Awesome Anatomy

The stages of normal human birth

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.

Uterine muscles form opposing spirals from the top of the upper segment of the uterus to its junction with the lower segment. During effacement, the cervix becomes incorporated into the lower segment of the uterus. During a contraction, these muscles contract causing shortening of the upper segment and drawing upwards of the lower segment, in a gradual expulsive motion. This draws the cervix up over the baby's head. Full dilatation is reached when the cervix is the size of the baby's head; at around 10 cm dilation for a term baby.

The duration of labour varies widely, but active phase averages some 8 hours for women giving birth to their first child ("primiparae") and 4 hours for women who have already given birth ("multiparae").

Second stage: delivery

This stage begins when the cervix is fully dilated, and ends when the baby is finally delivered. At the beginning of the normal second stage, the head is fully engaged in the pelvis; the widest diameter of the head has successfully passed through the pelvic brim. Ideally it has successfully also passed below the interspinous diameter. This is the narrowest part of the pelvis. If these have been accomplished, all that will remain is for the fetal head to pass below the pubic arch and out through the introitus. This is assisted by the additional maternal efforts of "bearing down". The fetal head is seen to 'crown' as the labia part. At this point the woman may feel a burning or stinging sensation. This is also known as the "ring of fire."

Delivery of the fetal head signals the successful completion of the fourth mechanism of labour (delivery by extension), and is followed by the fifth and sixth mechanisms (restitution and external rotation).

The second stage of labour will vary to some extent, depending on how successfully the preceding tasks have been accomplished.

Third stage: placenta

In this stage, the uterus expels the placenta (afterbirth). The placenta is usually delivered within 15-30 minutes of the baby being born. Maternal blood loss is limited by contraction of the uterus following delivery of the placenta. Normal blood loss is less than 600 mL.

Breastfeeding during and after the third stage, the placenta is visible in the bowl to the right.

The third stage can be managed either expectantly or actively. Expectant management (also known as physiological management) allows the placenta to be expelled without medical assistance. Breastfeeding soon after birth and massaging of the top of the uterus (the fundus) causes uterine contractions that encourage delivery of the placenta. Active management utilizes oxytocic agents and controlled cord traction. The oxytocic agents augment uterine muscular contraction and the cord traction assists with rapid delivery of the placenta.

A Cochrane database study suggests that blood loss and the risk of postpartum bleeding will be reduced in women offered active management of the third stage of labour. However, the use of ergometrine for active management was associated with nausea or vomiting and hypertension, and controlled cord traction requires the immediate clamping of the umbilical cord.