[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Reproduction, pregnancy and the fetus](https://durgical.com/en/physiology/#s-repro) › Parturition: onset and uterine contraction

[Physiology](https://durgical.com/en/physiology/) · [Reproduction, pregnancy and the fetus](https://durgical.com/en/physiology/#s-repro)

# 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](https://durgical.com/en/anatomy/myometrium/) · [Cervix](https://durgical.com/en/anatomy/cervix-of-uterus/) · [Body of uterus](https://durgical.com/en/anatomy/body-of-uterus/) · [Lower uterine segment](https://durgical.com/en/anatomy/isthmus-of-uterus/) · [Pituitary gland](https://durgical.com/en/anatomy/pituitary-gland/) · [Uterine artery](https://durgical.com/en/anatomy/uterine-artery/)

## Key points

- Progesterone keeps the uterus quiet; its functional withdrawal (receptor changes) and the estrogen-to-progesterone ratio start labor.
- Estrogen raises oxytocin receptors, connexin-43 gap junctions and prostaglandin synthesis in the myometrium.
- Ferguson reflex: cervical stretch → oxytocin release → stronger contractions → more stretch (positive feedback).
- Prostaglandins ripen (soften) the cervix and stimulate contraction.
- After delivery, contraction of the interlacing myometrial fibers compresses the spiral arteries: uterine atony is the leading cause of postpartum hemorrhage.

## 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](https://durgical.com/en/conditions/postpartum-haemorrhage/) | 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.

## Memory aid

Stretch calls oxytocin, oxytocin calls contraction, contraction calls more stretch: a loop that only the birth can break.

## 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.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/physiology/parturition-onset-and-uterine-contraction/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Myometrium](https://durgical.com/en/anatomy/myometrium/)
- [Cervix](https://durgical.com/en/anatomy/cervix-of-uterus/)
- [Body of uterus](https://durgical.com/en/anatomy/body-of-uterus/)
- [Lower uterine segment](https://durgical.com/en/anatomy/isthmus-of-uterus/)
- [Pituitary gland](https://durgical.com/en/anatomy/pituitary-gland/)
- [Uterine artery](https://durgical.com/en/anatomy/uterine-artery/)

## Related topics

- [The placenta and pregnancy hormones](https://durgical.com/en/physiology/the-placenta-and-pregnancy-hormones/)
- [The posterior pituitary: ADH and oxytocin](https://durgical.com/en/physiology/the-posterior-pituitary-adh-and-oxytocin/)
- [Smooth muscle contraction](https://durgical.com/en/physiology/smooth-muscle-contraction/)
- [Lactation](https://durgical.com/en/physiology/lactation/)
- [Homeostasis and feedback control](https://durgical.com/en/physiology/homeostasis-and-feedback-control/)

## Related conditions

- [Postpartum haemorrhage](https://durgical.com/en/conditions/postpartum-haemorrhage/)
- [Placental abruption](https://durgical.com/en/conditions/placental-abruption/)
- [Placenta praevia](https://durgical.com/en/conditions/placenta-praevia/)

## In Biochemistry

- [IP₃, DAG and calcium pathway](https://durgical.com/en/biochemistry/topics/ip-dag-and-calcium-pathway/)

## In Pharmacology

- [Nonsteroidal anti-inflammatory drugs (NSAIDs)](https://durgical.com/en/pharmacology/classes/nonsteroidal-anti-inflammatory-drugs/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/28-4-changes-during-pregnancy-labor-and-birth/) (OpenStax (Rice University), 2022); [Postpartum Hemorrhage](https://www.ncbi.nlm.nih.gov/books/NBK499988/) (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.

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Source: https://durgical.com/en/physiology/parturition-onset-and-uterine-contraction/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/parturition-onset-and-uterine-contraction/