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Physiological Changes During Pregnancy

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Summary Table - Maximum Changes at Term

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1. Central Nervous System (Neurological) Changes

• MAC decreases by ~40% for all inhalational anesthetics at term; returns to normal by day 3 post-delivery.
• Progesterone rises to 20x normal at term and is at least partly responsible (it is sedating at pharmacological doses).
• β-endorphin surge during labor also reduces anesthetic requirements.
• Enhanced sensitivity to local anesthetics - epidural dose requirements reduced by up to 30%.
• Engorgement of epidural venous plexus (from inferior vena cava compression by the uterus) has three effects:
  • Decreased spinal CSF volume
  • Decreased potential epidural space volume
  • Increased epidural pressure (becomes positive rather than negative)
  • These enhance cephalad spread of local anesthetics during spinal/epidural anesthesia.

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2. Respiratory Changes

• Oxygen consumption and minute ventilation increase progressively, up to 50% by term.
• Tidal volume increases (+40%), respiratory rate increases slightly (+15%).
• PaCO2 falls to 28-32 mmHg (physiological hyperventilation).
• Significant respiratory alkalosis is prevented by compensatory decrease in plasma bicarbonate.
• PaO2 increases slightly.
• 2,3-DPG rises to offset the Bohr effect - the P50 for hemoglobin shifts from 27 to 30 mmHg, enhancing O2 delivery to tissues.
• In the third trimester, the diaphragm is elevated but diaphragmatic motion is not restricted; the chest compensates with increased anteroposterior diameter.
• FRC decreases by up to 20% at term (returns to normal within 48 h post-delivery) - primarily due to reduced expiratory reserve volume.
• Vital capacity and closing capacity are minimally affected; airway resistance decreases.
• Physiological dead space decreases but intrapulmonary shunting increases.
• Critical clinical point: Decreased FRC + increased O2 consumption = rapid desaturation during apnea. Preoxygenation before GA induction is mandatory.

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3. Cardiovascular Changes

• Blood volume increases 35%, plasma volume increases 55% - this "dilutional anemia" is physiological.
• Cardiac output increases 40% - primarily from increased stroke volume early, then heart rate later.
• Heart rate increases 20%, stroke volume increases 30%.
• Systemic and pulmonary vascular resistance both fall; BP decreases slightly despite higher cardiac output.
• During labor, cardiac output increases a further 10-25% with each contraction.
• Immediately post-delivery, cardiac output transiently rises even further due to:
  • Relief of aortocaval compression
  • Autotransfusion from the uterus
• Aortocaval compression: The enlarged uterus in the supine position compresses the inferior vena cava, reducing venous return and cardiac output (supine hypotension syndrome). Left uterine displacement corrects this.

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4. Uteroplacental Circulation

• At term, uterine blood flow = ~10% of cardiac output = 600-700 mL/min (vs. 50 mL/min non-pregnant).
• 80% goes to the placenta; 20% to the myometrium.
• Uterine vasculature is maximally dilated - no autoregulation but remains sensitive to α-adrenergic agonists.
• Three major factors reduce uterine blood flow:
  1. Systemic hypotension (aortocaval compression, hypovolemia, sympathetic blockade)
  2. Uterine vasoconstriction (catecholamines, α-agonists like phenylephrine)
  3. Uterine contractions (elevated venous pressure + arterial compression)

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5. Hematological Changes

• Plasma volume increases more than RBC mass → physiological dilutional anemia (Hb decreases ~20%).
• Platelets decrease by ~10%.
• Clotting factors increase dramatically (30-250%) - pregnancy is a hypercoagulable state.
  • Factors I (fibrinogen), VII, VIII, IX, X, XII all rise.
  • This protects against hemorrhage at delivery but increases DVT/PE risk.
• WBC increases (neutrophilia).

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6. Renal Changes

• GFR increases 50% due to increased cardiac output and reduced renal vascular resistance.
• Serum creatinine and urea fall (normal pregnancy values are lower than non-pregnant norms).
• Glycosuria can occur due to tubular maximum being exceeded (not necessarily pathological).
• Uterine compression of the ureters may cause physiological hydronephrosis (right > left).

