Parturition and Labor

Parturition is the physiological process by which the uterus converts from a quiescent organ that retains the fetus to an actively contracting organ that expels it. Labour involves coordinated uterine contractions, softening and dilation of the cervix, and a series of hormonal signals that together time the onset of birth and drive its progression through delivery.

Definition

Parturition is the process of giving birth, comprising the transition of the uterus from quiescence to coordinated contractility, cervical ripening and dilation, and fetal expulsion, governed by an integrated endocrine and mechanical cascade at term.

Scope

This topic covers the physiology of the onset and progression of labour: the shift from uterine quiescence to activation, the roles of oxytocin, prostaglandins and the changing progesterone-to-oestrogen balance, cervical remodelling, and the stages of labour. It is reference physiology and does not provide guidance on the management, induction or augmentation of labour.

Core questions

• What keeps the uterus quiescent through most of pregnancy?
• What signals trigger the onset of labour and how is its timing controlled?
• How do oxytocin, prostaglandins and changing steroid signalling drive contractions?
• How does the cervix remodel and what defines the stages of labour?

Key concepts

• Uterine quiescence and activation
• Functional progesterone withdrawal
• Oxytocin and its receptors
• Prostaglandins in labour
• Cervical ripening (remodelling)
• Coordinated myometrial contractions
• Stages of labour

Mechanisms

For most of pregnancy the myometrium is held quiescent, an effect attributed in large part to progesterone action. The onset of labour reflects a shift from this quiescent state to an activated, contractile one. In humans this transition is associated with a functional withdrawal of progesterone action and a relative increase in oestrogen signalling, which upregulates contraction-associated proteins, including oxytocin receptors and gap junctions that synchronise the myometrium. Prostaglandins promote both cervical ripening and uterine contractility, and oxytocin reinforces contractions, with stretch and feto-placental signals contributing to the timing. The cervix remodels from a firm, closed structure into a soft, distensible one. Once active labour begins, coordinated contractions dilate the cervix and, together with maternal effort, expel the fetus and then the placenta, after which the uterus contracts to limit bleeding.

Clinical relevance

The physiology of parturition is the reference background for understanding why labour begins when it does and how its stages unfold. This entry describes normal physiology for educational orientation; it does not address the clinical assessment, induction or management of labour or its complications.

Evidence & guidelines

The mechanisms summarised here are drawn from integrative reviews of the endocrinology and physiology of parturition; this topic presents reference physiology rather than obstetric practice recommendations.

History

Research through the late twentieth and early twenty-first centuries reframed labour onset from a single trigger to an integrated transition from uterine quiescence to activation, emphasising functional progesterone withdrawal, oxytocin and prostaglandin signalling, and cervical remodelling, as synthesised in widely cited reviews of the control of labour and of parturition.

Debates

How is progesterone withdrawal achieved in human labour?

Unlike many species, humans do not show a clear fall in circulating progesterone before labour, so attention has focused on a functional withdrawal of progesterone action at the level of the myometrium and its receptors; the precise mechanism remains an active area of study.

Key figures

• Roger Smith
• Errol Norwitz
• John Challis
• Sam Mesiano

Seminal works

• smith-2007
• norwitz-1999
• mesiano-2022

Frequently asked questions

What triggers the onset of labour?

Labour reflects a coordinated shift of the uterus from quiescence to activation, associated in humans with a functional withdrawal of progesterone action, increased oestrogen and prostaglandin signalling, and rising responsiveness to oxytocin, rather than any single trigger.

Why does the cervix soften before birth?

The cervix undergoes remodelling, driven in part by prostaglandins, that changes it from a firm, closed structure into a soft and distensible one so that it can dilate during labour.