Progesterone, Corpus Luteum, and the Luteal Phase

Progesterone is a steroid hormone initially secreted by the corpus luteum, an area in the ovary that develops after ovulation. Prior to ovulation, progesterone levels are very low. They increase right after ovulation and reach levels above 10 ng/ml usually by 5-7 days after ovulation.

The time right after ovulation until the next menstrual period is called the “corpus luteum phase” or the “luteal phase.” During the luteal phase, the fertilized egg travels through the fallopian tube into the uterus where it implants usually 6-12 days after fertilization. The normal luteal phase lasts on average 14 days, and it can be anywhere from 12 to 17 days long.

Progesterone has the following functions:

• It prepares the endometrium, the lining of the uterus, for implantation and a possible pregnancy.
• It prevents the contraction of the uterus.
• It prevents the development of a new follicle.
• During pregnancy, it is being produced by the placenta and maintains the pregnancy until birth.

If pregnancy does not happen then production of progesterone by the corpus luteum decreases toward the end of the menstrual cycle, and menstruation begins.

If pregnancy does happen, then the placenta in addition to the corpus luteum will start producing progesterone. Progesterone levels increase at the time of implantation and by the 10^th to the 11^th week of the pregnancy the placenta takes over the main production of progesterone and the corpus luteum is no longer essential to maintain the pregnancy.