10. Onset of labor. Stages of labor

Explains how labour is defined and what is thought to trigger it, covering the interacting hormonal, uterine and mechanical factors and the preparatory signs that come before it. Presents the stages of labour in a table giving the cervical range and usual duration of each, followed by a table contrasting true and false labour. Ends with the point that diagnosis rests on cervical change rather than pain.

Inside: 3 sections · 2 tables · 16 flashcards · 2 min

Topic 10 of 61 in the MU Sofia syllabus for Obstetrics and Gynecology (Medicine, Year 5). The free AceMed note covers it in 3 sections, about 176 words (2 min of reading), with 16 flashcards.

Introduction

Labour is the process of regular uterine contractions that efface and dilate the cervix and expel the fetus and placenta. Normal term labour starts between 37⁺⁰ and 41⁺⁶ weeks.

Why labour starts

The exact trigger in humans is not fully known; several factors act together:

Functional progesterone withdrawal (change in progesterone receptors) while oestrogens rise.
Fetal signal: maturing fetal hypothalamus–pituitary–adrenal axis; placental CRH rises sharply near term.
More oxytocin receptors and gap junctions (connexin 43) in the myometrium, so the uterus contracts as one.
Prostaglandins (PGE₂, PGF₂α) from the membranes and decidua ripen the cervix and stimulate contractions.
Stretch of the uterus (explains earlier labour in twins and polyhydramnios); Ferguson reflex (pressure on the cervix → oxytocin release).

Preparatory signs

Lightening (the head descends), more Braxton Hicks contractions, cervical ripening, passage of the mucus plug ("show").

Stages of labour

True versus false labour

In a first labour the cervix effaces first and then dilates; in later labours effacement and dilatation happen together, so labour is shorter.

Labour is diagnosed by cervical change, not by pain: regular contractions with a dilating, effacing cervix.