Childbirth (also called labour, birth, partus or parturition) is the culmination of a human pregnancy or gestation period with the delivery of one or more newborn infants from a woman's uterus. The process of human childbirth is categorized in three stages of labour: the shortening and dilation of the cervix, descent and delivery of the infant, and delivery of the placenta.

The mechanics of birth

Because humans are bipedal with an erect stance and humans have relatively the biggest head and shoulders to the size of the pelvis of any species, humans fetuses are adapted to make birth possible.

The erect posture causes the weight of the abdominal contents to thrust on the pelvic floor, a complex structure which must not only support this weight but allow three channels to pass through it: the urethra, the vagina and the rectum. The relatively large head and shoulders require a specific sequence of manoeuvres to occur for the bony head and shoulders to pass through the bony ring of the pelvis. If these manoeuvres fail, the progress of labour is arrested. All changes in the soft tissues of the cervix and the birth canal are entirely dependent on the successful completion of these six maneuvers:

1. Engagement of the fetal head in the transverse position. The baby is looking across the pelvis at one or other of the mother's hips.
2. Descent and flexion of the fetal head
3. Internal rotation. The fetal head rotates 90 degrees to the occipito-anterior so that the baby's face is towards the mother's rectum.
4. Delivery by extension. The fetal head passes out of the birth canal. Its head is tilted backwards so that its forehead leads the way through the vagina.
5. Restitution. The fetal head turns through 45 degrees to restore its normal relationship with the shoulders, which are still at an angle.
6. External rotation. The shoulders repeat the corkscrew movements of the head, which can be seen in the final movements of the fetal head.

Latent phase

The latent phase of labour may last many days and the contractions are an intensification of the Braxton-Hicks contractions that start around 26 weeks gestation. Cervical effacement occurs during the closing weeks of pregnancy and is usually complete or near complete, by the end of latent phase. Cervical effacement is the thinning and stretching of the cervix. The degree of cervical effacement may be felt during a vaginal examination. A 'long' cervix implies that not much has been taken into the lower segment, and vice versa for a 'short' cervix. Latent phase ends with the onset of active first stage; when the cervix is about 3 cm dilated.

First stage: contractions

The first stage of labor starts classically when the effaced (thinned) cervix is 3 cm dilated. There is variation in this point as some women may have active contractions prior to reaching this point, or they may reach this point without regular contractions. The onset of actual labor is defined when the cervix begins to progressively dilate. Rupture of the membranes, or a blood stained 'show' may or may not occur at around this stage.