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7. Gastrointestinal Changes

• Gastric emptying is delayed; lower esophageal sphincter tone decreases (progesterone effect).
• Increased risk of aspiration - all pregnant patients at term are considered to have a full stomach for anesthetic purposes.
• Intragastric pressure is elevated by the gravid uterus.

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8. Key Hormonal/Metabolic Drivers

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• The airway is often edematous/friable - smaller ETT may be needed; Mallampati class worsens with labor.
• Delayed gastric emptying + reduced LES tone = aspiration risk; RSI + cricoid pressure is standard.
• Supine position → aortocaval compression → always use left lateral tilt.
• Rapid desaturation on induction - always pre-oxygenate thoroughly.
• Morgan and Mikhail's Clinical Anesthesiology, 7e, Chapter 40 - pp. 1572-1581

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Physiological Changes During Pregnancy - Detailed Notes

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1. GENITAL ORGANS

Uterus

• Non-pregnant uterus weighs ~60-70 g and has a capacity of ~10 mL.
• At term, it weighs ~1000 g with a capacity of ~5000 mL (500-fold increase in volume).
• Growth occurs by hypertrophy of existing myometrial cells (stimulated by estrogen and mechanical stretch) - NOT hyperplasia.
• Uterine blood flow at term = 600-700 mL/min (10% of cardiac output), vs. 50 mL/min when non-pregnant.
• 80% of uterine blood goes to the placenta; 20% to the myometrium.
• The uterine vasculature is maximally dilated - autoregulation is absent.
• Uterine vascular bed remains sensitive to α-adrenergic agonists (catecholamines, phenylephrine can cause vasoconstriction and fetal compromise).
• Three factors reduce uterine blood flow:
  1. Systemic hypotension (aortocaval compression, hypovolemia, regional anaesthesia)
  2. Uterine vasoconstriction (stress-induced catecholamine release)
  3. Uterine contractions (elevate venous pressure + compress arterial vessels)
• Softening of the cervix (Goodell's sign) - due to increased vascularity and edema; facilitated by relaxin.
• Hegar's sign - softening of the lower uterine segment (6-8 weeks).
• Osiander's sign - pulsation felt in lateral fornix due to increased uterine artery blood flow.

Vagina

• Increased vascularity and venous stasis → Chadwick's/Jacquemier's sign: mucosa changes from pink → violet → blue (from 4th month onward).
• Vaginal secretions increase (white, non-offensive leukorrhea of pregnancy).
• Vaginal pH decreases (more acidic) due to increased lactic acid from glycogen breakdown - protective against infection.

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2. BREASTS

First Trimester

• Estrogen (from corpus luteum, then placenta): ductal proliferation and elongation, involution of adipose tissue.
• Progesterone: stimulates lobule development.
• hCG peaks at 9 weeks - prevents corpus luteum involution, maintains estrogen + progesterone.
• Mononuclear inflammatory cell infiltration of breast tissue begins.
• Breast enlargement and tenderness (one of the earliest signs of pregnancy).

Second Trimester (by 20 weeks)

• Mammary glands sufficiently developed to produce milk components (prolactin-stimulated).
• Secretory substances accumulate in lobule acini epithelial cells.
• Myoepithelial cells become flattened; epithelial cells enlarge.
• Areola darkens; areolar glands (Montgomery's tubercles) become more prominent.

Third Trimester

• Increased prolactin promotes alveolar cell differentiation and initiates lactogenesis stage I.
• Colostrum is produced: thin yellowish fluid containing free fat globules and large phagocytic cells - present from ~16 weeks.
• Actual milk production is inhibited by high estrogen + progesterone (these fall rapidly post-delivery to allow let-down).
• Striae (silver lines/linea albicantes) appear on breasts after 6th month.
• Surface veins become prominent - visible as blue lines.

