Progesterone is a steroid hormone initially secreted by the corpus luteum, an area in the ovary which 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 prepares the endometrium, the lining of the uterus, for implantation and a possible pregnancy, it prevents contractions of the uterus and the development of a new follicle, and during pregnancy it is being produced by the placenta and maintains the pregnancy until birth.