Length of the Menstrual Cycle

OVULATION occurs on only one day in the cycle and is followed about 2 weeks later by menstruation, in the absence of pregnancy. Normally the time between ovulation and the next menstruation does not vary to any great extent. The length of the menstrual cycle is dependent upon variations in the time from the beginning of the cycle up to ovulation, as illustrated below.

The occurrence of ovulation determines the length of the cycle:

The length of time from the beginning of menstruation up to ovulation can vary. Ovulation is often delayed at times of stress, during lactation and at pre-menopause.

On this one day of ovulation in the cycle, one or more ova may become available for fertilization. The ovum lives for no more than 24 hours, and the sperm cells for a variable time. In the absence of satisfactory mucus the sperm cells are unlikely to survive beyond an hour or so, but with the support of good cervical mucus they may survive for up to 2 or 3 days, even rarely 4 or 5 days.

For further illustrations of variable cycle length click here.

Reference: Evelyn L. Billings, John J. Billings and Maurice Catarinich, Billings Atlas of the Ovulation Method, the Mucus Patterns of Fertility and Infertility, Ovulation Method Research and Reference Centre of Australia, Melbourne, 1989.