Stages of Labor
Obstetricians have divided labor into 3 stages that delineate milestones in a continuous process.
First stage of labor
- Begins with regular uterine contractions and ends with complete cervical dilatation at 10 cm
- Divided into a latent phase and an active phase
- The latent phase begins with mild, irregular uterine contractions that soften and shorten the cervix
- Contractions become progressively more rhythmic and stronger
- The active phase usually begins at about 3-4 cm of cervical dilation and is characterized by rapid cervical dilation and descent of the presenting fetal part
Second stage of labor
- Begins with complete cervical dilatation and ends with the delivery of the fetus
- In nulliparous women, the second stage should be considered prolonged if it exceeds 3 hours if regional anesthesia is administered or 2 hours in the absence of regional anesthesia
- In multiparous women, the second stage should be considered prolonged if it exceeds 2 hours with regional anesthesia or 1 hour without it
Third stage of labor
- The period between the delivery of the fetus and the delivery of the placenta and fetal membranes
- Delivery of the placenta often takes less than 10 minutes, but the third stage may last as long as 30 minutes
- Expectant management involves spontaneous delivery of the placenta
- The third stage of labor is considered prolonged after 30 minutes, and active intervention is commonly considered
- Active management often involves prophylactic administration of oxytocin or other uterotonics (prostaglandins or ergot alkaloids), cord clamping/cutting, and controlled traction of the umbilical cord
Mechanism of Labor
The mechanisms of labor, also known as the cardinal movements, involve changes in the position of the fetus’s head during its passage in labor. These are described in relation to a vertex presentation. Although labor and delivery occurs in a continuous fashion, the cardinal movements are described as the following 7 discrete sequences:
- Engagement
- Descent
- Flexion
- Internal rotation
- Extension
- Restitution and external rotation
- Expulsion
