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 3 stages of labour. The first stage accomplishes the shortening and then the dilation of the cervix. It is deemed to have started when the cervix is 3 cm dilated, and ends with full dilation. Contractions begin in the first stage of labour although they may be irregular and sporadic at first. The second stage, often called the pushing stage, starts when the cervix is fully dilated and ends with the expulsion of the fetus. In the third stage, the placenta detaches from the uterine wall and is expelled through the birth canal. Preceding the onset of labour is a period called the latent phase. This phase may last many days, and the contractions are an intensification of the Braxton Hicks contractions that start around 26 weeks gestation. Latent phase ends with the onset of active first stage labour.
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.
These movements are all due to the relationship of the bony head and shoulders of the fetus to the bony ring of the mother's pelvis and are independent of any changes in the maternal soft tissues.
The stages of normal human birth
The latent phase of labour causes confusion with many. Latent phase 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. Cervicaleffacement is the incorporation of the cervix to form the lower segment of the uterus. The muscular portion of the uterus is the upper segment, and is made of non-striated muscle. The lower segment of the uterus has no muscles and is comprised of the cervix itself, which becomes massively stretched and thinned out. This cervical effacement will usually be accomplished fully prior to the onset of labour. 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.
Mother and newborn with umbilical cord still attached after a water birth
First stage: contractions
The first stage of labor is an active stage and should not be confused with the latent phase of labor.
The first stage of labor starts classically when the effaced cervix is 3 cm dilated. There is variation in this point as some patients may present a little before this point with active contraction, or later, 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. 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.
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).
A newborn baby with umbilical cord ready to be clamped
The second stage of labour will vary to some extent, depending on how successfully the preceding tasks have been accomplished.