.