Labor and Delivery

Mechanistic triggers of parturition

• Endocrine (functional progesterone withdrawal): fetal HPA-driven cortisol shifts the progesterone:estrogen ratio, upregulating myometrial oxytocin receptors and connexin-43 — the final common pathway for term labor.
• Inflammatory/infectious: decidual and chorioamniotic infection (intraamniotic infection, bacterial vaginosis, pyelonephritis, periodontal disease) releases IL-1, IL-6, IL-8 and TNF-α, driving COX-2/prostaglandin synthesis and premature cervical remodeling — the classic mechanism of preterm labor.
• Mechanical/uterine overdistension: multiple gestation, polyhydramnios, and fibroids stretch myometrium, increasing gap junctions and contractility.
• Vascular/decidual hemorrhage: abruption generates thrombin, a potent uterotonic (bleeding plus contractions).
• Iatrogenic: prostaglandin ripening agents, oxytocin, amniotomy, and membrane sweeping.

Non-modifiable risk factors for abnormal labor (dystocia)

• Nulliparity: the strongest non-modifiable risk factor for labor dystocia; advanced maternal age also prolongs labor.
• Fetal factors: macrosomia, occiput posterior or transverse arrest, brow/face presentation, breech, hydrocephalus, anomalies.
• Maternal pelvis: android/platypelloid pelvis, prior pelvic fracture, short stature.
• Prior preterm birth or short cervix for preterm labor; prior cesarean for rupture during trial of labor.

Modifiable/preventable risk factors

• Obesity and excessive gestational weight gain: prolong the first stage and raise cesarean rates.
• Pregestational and gestational diabetes: macrosomia with disproportionate shoulder growth → shoulder dystocia.
• Induction with an unfavorable cervix: ACOG defines a Bishop score of 6 or less (dilation, effacement, station, consistency, position) as unfavorable, predicting failed induction unless ripening is used first.
• Smoking, substance use, short interpregnancy interval, untreated genitourinary infection: preterm labor risks amenable to intervention.
• Neuraxial analgesia: does not increase the risk of cesarean delivery (ACOG); it should not be withheld or delayed for that reason.

Initiation of Labor (The "Trigger" for Parturition)

• Fetal signals: Fetal hypothalamic-pituitary-adrenal (HPA) axis activation increases fetal cortisol, which stimulates placental conversion of pregnenolone to estrogen and decreases progesterone production, shifting the progesterone:estrogen ratio—the fundamental endocrine change initiating labor
• Placental and myometrial changes: Placental corticotropin-releasing hormone (CRH) increases exponentially in third trimester, amplifying fetal cortisol production and stimulating prostaglandin synthesis from placental and decidual tissues
• Progesterone withdrawal effect: Functional progesterone withdrawal occurs (due to decreased receptor responsiveness and altered metabolism) rather than absolute serum level decline, allowing increased uterine contractility and reduced myometrial quiescence

Myometrial Activation and Excitability

• Oxytocin receptor upregulation: Estrogen surge increases oxytocin receptor expression (OTR) in myometrium 100-200 fold, making uterine muscle responsive to endogenous and exogenous oxytocin
• Gap junction formation: Connexin-43 protein increases, forming intercellular gap junctions that allow electrical and chemical communication between myometrial smooth muscle cells, enabling coordinated contractions
• Prostaglandin signaling: Increased cyclooxygenase-2 (COX-2) activity in decidua and myometrium produces prostaglandins (PGE2 and PGF2α) that stimulate myometrial contractions and cervical ripening through membrane receptors

Cervical Ripening and Dilation

• Cervical remodeling: Decreasing collagen cross-linking and increased hyaluronic acid and water content reduce cervical stiffness; matrix metalloproteinases (MMPs) degrade collagen Type I and III
• Inflammation-mediated changes: Prostaglandins and cytokines (IL-6, IL-8, TNF-α) recruit inflammatory cells to cervix, further degrading extracellular matrix and promoting cervical softening
• Progressive dilation: Uterine contractions apply mechanical force to cervix during labor; cervix effaces (thins) first, then dilates from 0 to 10 cm (full dilation) over hours to days

Three Stages of Labor

• Stage 1 (Latent, Active, Transition phases): Begins with onset of labor (regular contractions + cervical change) and ends at 10 cm dilation; latent phase characterized by slow cervical dilation and mild contractions; active phase shows accelerated dilation (~1 cm/hour in nulliparas, ~1.5 cm/hour in multiparas); transition phase (8-10 cm) involves intense contractions
• Stage 2 (Expulsive stage): From complete cervical dilation to delivery of fetus; involves maternal pushing efforts combined with involuntary uterine contractions; fetus descends through pelvis via mechanism of labor (engagement, descent, flexion, internal rotation, extension, external rotation, expulsion)
• Stage 3 (Placental stage): From delivery of fetus to delivery of placenta; typically lasts 5-30 minutes; placental separation occurs via placental site bleeding and uterine contractions; placenta delivered either by active management (oxytocin, controlled cord traction, uterine massage) or expectant management

