How Birth Works: Labour and the First Breath

In numbers

• MeasuredThe cervix opens to about 10 cm
• EstimateA first labour often takes 12 hours or more, later ones less; it varies a great deal
• EstimateIn active labour, a contraction about every 2–3 minutes, lasting about a minute
• MeasuredAbout 1 in 5 babies in the world is born by caesarean section
• MeasuredThe hole between the heart's upper chambers closes with the first breaths; the vessel that bypassed the lungs closes within days
• MeasuredThe cord is best clamped after at least a minute
• DrawnThe section, the baby and the pace are drawn: a labour of hours is shown in about a minute

Every number is labelled measured, an estimate or drawn: how we know.

The science

Labour has three stages. In the first, contractions thin (efface) and open (dilate) the cervix: a latent phase, often slow, to about 5 cm, and then the active phase, from 5 cm to full dilation at about 10 cm. In active labour contractions usually come every 2–3 minutes and last about a minute. A first labour often takes 12 hours or more and later ones less, and it varies a great deal; the World Health Organization advises that the active first stage usually lasts up to 12 hours in a first labour and up to 10 in later ones. In the second stage, from full dilation to the birth, the head goes through the cardinal movements: it engages in the pelvis, comes down, flexes (chin to chest, so that its smallest diameter, about 9.5 cm, leads), turns from facing the mother's side to facing her back (internal rotation), extends under the pubic arch to be born, then turns back (restitution and external rotation), and the shoulders and the body follow. The bones of the skull are not yet joined: they overlap at their seams (moulding), and the fontanelles, the soft spots, let the head change shape a little. At birth the fluid in the lungs is cleared, partly squeezed out in the canal and mostly absorbed; the first breaths open the air sacs, the vessels of the lungs relax, and the pressure in the left side of the heart rises above the right, which closes the flap over the foramen ovale, the hole between the upper chambers; the ductus arteriosus, the vessel that carried blood past the lungs, closes over the next days. Most babies breathe on their own within about half a minute. Waiting at least a minute before clamping the cord gives the baby extra blood and iron. In the third stage the placenta comes away, usually within half an hour, and the womb's muscle clamps down and closes the vessels where it was. About 1 in 5 births in the world is by caesarean section, and the share is rising.

In Endo-View

This is chapter 13 of the journey to life, an interactive 3D simulation at true scale: watch it, slow it down or run it at real speed, and read every number with its label.

Watch it in 3D