Progesterone’s role in a woman’s fertility

The levels of progesterone in a woman’s body rise and fall dramatically with her monthly cycles. At ovulation, the production of progesterone rapidly rises from 2-3mg per day to an average of 22mg per day, peaking as high as 30mg per day a week or so after ovulation.

After ten or twelve days, if fertilisation does not occur, ovarian production of progesterone falls significantly. It is this sudden decline in progesterone levels (as well as estrogen levels) that triggers a period (menstruation), and another menstrual cycle will begin.

If pregnancy occurs, progesterone production increases and the shedding of the lining of the uterus is prevented, preserving the developing embryo. As pregnancy progresses, progesterone production is taken over by the placenta and its secretion increases gradually to levels of 300-400mg per day during the third trimester.

During pregnancy, rising progesterone levels prevent the premature shedding of the uterine lining (pro-gestation). If progesterone levels drop due to inadequate progesterone production, then a premature delivery could result, or bring about a miscarriage in the early trimesters.

Progesterone also influences the development of the breasts during pregnancy in preparation for producing milk after the birth. It has an impact on ligaments and muscles throughout the body as well, essentially to allow the suppleness and expansion necessary for giving birth. This also accounts for some of the problems which may be experienced during pregnancy - backache, constipation, and low-blood-pressure, for example.

Although the data are not entirely clear, it appears that progesterone may also have an effect on transport time of the ovum in the fallopian tube, and it may make the ovum more susceptible to sperm penetration.