Recognition of Labor Onset

• Regular uterine contractions occurring every 3-5 minutes with increasing intensity and duration (>30 seconds); must be accompanied by cervical change to distinguish from Braxton-Hicks contractions (painless, irregular, stop with movement or hydration)
• Cervical dilation and effacement documented by vaginal examination; transition from unfavorable cervix (firm, posterior, closed) to favorable cervix (soft, anterior, patent) occurs with cervical ripening
• Vaginal show: Blood-tinged mucoid discharge from cervical plug loss; may be accompanied by "bloody show" as cervical capillaries rupture with dilation

Progression Through Labor Stages

• Early latent phase: Mild to moderate contractions (every 5-30 minutes), patient often ambulatory and able to speak through contractions; cervical dilation 0-3 cm; can last many hours
• Active phase: Contractions every 3-5 minutes, moderate to strong intensity, patient focuses intensely on coping; cervical dilation 3-7 cm; nulliparous patients average 1.2 cm/hour dilation, multiparous average 1.5 cm/hour (Friedman curve)
• Transition phase: Very intense contractions every 2-3 minutes; cervical dilation 8-10 cm; patient may experience nausea, vomiting, shaking, and emotional vulnerability; often brief (30 min-2 hours)
• Second stage onset: Urge to push, sensation of pressure in rectum/perineum; may be passive descent initially (before active pushing) or active pushing from onset; duration typically <2 hours in nulliparas, <1 hour in multiparas

Fetal Descent and Station

• Engagement: Biparietal diameter of fetal head enters pelvic inlet (at -2 station in nulliparas; may occur later in multiparas); felt as "dropping" or "lightening" by mother
• Progressive descent: Measured in "stations" from -5 (above pelvic inlet) to +5 (at introitus); 0 station is level of ischial spines; descent typically accelerates in active labor
• Mechanism of labor: Sequence of position changes (flexion → internal rotation → extension → external rotation → expulsion) allows passage of largest fetal diameter through smallest pelvic diameter

Third Stage Findings

• Signs of placental separation: Cord lengthens, gush of blood, uterus becomes firm and rises in abdomen, often preceded by small contraction; typically occurs 5-30 minutes after fetal delivery
• Placental delivery: Spontaneous expulsion or assisted delivery with controlled cord traction; complete evacuation confirmed by inspection of placenta for completeness and maternal-fetal surface integrity
• Postpartum hemostasis: Uterine contractions compress spiral arteries at placental site; uterine involution begins immediately postpartum

Clinical Diagnosis of Labor Onset

• Essential criterion: Regular uterine contractions (documented on partograph or tocometry) AND objective cervical change (dilation and/or effacement) on serial vaginal examinations; contractions alone without cervical change ≠ labor (distinguish from prodromal labor or Braxton-Hicks)
• Timing distinction: True labor contractions occur at regular intervals with gradual intensification; Braxton-Hicks are irregular, painless, cease with movement/hydration, and do not cause cervical dilation

Assessment of Cervical Dilation and Effacement

• Vaginal examination: Systematic assessment documents cervical dilation (0-10 cm), effacement/thinning (0-100%), consistency (firm/medium/soft), position (posterior/mid/anterior), and cervical station of presenting part (-5 to +5)
• Cervical dilation assessment: Performed by digital exploration; examiner inserts examining fingers into vaginal canal and estimates diameter of cervical opening; requires experience and serial examinations to assess progress

Fetal Assessment During Labor

• Partograph/Partogram: Graphical tool plotting cervical dilation, fetal descent, and time; allows visual assessment of labor progress against expected curve (Friedman curve for nulliparas); identifies prolonged labor (>alert line) or arrested labor (>action line)

Admission and baseline management

• Confirm labor and assess fetus: ACOG accepts either continuous electronic fetal monitoring or structured intermittent auscultation in low-risk labor; high-risk labor (oxytocin, preeclampsia, meconium, prior cesarean) warrants continuous monitoring.
• Intrapartum GBS prophylaxis (ACOG, adopting CDC criteria): penicillins — penicillin G IV is first line; cefazolin for low-risk penicillin allergy (cephalosporin cross-reactivity is roughly 1–3%, driven by shared R1 side chains); clindamycin only if isolate is susceptible, otherwise vancomycin.
• Analgesia: neuraxial epidural is first line, and per ACOG maternal request alone is sufficient indication. Opioid alternatives exist but cause neonatal respiratory depression.

Ripening, induction, and augmentation

• Unfavorable cervix (Bishop ≤6): prostaglandins — misoprostol (PGE1) or dinoprostone (PGE2) — or a mechanical transcervical Foley balloon.
• Favorable cervix: oxytocin infusion titrated to adequate contractions, with amniotomy once the head is applied to the cervix.
• Arrest definitions (ACOG/SMFM Safe Prevention of the Primary Cesarean Delivery): active phase begins at 6 cm; first-stage arrest requires ≥6 cm with ruptured membranes and no cervical change over an adequate interval of adequate contractions. Second stage — do not diagnose arrest before ≥3 hours of pushing in nulliparas or ≥2 hours in multiparas; longer durations (an additional hour or more) may be appropriate with epidural analgesia, fetal malposition, or ongoing descent, provided maternal and fetal status are reassuring.

