Progestins and antiprogestins

Progestins are a group of synthetic progestogens that have similar effects to those of natural progesterone.

They can be divided into two categories: the progesterone derivatives and the testosterone derivatives.

They are mainly used as contraceptives, in hormone replacement therapy, or HRT, and in the treatment of various gynecologic conditions.

Now, antiprogestins act as progesterone antagonists and include mifepristone and ulipristal.

Mifepristone is used for medical abortion while ulipristal is used as an emergency contraceptive.

The hypothalamus secretes gonadotropin-releasing hormone, or GnRH, which travels to the nearby pituitary gland and stimulates it to secrete two hormones, follicle stimulating hormone, or FSH, and luteinizing hormone, or LH.

During the two weeks following ovulation, which is referred to as the ovarian luteal phase, the remnant of the ovarian follicle becomes the corpus luteum, which is made up of luteinized theca and granulosa cells, meaning that these cells have been exposed to the high luteinizing hormone levels that occur just before ovulation.

The luteinized cells secrete more progesterone than estrogen and progesterone becomes the dominant hormone.

Under the influence of progesterone, the uterus enters into the secretory phase of the endometrial cycle.

During this time, spiral arteries continue to grow, and the endometrial glands continue to produce more secretions that make the endometrium more receptive to the implantation of a fertilized gamete.

After day 15 of the cycle, the optimal window for fertilization begins to close and the corpus luteum gradually degenerates into the nonfunctional corpus albicans.

The corpus albicans doesn’t make hormones, so estrogen and progesterone levels slowly decrease.

When progesterone reaches its lowest level, the spiral arteries collapse and the functional layer of the endometrium prepares to shed during menstruation.