progesterone
Progesterone is the hormone of pregnancy preparation and maintenance. Its name says it all — it is literally pro-gestation. If estradiol builds up the uterine lining like laying down soil and plumbing, progesterone is what makes that lining hospitable and quiet, ready to receive and nourish an embryo.
It is a steroid made from cholesterol, secreted mainly by the corpus luteum (the structure left behind in the ovary after ovulation) during the second half of the cycle, and in much larger amounts by the placenta during pregnancy. Progesterone transforms the estrogen-primed endometrium into a secretory, nutrient-rich tissue, thickens cervical mucus into a barrier, quiets uterine muscle contractions, and helps prepare the breasts for milk production.
If pregnancy does not occur, the corpus luteum degenerates, progesterone falls, and the withdrawal of this hormonal support triggers shedding of the lining as menstruation. Progesterone also raises basal body temperature by a few tenths of a degree after ovulation — a real, measurable sign used in fertility tracking. An important caveat: the synthetic relatives used in contraceptives, called progestins, share many but not all of progesterone's effects, so the two are not interchangeable terms.
A woman tracking fertility notices her basal body temperature jumps and stays slightly elevated about a day after ovulation; this thermal shift reflects the rise in progesterone from the newly formed corpus luteum.
Progesterone from the corpus luteum raises basal body temperature after ovulation, a classic fertility-awareness sign.
Because progesterone keeps the uterus quiet, the abrupt fall in progesterone activity is a key trigger for the onset of labor at the end of pregnancy. Drugs that block its receptor can be used to terminate early pregnancy or induce labor.