Post-delivery

• After parturition, estrogen and progesterone fall → prolactin uninhibited → milk production begins.
• Colostrum is replaced by milk ~10 days post-delivery.
• Post-lactation: massive apoptosis, acinar involution, connective tissue remodelling (loose → dense structure), basement membrane thickening.

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3. CUTANEOUS CHANGES

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4. WEIGHT GAIN

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5. BODY WATER METABOLISM

• Total body water increases by 6-8 litres at term.
• Of this, approximately 3.5 L is in the fetus, placenta, and amniotic fluid; the rest is in the mother.
• Plasma volume increases by 55% (most disproportionate change - produces dilutional anemia).
• Red blood cell mass increases by 45%.
• Plasma osmolality decreases by 8-10 mOsm/kg (the osmotic threshold for ADH secretion and thirst is reset downward).
• Dependent edema of the ankles is physiological (occurs in up to 80% of pregnancies due to increased venous pressure from IVC compression).

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6. HEMATOLOGICAL CHANGES

Erythrocytes

• RBC mass increases by ~45% (less than plasma volume increase of 55%) → physiological dilutional anaemia.
• Hemoglobin at term usually remains >11 g/dL.
• Physiological anaemia of pregnancy - not true iron deficiency; Hb falls to ~11-12 g/dL.
• Iron supplementation is required because fetal demands + increased RBC production exceed normal dietary intake.
• Folate deficiency anaemia can develop if supplements are not taken (fetal utilization of folate).

White Blood Cells

• Leukocytosis up to 21,000/μL during third trimester (predominantly neutrophilia).
• Further leukocytosis during labor (up to 25,000-30,000/μL).

Platelets

• Platelet count decreases by ~10% (gestational thrombocytopenia); usually >100,000/μL and clinically insignificant.

Coagulation - Hypercoagulable State

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7. CARDIOVASCULAR CHANGES

Cardiac Output

• Increases 40% by term = largest single physiological change in the cardiovascular system.
• Mechanism: Stroke volume ↑30% + Heart rate ↑20%.
• Timeline:
  • Cardiac output rises early in the 1st trimester (by 8 weeks - mostly stroke volume driven).
  • Peaks in the 2nd trimester.
  • 3rd trimester: Relatively stable; does not appreciably rise further until labor.
• During labor: Each contraction adds 10-25% more output (autotransfusion of 300-500 mL from the contracting uterus).
• Immediately post-delivery: Cardiac output rises further (relief of IVC compression + uterine autotransfusion).
• Returns to normal 2 weeks after delivery.

Blood Pressure

• Systolic BP: decreases slightly (~5%).
• Diastolic BP: decreases more (~15%) - nadir in mid-2nd trimester.
• Fall in BP despite increased cardiac output is entirely due to peripheral vasodilatation (progesterone-mediated smooth muscle relaxation, local prostaglandins, nitric oxide).
• Returns toward baseline in 3rd trimester.

Vascular Resistance

• Peripheral vascular resistance: decreases 15%.
• Pulmonary vascular resistance: decreases 30% - pulmonary hypertension does not develop despite higher blood flow.

Blood Volume

• Total blood volume increases 35% (1000-1500 mL by term).
• Total blood volume at term = 90 mL/kg.
• Expected blood loss at delivery: 200-500 mL (vaginal), 800-1000 mL (cesarean).
• Blood volume normalizes 1-2 weeks post-delivery.

Structural Cardiac Changes

• Cardiac chambers enlarge; myocardial hypertrophy on echocardiography.
• Heart is displaced upward and leftward by the elevated diaphragm → chest X-ray shows apparent cardiomegaly + left axis deviation on ECG.
• T-wave changes and ST depression can occur normally.
• Physical exam:
  • Grade I-II systolic ejection (flow) murmur (due to increased blood flow - normal)
  • Exaggerated splitting of S1
  • S3 may be audible (normal in pregnancy)
  • Small asymptomatic pericardial effusion may occur
• Central venous pressure, pulmonary artery pressure, and PAOP (wedge pressure) remain unchanged despite expanded volume (due to reduced resistance).

