The initiation of labour can happen due to oxytocin stimulation, progesterone withdrawal, fetal triggers (adrenal, hypothalamic-pituitary axis, prostaglandins, or the withdrawal of labour inhibitors (progesterone, relaxin, prostacyclin, parathyroid hormone, nitric acid).
The uterine activity of labour is measured by assessing amplitude, frequency and duration of contractions. False contractions can occur, such as Braxton Hicks contractions, which are painless tightenings with no cervical changes. Braxton Hicks contractions can start at around 28 weeks of gestation. Ultimately, the baby is imminent when engagement — the fixation of the fetal head in the pelvis — occurs. In a primipara, engagement can occur approximately 2 weeks prior to labour, while a multipara can engage immediately before labour and birth.
The first stage is defined as full dilatation, and occurs in two phases. In the latent phase, the cervix is dialated 0-4cm, whereas in the active phase, it is dialated 4-10cm. During the first stage of labour, intermittent monitoring of the fetal heart rate (FHR) is conducted (every fifteen minutes during the first stage, then every five minutes in the second stage). In cases of high risk, or during a vaginal birth after caesarian (VBAC), continuous monitoring is used. Friedman curves predict that the first stage should last 6 - 18 hours in a primipara and 2 - 10 hours in a multipara.
The second stage is the delivery of fetus. Acording to Friedman curves, this should take 30 minutes to 3 hours in a primipara, or 5 to 30 minutes in a multipara.
In normal labour, the cardinal movements of the baby are:
- Internal rotation
- External rotation
The best outcomes of the second stage of labour happen for women with gynecoid or android-shaped pelvises, as the cardinal movements may be inhibited by a narrow or flat pelvis. However, the trial of labour is the only true test of pelvic adequacy.
The third stage of labour is the expulsion of the placenta after the delivery of the fetus, which usually lasts for 2-30 minutes. The WHO and SOGC both advocate active management for this phase, which represents a change in the conventional dogma, which was to do nothing.
Uterotonic agents such as Syntocinon or Ergot are used, and gentle traction is applied to the cord, along with a fundal massage. Signs that the placenta has separated include the new onset of bright bleeding, lenghthening of the cord, and "balling up" of the fundus.
In general, complications of birth can be the result of the passage, passenger or power (poor contraction pattern or poor pushing).
Another potential complication is postpartum hemorrhage.