Critical Periods in the Fertility Cycle A woman\’s fertility cycle usually lasts 28 days, but the specifics vary from person to person. Key periods in the reproductive cycle include ovulation, fertilization and implantation. Understanding these critical periods is important for women during pregnancy. Ovulation calculation Ovulation is the most important period in a woman\’s reproductive cycle, usually around day 14 of the cycle. However, depending on the length of your period and the effects of estrogen and progesterone, ovulation can occur early or late in the cycle. In order to accurately calculate the ovulation period, women can measure basal body temperature, observe changes in vaginal secretions, and use ovulation