Labour commences when the cervix starts to dilate and finishes with delivery of the baby and placenta.
The exact triggers that start the labour of pregnancy are unknown, but the hormones responsible come from the pituitary gland in the brain. There is some evidence that labour can be induced in the last week or two of pregnancy by an orgasm after sexual intercourse or by the constant stimulation of the nipples.
Labour proceeds through a number of stages that are identified by the movements of the baby’s head. The vagina (birth canal) is a curved cylinder and the baby’s head must move through various positions in order to pass through it. Labour is preceded by engagement, which is the fitting of the baby’s head into the pelvis. This is followed by flexion of the head, descent of the head, internal rotation, extension of the neck, external rotation and finally expulsion. These movements will differ if the baby’s head is in a different position to the normal one of coming out with the back of the head at the front of the mother.
The progress of labour is measured by the dilation of the cervix, which reaches a maximum of 10 cm. as the baby’s head passes through it, and by the stations of labour that measure the descent of the top of the baby’s head through the birth canal. A line between the spines on the ischial bone, which can be felt by a doctor when examining the vagina, is station zero. If the baby’s head is above this line the station is negative, and if below the station is positive. A station of plus two (+2) indicates that the top of the baby’s head is 2 cm. below the ischial.
Labour can also be measured by stages. In the first stage the cervix thins and starts to dilate. First stage ends with the full dilation of the cervix. It last on average 14 hours in a woman having her first baby and seven hours in a woman who has already had a baby. The first two-thirds of first stage labour is relatively quiet and comfortable in most women. In second stage the baby’s head descends further into the pelvis and lasts until the birth of the baby with forceful contractions of the uterus lasting from 60 to 90 seconds every two to five minutes. The patient develops an almost unbearable urge to push, which should be resisted until it can be timed with a contraction. The second stage lasts on average one hour in a first time mother and twenty minutes in a second time mother. The third stage of labour lasts from the birth of the baby to the expulsion of the placenta (afterbirth), which takes ten to fifteen minutes.
The baby moves down through the vagina and is expelled from the uterus by the force exerted by the powerful muscle contractions in the uterus, and is assisted by contractions of the muscles in the wall of the abdomen and in the diaphragm as the mother voluntarily pushes.
After the baby is delivered further contractions of the uterus over the next few minutes cause the placenta to separate from the wall of the uterus and be expelled.