Aortocaval Compression

• After week 20, the gravid uterus compresses the inferior vena cava when supine → reduces venous return → decreases cardiac output by up to 30%.
• 5% of women at term: Supine hypotension syndrome - hypotension + pallor + nausea/vomiting.
• The aorta is also compressed → reduces blood flow to lower limbs and uteroplacental circulation.
• Correction: left lateral tilt >15° (wedge under right hip).
• Chronic partial IVC obstruction in 3rd trimester: venous stasis, edema, DVT risk in lower limbs; distension of epidural veins.

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8. RESPIRATORY CHANGES

Volumes and Capacities

Blood Gases

Mechanisms

• Hyperventilation is driven primarily by progesterone (direct stimulatory effect on the respiratory centre).
• Elevated 2,3-DPG offsets hyperventilation's effect on O2-Hb affinity (keeps rightward shift intact).
• Compensatory metabolic acidosis (HCO3- falls) prevents frank respiratory alkalosis.

Diaphragm/Chest Wall

• Diaphragm is elevated by ~4 cm in 3rd trimester.
• Compensatory increase in anteroposterior chest diameter.
• Diaphragmatic motion is NOT restricted.
• CXR shows elevated diaphragm + prominent vascular markings.

Clinical Hazards

1. Rapid desaturation during apnea - decreased FRC + increased O2 consumption → preoxygenation mandatory before GA induction.
2. Atelectasis in supine position - closing volume can exceed FRC at term → atelectasis and hypoxemia.
3. Respiratory mucosa engorgement - upper airway edema and friability → trauma with laryngoscopy; use smaller ETT (6-6.5 mm); Mallampati score worsens.
4. Faster inhalational anesthetic uptake (decreased FRC + increased minute ventilation).

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9. RENAL CHANGES

• Renal plasma flow increases by 75% early in pregnancy.
• GFR increases 50% by 16 weeks (due to increased cardiac output + reduced renal vascular resistance).
• Serum creatinine falls to as low as 0.5 mg/dL (normal non-pregnant ~0.9 mg/dL).
• BUN may decrease to 9 mg/dL.
• Glycosuria (1-10 g/day): tubular reabsorption maximum is exceeded even at normal blood glucose - NOT necessarily pathological.
• Mild proteinuria (<300 mg/24h) - normal; >300 mg/24h = pre-eclampsia threshold.
• Plasma osmolality decreases by 8-10 mOsm/kg (osmostat is reset).
• Physiological hydronephrosis: ureteric compression by the gravid uterus (right > left due to dextrorotation) + progesterone-induced smooth muscle relaxation → stasis → increased UTI risk.
• Urinary frequency: in early pregnancy (uterine pressure on bladder) and near term.

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10. GASTROINTESTINAL CHANGES

• Gastric emptying delayed (progesterone inhibits smooth muscle).
• Lower esophageal sphincter (LES) tone reduced (progesterone) → gastroesophageal reflux and esophagitis are common.
• Stomach displaced upward and anteriorly by the uterus → promotes sphincter incompetence.
• Gastric acid and volume: Do NOT significantly change in normal pregnancy.
• Aspiration risk: All pregnant patients at term treated as "full stomach" - RSI + cricoid pressure required.
• Pseudocholinesterase (plasma cholinesterase) decreases 25-30% at term (dilutional + reduced hepatic synthesis) → rarely clinically significant prolongation of succinylcholine effect; normalizes 6 weeks post-partum.
• Liver: Hepatic function and blood flow are unchanged. Minor elevations of transaminases and LDH in 3rd trimester. Alkaline phosphatase rises (placental isoenzyme). Serum albumin decreases (dilutional) → reduced colloid oncotic pressure.
• Gallbladder: Progesterone inhibits cholecystokinin → incomplete gallbladder emptying → bile stasis → increased cholesterol gallstone risk.
• Nausea and vomiting (morning sickness): peaks at 8-12 weeks; correlates with hCG peak.

