Stages of labor

Main stages and their duration

Labor begins when regular contractile activity is established. The entire process is divided into three periods:

1. First stage (dilation stage) — shortening, effacement, and dilation of the cervix until full opening of the os.
2. Second stage (expulsion stage) — descent and birth of the fetus.
3. Third stage (placental stage) — separation and delivery of the placenta and membranes.

Due to modern active management and pain relief protocols, the total duration of labor has decreased. In the 19th and 20th centuries, 15–20 hours was considered normal; today, labor takes 12–16 hours in nulliparous women (with 10–12 hours spent on the first stage) and 8–10 hours in multiparous women (7–9 hours required for dilation).

Anatomical remodeling of the uterus

With the onset of labor, the uterus functionally and anatomically divides into specialized zones:

• Uterine body: the dense, actively contracting upper segment. Longitudinal and oblique muscle fibers predominate here, designed to dilate the cervix and expel the fetus.
• Lower segment: a thinned, passively stretching zone. It begins to form in the middle of pregnancy from the isthmus and cervix. Muscle fibers here are arranged circularly.
• Retraction ring: the border furrow between the actively working uterine body and the passive lower segment.

Triple descending gradient law

The physiological basis of uterine contractions was described by Caldeyro-Barcia and Poseiro (1960). A normal coordinated contraction follows the rule of three descending gradients:

1. Direction: the excitation wave starts at the top and travels downward.
2. Duration: muscle bundles in the upper segments contract longer than those in the lower segments.
3. Intensity: contraction force is maximal at the uterine fundus and gradually decreases downward.

The impulse is generated by the pacemaker — a specific group of cells with minimal $\beta$-reactivity. It appears exclusively during labor, most often in the uterine horn opposite the placental attachment site. The wave encompasses the entire uterus in 15–20 seconds, while the peak tension of all layers occurs absolutely synchronously. The outcome of this mechanism is the formation of the uterine fundus dominance.

Mechanisms of cervical dilation

Opening of the birth canal in the first stage is ensured by two interrelated mechanisms:

1. Muscular mechanism (retraction and distraction) The upper segments of the uterus contract 2–3 times stronger than the lower ones. During a contraction, the longitudinal fibers of the uterine body shorten and shift upward (retraction), without returning to their original position during intervals. The fundal wall progressively thickens. Simultaneously, these longitudinal muscles pull on the circular fibers of the cervix, forcing them to stretch (distraction). The principle of reciprocity applies: contraction of the longitudinal muscles of the body is invariably accompanied by relaxation of the transverse muscles of the lower segment.

2. Hydrodynamic mechanism (role of the amniotic sac) Contractions exert uniform pressure on the amniotic fluid, directing it toward the path of least resistance — the internal os. Under the pressure of the fluid, the lower pole of the membranes detaches from the uterine walls, forming the amniotic sac. It invaginates into the cervical canal and acts as a hydraulic wedge, gently expanding the cervix from within.

Class and exam questions

What else to study on this topic

• Tocometry: assessment of basal tone and contraction intensity
• Differences in the dynamics of the first stage in nulliparous and multiparous women
• The process of lower uterine segment formation from the isthmus
• Cellular mechanisms of pacemaker function
• Biomechanics of the second stage of labor (expulsion stage)

Other topics in the "Obstetrics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.