Non-reassuring tracing — intrauterine resuscitation

• Stop oxytocin, reposition to lateral decubitus, IV fluid bolus, treat epidural hypotension with a vasopressor; beta-agonist tocolytic (terbutaline) for tachysystole; amnioinfusion for recurrent variable decelerations. ACOG notes routine maternal oxygen has not shown benefit.

Definitive/operative

• Operative vaginal delivery (forceps or vacuum) requires full dilation, ruptured membranes, engaged head, known position, anesthesia, and empty bladder; otherwise cesarean delivery.
• Third stage: active management with oxytocin after delivery; ACOG endorses delayed cord clamping in vigorous term and preterm infants.

Intrapartum emergencies

• Shoulder dystocia (emergency): anterior shoulder impacted behind the pubic symphysis; turtle sign with retraction of the delivered head. Sequelae are brachial plexus injury (Erb-Duchenne, waiter's tip), clavicular/humeral fracture, and neonatal asphyxia. Fundal pressure is contraindicated.
• Umbilical cord prolapse (emergency): typically follows amniotomy at an unengaged station; abrupt prolonged deceleration or bradycardia with a palpable pulsatile cord — elevate the presenting part and proceed to immediate cesarean.
• Uterine rupture (emergency): most often a prior scar under oxytocin or prostaglandin exposure; sudden fetal bradycardia, loss of station, tearing pain, and hemodynamic collapse.
• Amniotic fluid embolism (emergency): fetal antigen entry triggers anaphylactoid response — abrupt hypoxia, cardiovascular collapse, seizures, then DIC.
• Uterine inversion (emergency): excessive cord traction on a fundal placenta; absent fundus with a mass at the introitus plus profound shock disproportionate to blood loss.

Hemorrhagic and infectious

• Postpartum hemorrhage: atony is the leading cause (overdistension, prolonged labor, prolonged oxytocin exposure causing receptor desensitization); the finding is a boggy, poorly contracted uterus. Retained placenta, accreta spectrum, and lacerations (firm uterus with ongoing bleeding) are the other categories.
• Intraamniotic infection (chorioamnionitis): prolonged rupture and repeated exams; maternal fever with uterine tenderness and fetal tachycardia; risks endometritis and neonatal sepsis.

Treatment-related

• Oxytocin: tachysystole → recurrent late decelerations; structural similarity to ADH causes hyponatremia with prolonged high-dose infusion in hypotonic fluid.
• Neuraxial analgesia: sympathectomy-induced hypotension → transient fetal bradycardia; also maternal fever and post-dural puncture headache.
• Episiotomy/operative delivery: midline episiotomy extends into third/fourth-degree lacerations → rectovaginal fistula and incontinence; vacuum causes cephalohematoma and subgaleal hemorrhage; forceps cause facial nerve palsy.

• Bishop score drives induction strategy: when the cervix is unfavorable, the best next step is cervical ripening (prostaglandin or Foley balloon), not straight-to-oxytocin.
• Slow dilation before 6 cm is latent labor, not arrest: the classic distractor is diagnosing "arrest" at 4–5 cm and taking the patient to cesarean (ACOG/SMFM Safe Prevention of the Primary Cesarean Delivery).
• Second-stage arrest thresholds: do not diagnose arrest before ≥3 hours of pushing in a nullipara or ≥2 hours in a multipara; allow an additional hour or more with epidural analgesia, fetal malposition, or ongoing descent when mother and fetus are reassuring.
• Deceleration decoding: early = head compression (vagal, benign); variable = cord compression (consider amnioinfusion); late = uteroplacental insufficiency (stop oxytocin, reposition, fluids). A sinusoidal tracing suggests fetal anemia and is Category III.
• Shoulder dystocia first move is McRoberts plus suprapubic pressure — never fundal pressure. The tested association is maternal diabetes with macrosomia; the tested sequela is Erb palsy.
• Postpartum hemorrhage: palpate the uterus first. Boggy uterus = atony → massage and oxytocin. Second-line uterotonics carry the classic contraindications examiners love: methylergonovine in hypertension/preeclampsia, carboprost (PGF2α) in asthma.
• Sudden fetal bradycardia with loss of station in a woman attempting VBAC is uterine rupture until proven otherwise — immediate laparotomy. Misoprostol is contraindicated with any prior uterine scar.
• Routine episiotomy is not recommended by ACOG; restricted use only. Midline heals better cosmetically but extends into the anal sphincter far more often than mediolateral.
• Penicillin G is first-line intrapartum GBS prophylaxis; cefazolin is acceptable for low-risk penicillin allergy, since true cephalosporin cross-reactivity is only about 1–3% and reflects shared R1 side chains, not the beta-lactam ring.