progesterone in pregnancy

Progesterone is the hormone that keeps a pregnancy calm and continuing, the chemical that tells the uterus “stay quiet, hold on, don't push yet.” Its very name hints at its job: pro-gestation, in favor of carrying the pregnancy.

Throughout pregnancy progesterone keeps the muscular wall of the uterus relaxed, suppressing the contractions that would otherwise expel the fetus. It also maintains the uterine lining that nourishes the early embryo, supports the developing breast tissue, and helps modulate the maternal immune system so the body tolerates the genetically half-foreign fetus. Early on it comes from the corpus luteum; from around the start of the second trimester the placenta becomes the dominant source.

Because high progesterone restrains contractions, the functional decline of its influence is one of the changes that permits labor to begin. In humans this is thought to occur more through changes in how the uterus responds to progesterone than through a simple fall in its blood level.

A clinical caveat: progesterone or related drugs are sometimes given to reduce the risk of preterm birth in selected pregnancies, but the evidence is nuanced and not every patient benefits, so it is not a universal preventive.