Summary
Highlights
Labor, also known as parturition, is the process of delivering a baby. It begins with uterine contractions that cause cervical changes, allowing the fetus to be delivered vaginally, and concludes with the delivery of the placenta. Labor typically starts between 37 and 42 weeks of gestation.
Before labor begins, a woman might experience a 'bloody show' or the rupture of the amniotic sac ('water breaking'). These can trigger the onset of true labor contractions, which are distinguished from milder Braxton Hicks contractions by their increasing frequency, duration, and intensity. True labor contractions cause the cervix to thin (efface) and open (dilate).
The first stage of labor is divided into two phases. The early or latent phase (up to 20 hours or 6 cm dilation) involves irregular contractions that gradually lead to the cervix dilating from 0 to 6 cm and effacing up to 80%. The active phase sees more intense contractions, dilating the cervix from 6 to 10 cm and effacing it to 100%. The amniotic sac often ruptures during this phase if it hasn't already.
To navigate the birth canal, the fetus undergoes several cardinal movements: descent into the pelvic inlet (measured by fetal station), flexion of the head against the chest, internal rotation of the shoulders, extension of the head as it passes under the pubic bone, restitution (external rotation of the head), and finally, expulsion of the rest of the body.
The third stage of labor occurs after the baby is delivered and involves the expulsion of the placenta. The uterus contracts, separating the placenta from its wall. It's crucial to ensure no placental remnants are left behind. Sometimes, a 'fourth stage' is recognized, referring to the hours after delivery when the mother's body adapts to blood loss and the uterus begins to return to its pre-pregnancy state.
The second stage, or pushing stage, begins when the cervix is fully dilated. This stage focuses on the baby's passage through the maternal pelvis, which depends on 'power' (uterine contractions), 'passenger' (the fetus), and 'passage' (the bony pelvis). The unique unfused skull of human babies aids in this process.
Several factors influence the ease of fetal passage, including fetal size (primarily head size), fetal attitude (the degree of flexion, with a fully flexed position being ideal), fetal lie (how the fetus is positioned in the uterus, with longitudinal being optimal), and fetal presentation (which part of the fetus presents first, with cephalic/head-first being most common and optimal).