Labor is the process of regular uterine contractions that cause progressive effacement (thinning) and dilation of the cervix and descent of the fetus, ending with birth of the infant and placenta.

Onset of labor — contributing mechanisms (the exact trigger in humans is not fully known)

• Functional progesterone withdrawal (progesterone maintains uterine quiescence)
• Rising estrogen effect → more oxytocin receptors, gap junctions, and prostaglandin production
• Prostaglandins (PGE2, PGF2α) → cervical ripening and contractions
• Oxytocin from the posterior pituitary → strong contractions; the Ferguson reflex (pressure of the presenting part on the cervix and vagina) increases oxytocin release
• Fetal cortisol, uterine stretch, and placental corticotropin-releasing hormone

Hormones and their roles

What uterine contractions do

• Efface and dilate the cervix
• Push the fetus downward (descent)
• After birth, compress uterine vessels at the placental site to control bleeding ("living ligatures")
• During a contraction, blood flow to the placenta decreases temporarily; it recovers during the rest period. Contractions therefore do not increase fetal oxygen supply, which is why an adequate resting interval matters

Maternal and fetal responses to labor

• Cardiac output rises (about 10–15% in the first stage, up to 30–50% in the second stage); BP rises during contractions, so measure it between contractions
• WBC may rise to about 25,000–30,000/mm³ without infection — interpret with other signs
• Rapid breathing can cause respiratory alkalosis (tingling, dizziness)
• Gastric emptying slows → nausea, vomiting, and aspiration risk with general anesthesia
• Mild temperature rise from exertion and dehydration; fluid loss through sweating and breathing
• Fetus: transient heart rate changes and reduced oxygen during contractions; head compression; lung fluid is squeezed out during vaginal birth

Premonitory signs (days to weeks before labor)

• Lightening — fetus drops into the pelvis; easier breathing, more urinary frequency (earlier in first pregnancies)
• Braxton Hicks contractions increase
• Bloody show — blood-tinged mucus plug passes as the cervix ripens
• Rupture of membranes may occur before labor
• Burst of energy ("nesting"), small weight loss, backache, loose stools

True vs. false labor

The five Ps of labor

1. Passenger (fetus and placenta)
   • Fetal skull: sutures and fontanels allow molding; the anterior fontanel is diamond-shaped, the posterior triangular
   • Lie: longitudinal or transverse; attitude: flexion is normal
   • Presentation: cephalic (most common, vertex), breech, shoulder
   • Position: relation of the presenting part (occiput in vertex) to the maternal pelvis — occiput anterior (LOA/ROA) is most favorable; occiput posterior causes back labor and slower progress
   • Station: level of the presenting part relative to the ischial spines (0 station); measured from about −5 to +5 cm. Engagement = the widest diameter of the presenting part has passed the pelvic inlet (usually station 0)
2. Passage (birth canal)
   • Gynecoid pelvis is the most favorable shape; android (heart-shaped) and platypelloid (flat) are less favorable
   • The diagonal conjugate (≥ 11.5 cm) is measured clinically; the obstetric conjugate is estimated about 1.5–2 cm less
3. Powers
   • Involuntary uterine contractions (primary power) and voluntary bearing down (secondary power — used only after full dilation)
   • Assess frequency (start to start), duration (start to end), intensity (mild, moderate, strong by palpation or IUPC), and resting tone
4. Position of the mother — upright and lateral positions use gravity and improve uterine blood flow; the supine position compresses the vena cava
5. Psychological response — fear and anxiety increase catecholamines, which can slow labor and increase pain

• Vaginal (cervical) examination: dilation (0–10 cm), effacement (0–100%), station, presentation and position, membrane status. In first pregnancies the cervix usually effaces before it dilates; in later pregnancies both happen together
• Leopold maneuvers: fetal lie, presentation, and position; locate the fetal back for FHR monitoring
• Contraction monitoring: palpation (fingertips on the fundus) and tocodynamometer or IUPC
• Testing for rupture of membranes: sterile speculum exam for pooling, nitrazine (alkaline amniotic fluid turns paper blue; false positives with blood, semen, or bacterial vaginosis), ferning under the microscope, or commercial protein assays
• Ultrasound to confirm presentation when uncertain

Stages of labor (current definitions from ACOG and consensus guidance)

• Active labor now begins at 6 cm (older teaching used 3–4 cm); progress slower than the old Friedman curve is often normal
• Latent-phase clients who are coping and have reassuring fetal status are commonly encouraged to stay home or be ambulatory
• WHO places the start of the active first stage at 5 cm (ACOG uses 6 cm). WHO notes that this stage usually lasts no more than 12 hours in a first labour and no more than 10 hours in later labours, and that labour may not speed up naturally before 5 cm

