The menstrual cycle is an average of 28 days, although it is normal to vary between 23 and 35 days. The cycle is counted from the first day of your period.
Ovulation is the release of the egg from the ovary.
This usually occurs at about day 14 of the cycle. Once released, the egg is fertile for up to 48 hours. If the egg is not fertilized, then the lining of the uterus will disintegrate, causing a period. By knowing your menstrual cycle, it is possible to approximate when you will be most fertile. This is caused by a decline in the hormones estrogen and progesterone, which occurs when the egg from the previous cycle is not fertilized.