Biomechanics of cervical dilation
The primary objective of the first stage is to prepare the birth canal. The process of cervical dilation (cervix uteri) directly depends on parity.
In nulliparous women, changes occur in a strictly sequential manner and are divided into four stages:
- Stage A: the cervix is preserved, the cervix itself, the isthmus (isthmus uteri), and the internal os (ostium uteri internum) are clearly differentiated.
- Stage B: the process of effacement begins.
- Stage C: the cervix is fully effaced (thinned out).
- Stage D: complete opening of the external os occurs.
In multiparous women, the biomechanics are different: the stages of shortening (effacement) and dilation of the cervical canal occur simultaneously. After this, full dilation immediately ensues.
The outcome in both cases is dilation of the os by 10–12 cm, which signifies that the birth canal is ready for fetal expulsion.
Fetal descent and division of fluid
The descent of the presenting part into the pelvic cavity starts with the first contractions and proceeds concurrently with cervical dilation. By the time of full dilation, the fetal head is usually already in the pelvic cavity or engages with its largest diameter at the pelvic inlet.
In cephalic presentation, the fetus presses the lower uterine segment against the pelvic bones. The internal girdle of contact is formed — a ring of tight tissue adherence to the head. It performs an important function: it isolates the amniotic fluid, dividing it into forewaters (located below the girdle) and hindwaters (above it).
Rupture of amniotic membranes
Normally, the amniotic sac performs a hydraulic function, but by the time of full (10 cm) or nearly full ($\ge$ 8 cm) dilation, it loses its physiological significance and ruptures. This is timely rupture of membranes.
In practice, other scenarios are also encountered:
- Premature (antepartum) rupture: membranes rupture before the onset of labor activity.
- Early rupture: rupture occurs against the background of contractions, but before full dilation of the os.
- Late rupture: the membranes are too tough and do not rupture spontaneously at full dilation. Amniotomy (artificial rupture) is required; otherwise, the baby may be born enveloped in the membranes ("born in a caul").
- High leak: membranes rupture above the external os. If the fetal head is tightly applied to the pelvic inlet, it acts as a tampon at the rupture site, and upon vaginal examination, the physician may palpate an intact, tense amniotic sac.
Formation of the caput succedaneum
While the amniotic sac is intact, pressure on the presenting part is distributed evenly. As soon as the fluid escapes, the balance is disturbed: intrauterine pressure becomes significantly higher than external (atmospheric) pressure.
Due to this pressure difference, venous outflow is impeded in the soft tissues of the head located below the girdle of contact. As a result, a local edema — the caput succedaneum — forms in the area of the leading point.
Transition to the second stage (expulsion stage)
As soon as the cervix is fully dilated, the first stage ends and the second stage of labor (the expulsion stage) begins, lasting until the birth of the baby.
In this phase, uterine contractions are joined by pushing efforts (bearing down) — the coordinated contraction of the abdominal wall muscles, diaphragm (diaphragma), and pelvic floor. The diaphragm descends, and pressure in the abdominal cavity (cavitas abdominis) increases. The uterine body (corpus uteri) contracts actively, while the thinned lower segment passively stretches, forming a single birth passage. The net vector of expulsion forces is directed downward and anteriorly (along the pelvic axis), pushing the fetus outward.
The duration of the expulsion stage depends on parity and pain management:
- Without epidural anesthesia: 1–2 hours in nulliparous women and from 20–30 minutes to 1 hour in multiparous women.
- With epidural anesthesia: the process may be prolonged up to 3 hours in first-time mothers and up to 2 hours in multiparous women.
To remember the difference in cervical dilation: in nulliparous women, the process proceeds sequentially (step by step), whereas in multiparous women, it proceeds in parallel (effacement and dilation occur simultaneously).
Class and exam questions
What is the internal girdle of contact?
This is the zone of tight engagement of the fetal head by the walls of the lower uterine segment. It isolates the amniotic fluid, dividing it into forewater and hindwater portions.
Why is amniotomy necessary in delayed rupture of membranes?
If the fetal membranes are too tough, the sac does not rupture spontaneously even with complete cervical dilation. Without artificial puncture (amniotomy), the baby may be born directly inside the amniotic sac — "in a caul."
Why does a caput succedaneum form on the fetal head?
After the escape of fluid, intrauterine pressure becomes higher than atmospheric pressure. Because of this, venous outflow is disrupted in the soft tissues of the presenting part located below the girdle of contact, resulting in edema.
What else to study on this topic
- Anatomical landmarks of the birth canal: the isthmus, internal os, and external os.
- Topography and vectors of expulsion forces in the second stage of labor.
- The effect of epidural anesthesia on the duration of the expulsion stage.
- Diagnosis of a high leak of amniotic membranes during vaginal examination.
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