Obstetrics and Gynaecology · MU Sofia

11. Mechanism of labor. Management of normal labor

Explains the cardinal movements of the fetal head in an occipito-anterior presentation, from engagement through expulsion, with a table of fetal head diameters and their presentations. Also covers the management of normal labour across the first, second and third stages, including admission, partogram use, delivery of the head and shoulders, and active management of the third stage.

Inside: 3 sections · 1 table · 16 flashcards · 3 min

Topic 11 of 61 in the MU Sofia syllabus for Obstetrics and Gynaecology (Dental Medicine, Year 4). The free AceMed note covers it in 3 sections, about 339 words (3 min of reading), with 16 flashcards.

In this note

  1. Introduction
  2. Cardinal movements
  3. Managing normal labour

Introduction

The mechanism of labour is the series of movements the fetal head makes to pass the curved birth canal. It is described for the commonest case, the occipito-anterior vertex presentation.

Cardinal movements

  1. Engagement: the biparietal diameter passes the pelvic inlet, usually in the transverse or oblique diameter.
  2. Descent: continues throughout labour.
  3. Flexion: the chin meets the chest, so the smallest diameter, the suboccipitobregmatic (9.5 cm), leads.
  4. Internal rotation: at the pelvic floor the occiput turns anteriorly under the symphysis.
  5. Extension: the head pivots under the symphysis and is born by extension.
  6. Restitution and external rotation: the head turns back to line up with the shoulders, which rotate into the AP diameter.
  7. Expulsion: anterior shoulder under the symphysis, then the posterior shoulder and body.

Fetal head diameters

Managing normal labour

First stage

  • Admission: history, observations, abdominal and vaginal examination, CTG assessment.
  • Vaginal examination about every 4 hours: dilatation, effacement, station (relative to the ischial spines), position, membranes, liquor colour.
  • Mother's pulse hourly, BP and temperature every 4 hours, fetal heart every 15–30 minutes (intermittent auscultation) or continuous CTG if at risk.
  • Encourage mobility, fluids, emptying the bladder; offer pain relief.
  • Partogram: a graph of dilatation, descent, contractions, fetal heart and maternal signs; crossing the action line shows slow progress early.

Second stage

  • Pushing with contractions; fetal heart after every contraction.
  • Guard the perineum, control delivery of the head; episiotomy only when needed (fetal distress, operative delivery, threatened severe tear).
  • Check for the cord around the neck; deliver the shoulders; delay cord clamping for at least 1 minute in a healthy baby.

Third stage: active management

  • Oxytocin 10 IU IM (or IV) after birth of the baby.
  • Controlled cord traction once there are signs of separation (cord lengthens, gush of blood, uterus rises and firms; Küstner's test: the cord does not retract when the suprapubic area is pressed).
  • Inspect the placenta and membranes for completeness; check the birth canal; massage the uterus if soft.

Active management of the third stage, with oxytocin given straight after the birth, is the single most effective step against postpartum haemorrhage.

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