• ChatGPT - available at chat.openai.com or on iOS/Android
• Claude - available at claude.ai or on iOS/Android
• Gemini - available at gemini.google.com or on iOS/Android
• Copilot - available at copilot.microsoft.com

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🏥 LABOR - Deep Concept Note for Medical Students

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1. DEFINITION

• Normally commences at 40 ± 2 weeks after the last menstrual period
• Over 95% of births are cephalic (head-first) presentations
• Buttocks-first = breech presentation

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2. PHYSIOLOGY - INITIATION OF LABOR

A. Hormonal Factors

Key concept: Circulating oxytocin levels do NOT always rise at the onset of labor - but the NUMBER of myometrial oxytocin receptors rapidly increases, making the uterus more sensitive.

B. Mechanical Factors

• Uterine stretch - As the fetus grows, stretching smooth muscle increases contractility. Twins are born ~19 days earlier than singletons, proving stretch plays a role
• Cervical stretch/irritation - Stretching the cervix triggers reflex oxytocin release (the Ferguson reflex) and myogenic signal transmission to the uterine body
• Rupture of membranes causes direct head pressure on the cervix, amplifying this effect

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3. PRODROMAL (PRE-LABOR) EVENTS

1. Lightening - The fetal presenting part settles into the pelvis; the fundal height drops; the woman notices reduced upper abdominal pressure but increased pelvic pressure
2. Braxton Hicks contractions - Sporadic, irregular contractions that are characteristically irregular in frequency, duration, and intensity ("false labor"). They help with cervical effacement
3. Cervical effacement - Cervix softens, thins, and moves anteriorly
4. Bloody show - Cervical mucous plug (often blood-tinged) is expelled. Most women enter true labor within 3 days of bloody show

Bloody show vs. pathological bleeding: Bloody show is mucoid and lightly blood-tinged. Heavy bleeding (like the start of a menstrual period) is pathological - consider placenta previa or abruptio placentae.

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4. TRUE vs. FALSE LABOR

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5. STAGES OF LABOR

Overview Table

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FIRST STAGE - Cervical Dilation

Latent Phase

• Onset of regular contractions to ~4 cm dilation
• Slow cervical dilation but major effacement occurs
• Duration: up to 20 hours in nulliparas, 14 hours in multiparas
• Contractions: irregular at first, becoming more regular
• Best managed at home with clear liquids

Active Phase

• From ~4 cm to full dilation (10 cm)
• Expected dilation rate:
  • Nulliparas: ≥1.2 cm/hr
  • Multiparas: ≥1.5 cm/hr
• Contractions: every 3-5 minutes, lasting ~1 minute, intensity ~40 mmHg
• Progress depends on: uterine contraction strength, fetal head size/position, and pelvic dimensions (the 3 Ps: Powers, Passenger, Passage)

• Acceleration phase
• Phase of maximum slope
• Deceleration phase

First Stage Duration:

• Nulliparas: 8-12 hours
• Multiparas: 5-8 hours

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LABOR PROGRESS CURVES

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SECOND STAGE - Expulsion of the Fetus

• From complete dilation to delivery of the baby
• Contractions: every 1.5-2 minutes, lasting 1-1.5 minutes
• The patient actively bears down (pushes), greatly augmenting intrauterine pressure
• Duration:
  • Multiparas: ~20 minutes (up to 120 min)
  • Nulliparas: ~50 minutes (up to 120 min without epidural, 3 hours with epidural)

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THIRD STAGE - Placental Delivery

• From delivery of the baby to delivery of the placenta
• Duration: 15-30 minutes (up to 45 min)
• Uterus continues contracting, shearing the placenta from its implantation site
• Signs of placental separation: gush of blood, cord lengthening, uterus rises and becomes globular
• Average blood loss: ~350 mL

How bleeding is controlled after placental delivery:

1. Uterine smooth muscle fibers are arranged in figure-of-eight patterns around blood vessels
2. Uterine contraction mechanically constricts the vessels (living ligature mechanism)
3. Vasospasm from local prostaglandins and other mediators

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6. MECHANICS OF PARTURITION - UTERINE CONTRACTIONS

• Contractions begin at the uterine fundus and spread downward
• Strongest at the top and body; weakest at the lower uterine segment near the cervix
• This polarity means each contraction pushes the fetus toward and through the cervix
• Early labor: contractions every 30 minutes
• Late labor: contractions every 1-3 minutes
• Combined uterine + abdominal force: approximately 25 pounds per contraction

Warning: Strong uterine contractions impede blood flow through the placenta. Continuous contractions (e.g., from oxytocin overuse causing uterine spasm) can cause fetal death from hypoxia. This is why intermittent contractions are physiologically protective.

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7. CARDINAL MOVEMENTS OF LABOR (Vertex/Cephalic Presentation)

The 7 Cardinal Movements:

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8. FETAL MONITORING DURING LABOR

• Tocodynamometer (external) or intrauterine pressure catheter (IUPC, internal) measures uterine contractions
• Fetal station = level of descent of presenting part relative to the ischial spines in cm (e.g., -3 to +3, or -5 to +5)
• Electronic Fetal Monitoring (EFM) looks at baseline FHR, variability, accelerations, and decelerations

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9. DYSTOCIA - ABNORMAL LABOR (High-Yield)

• Powers - inadequate uterine contractions
• Passenger - fetal malpresentation, macrosomia, deflexion
• Passage - cephalopelvic disproportion (CPD), pelvic abnormality

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10. APGAR SCORE (assessed at 1 and 5 minutes)

• Score 7-10: Normal
• Score 4-6: Moderate depression - stimulate, supplemental O2
• Score 0-3: Severe - resuscitation required

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11. QUICK CLINICAL PEARLS

• Rupture of membranes before onset of labor = PROM (Premature Rupture of Membranes). If before 37 weeks = PPROM
• Induction of labor uses cervical ripening agents (PGE2, misoprostol) and oxytocin (Pitocin)
• Epidural analgesia given in latent phase may slow progress; does not affect active phase
• Episiotomy is NOT routine - only for fetal distress, shoulder dystocia, or operative delivery
• Crowning = fetal head visible and no longer retracts between contractions
• The Ferguson reflex = cervical stretch → reflex oxytocin release → stronger contractions (positive feedback loop that drives labor to completion)

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