A woman's menstrual cycle begins, as it has been arbitrarily assigned, with menses. Next is the follicular phase where estrogen levels build as an ovum matures (due to the follicular stimulating hormone, or FSH) within the ovary. When estrogen levels peak, it spurs a surge of luteinizing hormone (LH) which finishes the ovum and enables it to break through the ovary wall.
This is ovulation. During the luteal phase, which follows ovulation LH and FSH cause the post-ovulation ovary to develop into the corpus luteum which produces progesterone. The production of progesterone inhibits the LH and FSH hormones which (in a cycle sans pregnancy)causes the corpus luteum to atrophy, and menses to begin the cycle again.
These periods and intervals are important factors for couples using the rhythm method of contraception.
Men and women have hormonal cycles which determine both when a woman can achieve pregnancy and when a man is most virile. The female cycle is approximately twenty-eight days long, but can deviate greatly from this "norm". The male cycle is also variable. Men can ejaculate and produce sperm at any time of the month, but their sperm quality dips occasionally, which scientists guess is in relation to their internal cycle.
Furthermore, age also plays an equally significant role for both men and women.
In demographic contexts, fertility refers to the actual production of offspring, rather than the physical capability to produce which is termed fecundity. While fertility can be measured, fecundity cannot be. Demographers measure the fertility rate in a variety of ways.
which can be broadly broken into "period" measures and "cohort" measures. "Period" measures refer to a cross-section of the population in one year. "Cohort" data on the other hand, follows the same people over a period of decades. Both period and cohort measures are widely used.(wikipedia.org)