Progesterone, a steroid hormone, is produced in small amounts by the ovary at the beginning of the menstrual cycle. The level of progesterone at the beginning of the menstrual cycle is usually less than 1.0 ng/ml in most women, MedlinePlus states, and increases after ovulation. When a woman becomes pregnant, progesterone production greatly increases.
Function
Progesterone helps thicken and prepare the uterine lining so a developing embryo has an adequate blood supply for implantation. If pregnancy occurs, progesterone also suppresses inflammation at the site of implantation and blocks immune reactions that would attack the growing embryo as a foreign substance, according to infertility specialist Alan Beer, M.D. Progesterone also thickens the mucus in the cervix, keeping sperm and bacteria from entering the uterus. Additionally, the body temperature rises after ovulation as a side effect of progesterone. A drop in temperature just before the start of the menstrual period can indicate that a woman isn't pregnant, the Merck Manuals Online Medical Library explains.
Timing
The first action of progesterone after ovulation is to prepare the uterine lining for implantation. Progesterone production after ovulation occurs in the corpus luteum, the leftover remainder of the follicle that once contained the ovulated egg. Progesterone starts to rise right after ovulation, with levels normally averaging 12 to 20 ng/ml in the luteal phase of the menstrual cycle, the period after ovulation, and in early pregnancy, according to the American Pregnancy Association.
Production
Immediately after ovulation, the corpus luteum produces progesterone, but once an embryo implants, the growing placenta begins to produce progesterone as well. By the eighth week of pregnancy, the placenta assumes the role of progesterone production for the duration of the pregnancy, according to Dr. Beer.
Levels
Progesterone levels continue to rise throughout pregnancy, rising to a mean of 39.5 ng/ml by the end of the first trimester, 90.4 ng/ml by the end of the second trimester and 166.2 ng/ml by the 40th week of pregnancy, Dr. Beer reports.
Supplementation
Low progesterone levels after ovulation may interfere with embryo implantation or continued growth in some cases. If levels are low, some medical personnel give supplemental progesterone in the form of injection, micronized pills or suppositories to boost progesterone levels after ovulation. Supplementation may continue up to the 16th week of pregnancy, Dr. Beer states.
