Situation: A 24-year-old primigravida at 39 weeks' gestation comes to a lying-in clinic at 02:00 because of abdominal tightening since midnight. Her membranes are intact, she has no vaginal bleeding, and she reports active fetal movement. Her prenatal course has been uncomplicated. While waiting, the client asks why labor starts at the end of pregnancy. Among the proposed mechanisms of labor onset, the functional withdrawal of which hormone is believed to remove the main inhibition of uterine contractions?

Progesterone maintains uterine quiescence during pregnancy. A functional withdrawal of its effect near term, together with a rising estrogen effect, prostaglandins, and oxytocin, is among the proposed mechanisms of labor onset. The exact trigger in humans is not fully known, but progesterone is the hormone that inhibits contractions.

Why labor begins near term

The question asks which hormone, when its effect is “functionally withdrawn” at the end of pregnancy, removes the main brake on uterine contractions. The answer is progesterone.

Progesterone is the dominant hormone that maintains uterine quiescence throughout pregnancy. It suppresses myometrial excitability, reduces gap junction formation, and inhibits the expression of contraction-associated proteins. Near term, the myometrium becomes less responsive to progesterone even though the circulating level does not drop in humans. This phenomenon is called functional progesterone withdrawal ^[1][2][3].

A key point for licensure exams is the contrast between humans and most other mammals. In many animals, labor is preceded by a sharp fall in maternal progesterone concentration. In humans, however, maternal, fetal, and amniotic fluid progesterone concentrations remain elevated up to and during labor ^[3][4]. Therefore, the trigger is not a numerical decrease in the hormone level but a loss of its biological effect at the tissue level.

Several mechanisms have been proposed to explain this functional withdrawal. They include changes in the relative expression of progesterone receptor isoforms, altered regulation of progesterone-responsive genes through nuclear factor kappaB (NF-kB) promoter sites, and non-genomic actions of progesterone ^[1]. Epigenetic regulation of progesterone receptors has also been implicated in shifting the myometrium from a quiescent to a contractile state ^[3].

Watch out! Do not confuse the hormone that maintains pregnancy with the hormones that actively drive contractions. Oxytocin and prostaglandins promote uterine contractions, and estrogen enhances myometrial excitability and oxytocin receptor expression. They all rise or become more effective near term, but the question asks which hormone’s withdrawal removes the main inhibition. That is progesterone.

Key point! The exact trigger of human labor remains incompletely understood, but the proposed framework is that a functional loss of progesterone action, combined with rising estrogen, prostaglandin, and oxytocin effects, shifts the uterus from quiescence to coordinated contractions ^[1][2][4].

The table below summarizes the roles of the four hormones in labor onset.

Hormone	Role in pregnancy and labor	Change near term in humans
Progesterone	Maintains uterine quiescence; inhibits contractions	Circulating level stays high; tissue responsiveness decreases (functional withdrawal)
Estrogen	Promotes myometrial excitability; increases oxytocin receptors and gap junctions	Effect increases relative to progesterone
Oxytocin	Stimulates rhythmic uterine contractions	Receptor sensitivity increases near term
Prostaglandin	Promotes cervical ripening and myometrial contraction	Production increases near term

In this client’s clinical picture—term primigravida with intact membranes, no bleeding, active fetal movement, and abdominal tightening since midnight—the nurse can explain that the onset of labor reflects a shift in the hormonal balance that has maintained the pregnancy. The functional withdrawal of progesterone is the central event that removes the primary inhibition of uterine contractions ^[1][2][3][4].