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Pregnancy and Neonatal Physiology
TEXTObstetric + Pediatric · 10 min read
Every organ system shifts in pregnancy. The neonate then reverses several of them. Both lists are mandatory.
After this lesson you can
3 min read8 sections- Recall key maternal CV, pulm, GI, heme changes in pregnancy.
- Apply changes to anesthetic decisions.
- Describe neonatal transitional physiology.
- Identify aortocaval compression and prevent.
Maternal CV shifts
Plasma volume rises 40-50%, RBC mass 20-30% dilutional anemia (target hemoglobin 11+).
SVR drops 20% (progesterone-mediated).
MAP slightly lower in T2, recovers T3.
Intra-partum: another 30% CO rise from contractions + pain.
Peak CO immediately post-delivery (autotransfusion + decompression).
High-risk MS/cardiomyopathy patients are most vulnerable in the first 24-48 hr postpartum.
Maternal pulmonary shifts
- PaCO₂ ~
30 mmHg - compensatory HCO₃ ~
20 mEq/L - pH ~7.42-7.44
FRC drops 20% (uterus pushes diaphragm up).
O₂ consumption up 20-30%.
Combined: rapid desaturation during apnea (1-2 min vs 5+ in non-pregnant).
Pre-O₂ aggressively.
Difficult intubation rate ~1:200 (vs 1:2000 non-pregnant) from edema + breast enlargement + time pressure.
Aortocaval compression
Supine position compresses IVC (reduces venous return) and aorta (reduces uteroplacental flow).
Maternal hypotension + fetal acidosis result.
Left lateral tilt 15° (or right hip wedge) mandatory for any supine intervention.
Even if mother feels fine, fetal compromise can be silent.
Universal precaution: never leave a pregnant patient (≥20 weeks) flat supine without tilt.
Maternal hematologic shifts
VTE risk 4-5× baseline.
Mild thrombocytopenia common (gestational thrombocytopenia) — usually >100k, no bleeding risk.
ITP, HELLP, preeclampsia drop platelets further.
WBC mildly elevated baseline (10-15k).
Plan for these shifts when interpreting labs — abnormal-looking values may be physiologic.
Fetal circulation
Ductus venosus — umbilical IVC bypassing liver.
Foramen ovale — right atrium left atrium bypassing pulmonary circulation.
Ductus arteriosus — pulmonary artery aorta, again bypassing lungs.
After birth: lung expansion + ↑PaO₂ + ↓PGE close DA (functionally 24-72 hr); pressure shift across atria closes FO.
Persistent fetal circulation in stressed neonates.
Neonatal physiology
RR 30-60.
FRC small relative to O₂ consumption 15-20 sec apnea = desaturation.
No shivering — heat from brown fat non-shivering thermogenesis (NE-stimulated UCP-1, mitochondrial uncoupling).
Immature GFR (~25 mL/min/1.73m²) + immature CYP enzymes cautious drug dosing.
HbF P50 ~19 (left-shifted, tighter O₂ binding, less tissue release); replaced by HbA over 6 months.
Pulse ox on RIGHT hand for pre-ductal saturation (left is post-ductal).
Maternal GI + airway changes
Gastric emptying delayed (especially in labor + after opioids).
Higher gastric volume + lower pH.
ALL pregnant patients ≥20 weeks treated as full stomach.
- weight gain
- breast enlargement
- mucosal edema
- capillary engorgement
Mallampati class often progresses 1 grade through pregnancy.
Difficult intubation rate ~1:200 (vs 1:2000 non-pregnant).
Smaller ETT (6.5-7.0) on hand.
Video laryngoscopy first-line for predicted difficult OB airway.
Drug pharmacology in pregnancy + lactation
Increased Vd for water-soluble drugs from increased plasma volume.
Most anesthetics cross placenta (low MW + lipid-soluble + non-ionized).
- propofol
- fentanyl
- ketamine OK in moderate doses for non-OB surgery
AVOID NSAIDs in 3rd trimester (premature ductus closure), avoid 1st-trimester high-dose benzos, avoid bupivacaine 0.75% (cardiotoxicity, removed from OB labels).
⚠ Common pitfalls
- Forgetting the term parturient has reduced FRC and increased O₂ consumption — desats in <2 min apnea.
- Standard NPO times skipping — pregnancy delays gastric emptying; treat as full stomach.
- Aspirating during induction — left tilt 15° + cricoid pressure considered (DAS softened, but OB still uses).
- Treating maternal HCT 32 as anemia — physiologic dilution; not transfusion trigger alone.
💎 Clinical pearls
- Maternal blood volume ↑40-50% by term — explains why bleeding tolerance is higher initially.
- Maternal MAC drops ~30% — adjust volatile + IV doses accordingly.
- Aortocaval compression after 20 weeks → left uterine displacement always.
- Fetal Hb shifts curve left → high O₂ affinity → easy uptake from placenta, harder offloading at fetal tissues.
Recap
- Maternal blood volume ↑40-50% by term — explains why bleeding tolerance is higher initially.
- Maternal MAC drops ~30% — adjust volatile + IV doses accordingly.
- Aortocaval compression after 20 weeks → left uterine displacement always.
- Fetal Hb shifts curve left → high O₂ affinity → easy uptake from placenta, harder offloading at fetal tissues.
Mark each section done to complete the module.