How the menstrual cycle works

Every NFP method rests on the same biology. Before learning any particular set of charting rules, it helps both spouses to understand what is actually happening in a woman's body across a cycle. This knowledge is not only for the wife. Husbands who understand the cycle tend to be more patient, more supportive and better partners in decision making.

The two halves of the cycle

A cycle is counted from the first day of menstrual bleeding to the day before the next period begins. It has two main parts, divided by ovulation.

• The follicular phase begins with menstruation. Under the influence of follicle stimulating hormone, several follicles in the ovaries begin to mature, and usually one becomes dominant. The growing follicle produces rising amounts of estrogen. This phase is the part of the cycle that varies most in length, which is why cycles of different lengths are normal.
• The luteal phase begins after ovulation. The emptied follicle becomes the corpus luteum, which produces progesterone. This phase tends to be more consistent in length from cycle to cycle for a given woman. If pregnancy does not occur, progesterone falls and menstruation begins.