Labor (Parturition): Stages, Phases and Cardinal Movements in Birth

Labor, or parturition, is the sequence by which a fetus is delivered through the birth canal. The video explains that labor begins with uterine contractions that cause the cervix to efface and dilate, enabling vaginal delivery, and ends with the delivery of the placenta. True labor contractions increase in frequency, duration, and intensity, while Braxton Hicks contractions are irregular and milder. The entire process is divided into three stages, with the first stage itself split into latent and active phases. The second stage is the pushing phase, during which the baby navigates the pelvis guided by the three Ps: power, passenger, and passage. After birth, the third stage involves placental delivery, and the fourth stage refers to postpartum stabilization. A recap covers the three stages.

Overview of labor and its stages

Labor, or parturition, occurs at term (approximately 37 to 42 weeks of gestation) and begins with uterine contractions that facilitate cervical changes to allow vaginal delivery, ending with the delivery of the placenta. The video distinguishes true labor from Braxton Hicks contractions and notes that real contractions progressively increase in frequency, duration, and intensity. The entire process is divided into three stages, with the first stage subdivided into latent and active phases. Typical durations are given for first pregnancies and subsequent pregnancies, though individual variation is common.

Stage 1: Latent and Active Phases

The first stage is the longest and involves the cervix gradually opening and thinning. The latent phase lasts up to about 20 hours, ending when the cervix dilates to around 6 centimeters. In this phase contractions are irregular, occurring every 5 to 30 minutes and lasting about 30 seconds, with dilation progressing from 0 to about 3 centimeters and effacement from 0% to roughly 30%. The subsequent active phase features more regular contractions, typically every 3 to 5 minutes and lasting 1 minute or more, driving cervical dilation from 3 to 6 centimeters and effacement toward around 80%.

Stage 1 transitions into Stage 2

As the cervix reaches full dilation (10 centimeters) and effacement is complete, labor enters the second stage, the pushing stage, where the fetus advances through the birth canal aided by strong, rhythmic uterine contractions and the body’s mechanisms for progression through the pelvis.

Stage 2: Pushing and Cardinal Movements

Stage 2 centers on the baby and particularly the head negotiating the maternal pelvis. The video emphasizes the three essential factors for labor progress: power, passenger, and passage. It also explains fetal attitude and presentations, noting that a fully flexed head and a longitudinal lie optimize passage, while breech or shoulder presentations can complicate descent. The cardinal movements describe the stepwise progression of birth: descent, flexion, internal rotation, extension, restitution, and expulsion. Each movement aligns the fetal head and shoulders with the pelvic inlet and outlet, culminating in the head’s emergence and the rest of the body following the head through expulsion.

• Descent – downward movement into the pelvic inlet, with fetal station described relative to the ischial spines (engagement occurs around -5 to 0).
• Flexion – the chin tucks to the chest as resistance from the pelvic floor is met.
• Internal rotation – shoulders rotate to align with the widest part of the pelvic inlet (about 45 degrees).
• Extension – after the head passes under the symphysis pubis, the head extends from flexion to allow passage through the birth canal.
• Restitution – the head externally rotates to align with the shoulders for passage through the pelvic outlet.
• Expulsion – the anterior shoulder slips under the symphysis pubis, followed by the posterior shoulder and the rest of the body.

Stage 3: Placenta Delivery

Following the birth, the uterus contracts again to detach the placenta from the uterine wall. The placenta is then delivered and any remnants are removed to prevent complications. The video notes a fourth stage, which encompasses the early postpartum period and adaptations to blood loss, as the uterus begins involution and returns toward its pre-pregnant state.

Key fetal positional factors and recap

Fetal size, attitude, lie, and presentation influence how easily the fetus can pass through the pelvis. The video explains that a fully flexed head (vertex presentation) and a longitudinal lie are optimal for delivery, while larger fetuses or malpositions may complicate labor. The cardinal movements illustrate the coordinated steps the fetus makes to reach birth. In summary, labor consists of three stages, with the first stage containing latent and active phases, the second stage involving pushing, and the third stage delivering the placenta, followed by a brief fourth stage of postpartum adaptation.