Physiology · Reproduction, pregnancy and the fetus
Parturition: onset and uterine contraction !
- Regulation
- Exam priority: medium
At term the quiet uterus switches on: hormones and stretch make it contract more and more strongly until the baby is born.
Updated 2026-11-01
Also called: Labor, Ferguson reflex
See it in 3D: Myometrium · Cervix · Body of uterus · Lower uterine segment · Pituitary gland · Uterine artery
What it is
For most of pregnancy progesterone keeps the myometrium quiet. Near term, functional progesterone withdrawal, rising estrogen, placental CRH and stretch make the uterus excitable: more oxytocin receptors, gap junctions and prostaglandins. Once the cervix is stretched, a positive feedback loop of oxytocin and contraction drives labor to delivery, and contraction after birth stops the bleeding.
How it works, step by step
- Rising placental CRH, fetal cortisol and estrogen near term shift the uterus from quiet to excitable.
- Progesterone receptor isoforms change so progesterone loses its relaxing effect, without a large fall in plasma level.
- The myometrium gains oxytocin receptors and gap junctions, so contractions spread as one coordinated wave from the fundus.
- Prostaglandins (PGE₂, PGF₂α) ripen the cervix by remodeling collagen and increase contractility.
- The fetal head stretches the cervix; afferent nerves trigger oxytocin release from the posterior pituitary.
- Stronger contractions push the head further, increasing stretch and oxytocin: the loop continues until delivery.
- After the baby and placenta are delivered, sustained contraction clamps the uterine vessels and limits bleeding.
Typical values
| Parameter | Value | Note |
|---|---|---|
| Term pregnancy | 37–41 weeks | from the last menstrual period |
| Uterine blood flow at term | ≈500–700 mL/min | ≈10–15% of cardiac output |
| Postpartum hemorrhage | ≥1000 mL | or bleeding with signs of hypovolemia |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Regulation
- +Oxytocin (posterior pituitary): Contraction via Gq–IP₃–Ca²⁺; prostaglandin release.
- +Prostaglandins E₂ and F₂α: Cervical ripening and myometrial contraction.
- +Estrogen and placental CRH: Raise oxytocin receptors, gap junctions and prostaglandin synthesis.
- +Uterine and cervical stretch: Excitability and the Ferguson reflex.
- −Progesterone: Myometrial quiescence through most of pregnancy.
- −Relaxin, nitric oxide, β₂-adrenergic tone: Relax the myometrium.
Stages of labor
| Stage | From | To |
|---|---|---|
| First | Regular contractions that change the cervix | Full dilation (10 cm) |
| Second | Full dilation | Birth of the baby |
| Third | Birth of the baby | Delivery of the placenta |
Clinical notes
- Postpartum hemorrhage: causes are tone (atony, most common), trauma, tissue (retained placenta) and thrombin (coagulopathy).
- Preterm labor and premature rupture of membranes often involve infection and inflammation that release prostaglandins.
- Placental abruption causes painful bleeding and a tense, contracting uterus.
Test yourself
Think of the answer, then tap the question to see it.
Why is labor a classic example of positive feedback?
Contractions push the fetus against the cervix; stretch triggers more oxytocin, which strengthens contractions, increasing stretch further, until delivery ends the loop.
What makes the myometrium contract as a coordinated unit near term?
Estrogen raises connexin-43 gap junctions and oxytocin receptors, so electrical activity spreads from cell to cell and the whole uterus contracts together.
Why is uterine atony dangerous after delivery?
The placental bed bleeds from open spiral arteries; only sustained contraction of the myometrial fibers compresses them. A relaxed uterus keeps bleeding.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- The placenta and pregnancy hormones
- The posterior pituitary: ADH and oxytocin
- Smooth muscle contraction
- Lactation
- Homeostasis and feedback control
Related conditions
In Biochemistry
In Pharmacology
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 28.4 Changes During Pregnancy, Labor, and Birth (OpenStax (Rice University), 2022); Postpartum Hemorrhage (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 83). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.