Normal labour

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Normal Labour

Definition

Labour is a clinical diagnosis characterized by regular, painful uterine contractions that increase in frequency and intensity, accompanied by progressive cervical effacement and dilation. Uterine contractions alone, without cervical change, do not meet the criteria for labour. - Creasy & Resnik's Maternal-Fetal Medicine, p. 122

Term and Timing

Mean duration of a human singleton pregnancy: 280 days (40 weeks) from the first day of the last normal menstrual period (LMP)
Term = 37 to 42 weeks of gestation
The fetoplacental unit - not the mother - is the primary controller of labour timing in humans

The Three P's: Powers, Passenger, and Passage

Labour is not a passive mechanical process. Three critical variables determine successful vaginal delivery:

Variable	Description
Powers	Forces generated by uterine musculature (contractions)
Passenger	Size, attitude, lie, and presentation of the fetus
Passage	Size, shape, and resistance of the bony pelvis and soft tissues of the pelvic floor

Because the fetal head and maternal pelvis are asymmetrical in shape, the fetus must execute a series of coordinated rotations - the seven cardinal movements of labour - to successfully negotiate the birth canal. - Creasy & Resnik's Maternal-Fetal Medicine, p. 137

Stages of Labour

First Stage - Cervical Effacement and Dilation

Begins with onset of labour and ends with full (10 cm) cervical dilation.

Latent phase: irregular contractions, slow cervical change
Active phase: regular contractions, rapid cervical dilation (typically from ~6 cm)

Second Stage - Expulsion of the Fetus

From full cervical dilation to delivery of the baby.

Third Stage - Delivery of the Placenta

From delivery of the baby to expulsion of the placenta and membranes.

Cervical Remodeling in Labour

Cervical changes begin early in pregnancy and are divided into four overlapping phases:

Figure: Cervical remodeling phases in human pregnancy. - Creasy & Resnik's Maternal-Fetal Medicine, Fig. 6.1

Phase	Timing	Key Features
Softening	Throughout most of pregnancy (from ~6 weeks)	Increased tissue compliance; tensile strength maintained; proliferation of epithelial cells and fibroblasts; mucus obstructs cervical canal
Ripening	Last 1-2 weeks of gestation (~36 weeks onward)	Maximal tissue compliance; loss of tissue integrity; increased vascularization, water content, hyaluronan synthesis; disorganization of collagen fibers
Dilation	Active labour (~40 weeks)	Mucus plug expelled; cervix expands to 10 cm diameter
Postpartum Repair	After delivery	Reduced tissue compliance; tissue integrity regained; increased proinflammatory cells

Cervical remodeling is mediated by:

Mechanical signals from the fetal presenting part pressing against the cervix
Endocrine/paracrine factors: progesterone, estrogen, oxytocin, and relaxin
Biochemical changes: increased hyaluronan synthesis, local progesterone metabolism
Traction from myometrial contractions and descent of the fetal head - Creasy & Resnik's Maternal-Fetal Medicine, p. 122-123

Hormonal Control of Labour

The hormonal cascade initiating labour involves the following key players:

Oxytocin

Produced by the maternal posterior pituitary and by the fetus
Acts on myometrial oxytocin receptors (OTR), whose expression increases markedly at term
Stimulates prostaglandin synthesis in the decidua
Upregulates gap junction formation between myocytes (connexin-43), facilitating coordinated contractions

Prostaglandins (PGE2 and PGF2α)

Produced in the decidua, fetal membranes, and myometrium
PGF2α acts as a uterotonic; PGE2 acts as both a cervical ripening agent and uterotonic
Stimulate increases in intracellular calcium in myocytes

Progesterone

Maintains uterine quiescence throughout pregnancy via progesterone receptors (PR-A and PR-B)
A functional progesterone withdrawal occurs at term in humans - not a fall in circulating levels, but a shift in PR isoform ratio (PR-A dominance over PR-B) and local metabolism of progesterone in the cervix and myometrium
This removes the inhibitory effect, allowing the onset of labour

Estrogen

Promotes expression of contraction-associated proteins (CAPs) including gap junctions (connexin-43), oxytocin receptors, and prostaglandin receptors
Counters the quiescent effects of progesterone

CRH (Corticotropin-Releasing Hormone)

Unlike in other mammals, CRH in humans is produced by the placenta (not just the hypothalamus)
Circulating CRH rises exponentially in late pregnancy
Stimulates fetal HPA axis, prostaglandin production, and promotes labour onset
Creasy & Resnik's Maternal-Fetal Medicine, p. 123-130

Myometrial Contraction Mechanism

Myometrial contraction follows the actin-myosin cross-bridge model:

Rise in intracellular calcium (from extracellular influx and sarcoplasmic reticulum release)
Calcium binds calmodulin (CaM)
Calcium-CaM complex activates myosin light-chain kinase (MLCK)
MLCK phosphorylates the 20-kDa myosin light chain
Phosphorylated myosin binds actin, forming cross-bridges
ATP hydrolysis causes the "power stroke" - myosin heads slide along actin filaments, shortening the cell
Repeated asynchronous cross-bridge cycles generate continuous smooth force

Gap junctions (connexin-43) between myocytes allow synchronized electrical coupling across the uterus, coordinating contractions. - Creasy & Resnik's Maternal-Fetal Medicine, p. 130-135

Inflammatory Mechanisms in Labour

Labour at term involves a sterile physiologic inflammatory process:

Recruitment of maternal immune cells (neutrophils, macrophages) into myometrium, cervix, and fetal membranes
Pro-inflammatory cytokines (IL-1β, IL-6, IL-8, TNF-α) upregulate prostaglandin synthesis and matrix metalloproteinases (MMPs)
MMPs degrade cervical extracellular matrix collagen, facilitating ripening
This "inflammatory activation" is a normal, necessary component of term parturition and mirrors infection-induced preterm labour - but at term, it is controlled and timely

Seven Cardinal Movements of Labour

These are the sequential rotations and movements the fetal head makes to navigate the maternal pelvis:

Engagement - fetal head enters the pelvic inlet (biparietal diameter below pelvic inlet)
Descent - progressive downward movement throughout labour
Flexion - head flexes so the chin meets the chest (presenting the smallest diameter)
Internal rotation - occiput rotates anteriorly from transverse to OA (occiput anterior) position
Extension - as head passes under the pubic symphysis, it extends (crowning and delivery of head)
Restitution/External rotation - head rotates back to align with fetal shoulders
Expulsion - delivery of anterior shoulder, then posterior shoulder, then rest of body

Labour Monitoring

The primary goal of labour monitoring is to achieve delivery of a healthy infant from a healthy mother with as little trauma as possible. Key elements:

Continuous electronic fetal monitoring (CTG/EFM)
Assessment of contraction frequency, duration, and strength
Cervical examination for dilation, effacement, station, and membrane status
Maternal vital signs and hydration
Fetal heart rate patterns
Ambulation and positions of comfort are physiologically beneficial
Family presence reduces anxiety, may shorten labour duration, and reduces need for medications
A 1:1 nurse-patient ratio during active labour is ideal - Creasy & Resnik's Maternal-Fetal Medicine, p. 969

Key Points Summary

Feature	Normal Value
Gestational age at term	37-42 weeks
Full cervical dilation	10 cm
Cardinal movements	7
Primary hormones	Oxytocin, PGE2, PGF2α, CRH, Estrogen
Myocyte calcium mediator	Calmodulin / MLCK
Gap junction protein	Connexin-43
Cervical remodeling phases	Softening → Ripening → Dilation → Postpartum Repair

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