Why labor is hard to start
For nine months the uterus has been kept deliberately quiet, mostly by progesterone. Starting labor therefore means overcoming that brake. As term approaches, the balance shifts: estrogen rises relative to progesterone, and the uterine muscle starts building the machinery of contraction — including more receptors for oxytocin. The muscle becomes primed, like a stage set up before the performance.
The exact trigger that says 'now' is still not fully understood in humans, and it likely involves signals from the fetus itself. But once the uterus is primed and the brake is lifted, two molecules drive the contractions: oxytocin and the prostaglandins.
The runaway loop
Oxytocin is released from the posterior pituitary and makes the uterine muscle contract. Here is the twist: each contraction pushes the baby's head against the cervix, stretching it, and that stretch signals the brain to release even more oxytocin. More oxytocin means stronger contractions, which means more stretch, which means more oxytocin. This is textbook positive feedback — a loop that amplifies itself instead of settling down.
FERGUSON REFLEX (positive feedback in labor)
+------------------------------------+
| |
v |
contraction --> baby's head pushes --> cervix STRETCHES
^ |
| v
uterine muscle <-- OXYTOCIN released <-- brain (signal up)
^ |
| v
prostaglandins <------+ (oxytocin boosts prostaglandins,
which further strengthen contractions)
The loop AMPLIFIES until -> BIRTH breaks the cycle
(no baby, no head on cervix, no stretch -> loop stops)The prostaglandins are local eicosanoid signals made right in the uterus and cervix. They do two jobs at once: they ripen and soften the cervix so it can open, and they make the muscle contract more forcefully, working hand in hand with oxytocin. This is why prostaglandin medicines are used clinically to induce or augment labor.
After the baby: the loop reverses
Once the baby and placenta are delivered, the stretch stops and the positive-feedback loop ends — exactly as designed. But oxytocin still has work to do: continued contractions clamp the blood vessels where the placenta detached, limiting bleeding, and over the following weeks the uterus shrinks back toward its original size, a process called uterine involution. The same hormone that opened the body now helps it close.