Monitoring labour progress — WHO tools

Current tool: WHO Labour Care Guide (LCG, 2020)

• Started only once the woman is in the active first stage (cervix 5 cm or more), whatever her parity or membrane status; it is not used in the latent phase, although latent-phase care and monitoring still continue
• Seven sections: (1) identifying information and labour characteristics at admission, (2) supportive care, (3) care of the baby, (4) care of the woman, (5) labour progress, (6) medication, (7) shared decision-making
• Each observation has a reference value; an "Alert" column flags a finding outside it so that the team looks for the cause and records a shared plan with the woman
• Time limits per centimetre replace the old straight line. An alert is triggered when the cervix stays at the same dilation for: 5 cm — 6 hours; 6 cm — 5 hours; 7 cm — 3 hours; 8 cm — 2.5 hours; 9 cm — 2 hours
• Second stage alert: no birth after 3 hours of active second stage in a nullipara or 2 hours in a multipara
• Cervical dilation is normally assessed every 4 hours in the active first stage unless there is a reason to examine sooner

Older tool still met in practice: modified WHO partograph

• WHO 2018 recommends against using the 1 cm per hour rate (the partograph alert line) to identify women at risk of poor birth outcomes; many healthy women dilate more slowly and still give birth normally
• A dilation rate slower than 1 cm per hour on its own is not a routine reason for obstetric intervention, and oxytocin augmentation or cesarean birth to speed labour before 5 cm is not recommended when mother and fetus are well
• Exam tip: an alert line or action line means the question is about the older partograph; current WHO practice uses the LCG's 5 cm start and per-centimetre time limits

Cardinal movements (mechanism of labor) in a vertex presentation

1. Engagement
2. Descent (continues throughout)
3. Flexion — chin to chest, smallest diameter presents
4. Internal rotation — occiput turns anterior
5. Extension — head is born under the symphysis
6. Restitution and external rotation — head realigns with the shoulders
7. Expulsion — anterior then posterior shoulder, then the body

Placental separation (third stage)

• Signs: a gush of blood, lengthening of the cord, the uterus becoming firm and globular and rising in the abdomen, and the cord no longer retracting when the lower uterus is pressed
• Schultze mechanism: shiny fetal side first; Duncan mechanism: rough maternal side first (more risk of retained fragments)

Listed in priority order.

1. Fetal and maternal safety
   • Assess FHR and contractions; check FHR immediately after membranes rupture
   • Monitor maternal vital signs, fluid intake, and bladder (a full bladder slows descent)
2. Assess labor progress
   • Time contractions over at least 10 minutes (frequency and duration) and palpate intensity
   • Limit vaginal exams to those needed, especially after rupture of membranes (infection risk); none if unexplained bleeding (possible previa)
3. Promote progress
   • Encourage walking, upright and changing positions, and voiding every 2 hours
   • Avoid prolonged supine position
4. Second stage
   • Encourage pushing with the urge to push after full dilation; pushing before full dilation can cause cervical edema and exhaustion
   • Normal: the head may appear to "go back in" between contractions — reassure and encourage rest between contractions
5. Fourth stage
   • Frequent assessment of fundus, lochia, vital signs, and bladder to detect hemorrhage early
6. Psychological support — explain progress, encourage the support person

• Go to the birth facility (or call) for regular contractions about every 5 minutes for an hour in a first pregnancy (sooner for later pregnancies or long travel distance), rupture of membranes, bright red bleeding, decreased fetal movement, or constant severe pain
• Latent labor: rest, hydrate, shower, walk, and use breathing techniques at home if no warning signs
• Report fluid leakage and note its color, odor, amount, and time (COAT)
• Push only when the nurse or provider confirms full dilation and with the natural urge

• Oxytocin stimulates contractions; progesterone inhibits them; prolactin is for milk production
• Contractions reduce placental blood flow temporarily; the resting interval restores it
• True labor = progressive cervical change
• Active phase begins at 6 cm (ACOG); WHO uses 5 cm
• WHO Labour Care Guide (current): starts at 5 cm, time limits per centimetre (5 cm 6 h down to 9 cm 2 h); the old partograph alert line (1 cm per hour from 4 cm, action line 4 hours to the right) is not recommended for identifying women at risk
• Station 0 = presenting part at the ischial spines; engagement
• Cardinal movements: engagement, descent, flexion, internal rotation, extension, external rotation, expulsion
• Frequency is measured start to start; duration start to end
• Placental separation: gush of blood, cord lengthens, uterus rises and becomes globular
• Fourth stage: fundus and bleeding are the priority
• Push with the urge after complete dilation