What is progesterone?

During ovulation, when a mature egg is released from the follicle, the follicle turns into what is called the corpus luteum. The corpus luteum produces progesterone. Women who are charting, using the Billings Ovulation Method, will draw an X on Peak Day. The reason an X is used is to signify that the hormones estrogen and progesterone have crossed and now progesterone is taking over, causing a dry or sticky sensation at the vulva. Progesterone prepares the lining of the uterus (the endometrium) for possible pregnancy. Progesterone, after ovulation, causes the basal body temperature to rise and also prevents further ovulation from occurring that cycle.

During pregnancy, progesterone levels continue to rise as the mother's placenta develops. Thus, it's frequently called the "pregnancy hormone".

How does it affect fertility?

Having good levels of progesterone indicates that a woman is not only potentially fertile but, that she will have a good chance of carrying a pregnancy. If progesterone levels are low, particularly during the early stages of pregnancy, the woman's risk of miscarriage is greatly increased.

How will I know if my progesterone levels are adequate?

Women who chart their cycles will be able to tell, at a glance, if their progesterone levels are most likely adequate or if they need to be checked. By counting the number of days, starting with the first day past Peak, fertile women should be able to count 11 to 16 days until the next menses. If the number consistently falls below 9 or 10 days, and she wishes to become pregnant, she should ask her physician to take a simple blood test to determine her progesterone levels.

Can progesterone levels be checked during any part of the cycle?

According to Dr. Paul Spencer, a family practice physician and medical consultant in Natural Family Planning, in White Bear Lake, Minnesota, progesterone levels should be checked - minimally on Peak plus 7, 9 and 11 in order to get an accurate assessment. If levels are low (less than 15 at Peak plus 7) or drop drastically on Peak plus 9 or 11, natural progesterone supplements can be prescribed. They may be given in the form of oral pills, vaginal suppositories or injections. Levels are then rechecked every two weeks, until the physician is confident an adequate level is being maintained. Blood tests for progesterone are much easier and more accurate in detecting luteal phase defects than endometrial biopsies.