The gestagen progesterone is a steroid hormone which is mainly formed in the cells of the corpus luteum and during pregnancy in the placenta.

The progesterone concentration correlates with the development and regression of the corpus luteum. Whereas progesterone is barely detectable in the follicular phase of the female cycle, a rise in the progesterone level is observed one day prior to ovulation.

Increased progesterone synthesis occurs during the luteal phase. In the second half of the cycle, pregnanediol is excreted in urine as the main degradation product of progesterone. Progesterone brings about the conversion of the uterine mucosa into a tissue rich in glands (secretion phase), in order to prepare for the intrauterine implantation of the fertilized ovum.

During pregnancy, progesterone inhibits the contraction of the myometrium. In the mammary gland, progesterone (together with estrogens) promotes the proliferation, secretion and disposition of the alveoli. The determination of progesterone is utilised in fertility diagnosis for the detection of ovulation and assessment of the luteal phase.