Normal labour

1st Stage

The first stage lasts from diagnosis of labour until the cervix is fully dilated (10cm).

This man is a big fan of labour.

Latent phase

The first part of this is the Latent phase which is usually quite slow and involves dilating to 3cm. It can take quite a few hours. Unsurprisingly, the physiology of the initiation of labour is poorly understood.

1. Both the foetus and the mother produce prostaglandins, which probably decrease cervical resistance and increasing oxytocin production.
2. Oxytocin aids stimulation of contractions. Involuntary contractions (Braxton-Hicks) are common during the third trimester.
3. Eventually, painful, regular contractions lead to effacement (gets shorter) and dilatation (gets wider) of the cervix. Dilatation is hopefully self-explanatory but effacement is the withdrawal of the normally tubular cervix.
4. This results in a 'show' - the cervix appears pink or white - and/or by rupture of the membranes which causes release of liquor (waters 'breaking').

Active phase

The part after this is referred to as the active phase which lasts from 3-10cm dilation.

• Nulliparous women progess at a rate of about 1cm/h during this phase, multiparous women at 2cm/h.

The entire first stage should last no longer than 12 hours.

2nd Stage

This lasts from full dilatation to delivery. Essentially, the foetus descends into the pelvic cavity, flexes its neck so as to enter the pelvic inlet and rotates its head in order to exit the pelvic outlet.

Passive stage

The first part of this is the passive stage which lasts from full dilatation to when the head reaches the pelvic floor. At this point the mother should feel the desire to push and rotation and flesxion have been completed.

Active stage

The active stage, as the next part is called, lasts until delivery. With the head pushing against the pelvic floor, mother should feel a desire to push although this may not be present if an epidural has been given. This stage should not last <1h - if it does, spontaneous delivery is unlikely. 40 min (nulliparous) and 20 min (multiparous) are the averages for duration of the active stage.

Delivery

When the head reaches the perineum it extends out of the pelvis. (Assuming this is in the usual OA position, this means the head moves towards the abdomen.) As it leaves it the restitutes i.e. it turns back 90^o to the same position as it was in when it entered the pelvic inlet.

Once the head is delivered, the shoulders must follow. The anterior shoulder comes first and comes out from under the symphisis pubis, aided by lateral body flexion posteriorly. The opposite occurs when the posterior shoulder is delivered and then the rest of the body follows.

3rd Stage

This is the time from foetal delivery to placental delivery. It generally lasts 15 mins and mothers usually lose about 500ml. The uterus contracts to compress the vessels formerly supplying the placenta.