A complete menstrual cycle includes the full interval from the first day of bleeding—menses—in one cycle, to the first day of bleeding in the next cycle. The menstrual cycle includes two phases: the follicular phase and the luteal phase. In the follicular phase, which begins at the onset of menses, the lining of the uterus (endometrium) thickens and an egg is recruited and matures. The follicular phase ends at ovulation, when the mature egg leaves the ovaries. Then the luteal phase begins. During the luteal phase, the cells accompanying the egg become a corpus luteum (literally “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 or 15. The highest fertility rates are associated with a 30- or 31-day menstrual cycle in which there are 5 days of-bleeding, with ovulation occurring on day 14, 15 or 16.
Accurately predicting ovulation and the fertility window can be difficult in individual women because of the following:
1. Menstrual cycle length is highly variable, even between similarly aged young women, ranging from 25 to 34 days. The key factor in this variation is the length of the follicular phase.
2. Throughout their lives, women experience changes in the length of their menstrual cycle. This is particularly true 5 years after the first period, and again at some point in their 40s (2–5 years before menopause).
3. Wide ranges in the follicular phase (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 phase and a 14-day luteal phase. However, once the cycle is set in maturity, luteal phase length remains relatively constant until menopause.