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11. METABOLIC CHANGES

Carbohydrate Metabolism - "Accelerated Starvation / Facilitated Anabolism"

• Pregnancy is diabetogenic despite fasting hypoglycemia - an apparent paradox.
• Fasting state: Blood glucose ↓, amino acids ↓, insulin ↓ → exaggerated fasting hypoglycemia.
• Fed state: Hyperinsulinemia + insulin resistance → glucose and amino acids are preferentially shunted to the fetus.
• Insulin resistance caused by:
  • Human chorionic somatomammotropin (hCS/hPL) - most important
  • Human placental growth hormone (hPGH)
  • Elevated cortisol
  • Elevated estrogen + progesterone
• Pancreatic β-cell hyperplasia occurs to compensate for insulin resistance → insulin levels progressively rise.
• Rapid return to normal glucose metabolism after delivery = evidence that fetoplacental hormones are the diabetogenic drivers.

Metabolic Fuel Values (Late Pregnancy vs Non-Pregnant Fasting)

Fat Metabolism

• Total plasma lipids increase significantly from 24 weeks onward.
• Triglycerides, cholesterol, and free fatty acids all increase markedly.
• HDL cholesterol increases early; LDL cholesterol increases later.
• Free fatty acids and ketones elevated (fuel for mother) to spare glucose for fetus.

Protein Metabolism

• Plasma amino acid levels fall (fetal utilization).
• Prolonged fasting → exaggerated hyperketonemia and hypoglycemia.
• Gluconeogenesis is NOT adequately increased to compensate for glucose loss to the fetus.

Thyroid

• hCG and elevated estrogens promote thyroid gland hypertrophy.
• Thyroid-binding globulin (TBG) increases → total T4 and T3 increase.
• Free T4, free T3, and TSH remain normal (euthyroid state).
• Clinically: goiter may develop; do not treat total T4 elevation as hyperthyroidism.

Calcium

• Total serum calcium decreases (dilutional, lower albumin).
• Ionized calcium remains normal (physiologically active form - no clinical consequence).

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12. MUSCULOSKELETAL CHANGES

• Relaxin (secreted by corpus luteum, decidua, and placenta):
  • Softens and ripens the cervix.
  • Relaxes pubic symphysis and sacroiliac joints.
  • Inhibits uterine contractions.
  • Increases ligamentous laxity throughout the spine.
• Lumbar lordosis increases to shift the center of gravity.
• Diastasis recti - separation of rectus abdominis muscles.
• Carpal tunnel syndrome - fluid retention compresses the median nerve at the wrist (common in 3rd trimester).
• Back pain is very common (ligamentous laxity + postural changes).

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13. CENTRAL NERVOUS SYSTEM CHANGES

• MAC (Minimum Alveolar Concentration) for all inhalational anesthetics decreases progressively:
  • By term: decreases 40%.
  • Returns to normal by day 3 post-delivery.
  • Mechanism: progesterone (sedating in pharmacological doses, rises 20x normal at term) + β-endorphin surge during labor.
• Sensitivity to local anesthetics increases:
  • Epidural dose requirements: reduced by up to 30%.
  • MLAC (minimum local analgesic concentration) concept.
  • Mechanisms: hormonal sensitization + physical epidural space changes.
• Epidural space changes (from IVC compression → epidural vein engorgement):
  • Decreased CSF volume in the spinal canal.
  • Decreased potential epidural space volume.
  • Epidural pressure becomes positive (normally negative).
  • Enhanced cephalad spread of epidural/spinal blocks.
  • Increased risk of inadvertent intravascular injection of epidural catheter.

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Key Hormonal Drivers Summary

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• Morgan and Mikhail's Clinical Anesthesiology, 7e - Chapter 40 (pp. 1572-1581)
• Creasy & Resnik's Maternal-Fetal Medicine - Chapter 10 (Endocrine-Metabolic Changes)
• Current Surgical Therapy, 14e - Pregnancy Changes to the Breast
• Parikh's Textbook of Medical Jurisprudence - Presumptive Signs of Pregnancy