Physiologic changes of pregnancy
Why examiners ask this
The physiology is rarely a stand-alone question; it is the "why" behind every obstetric answer. The examiner checks that you can attach a change to a decision: smaller tube because of airway edema, faster desaturation because of a smaller FRC, lower doses because of neuraxial sensitivity, left tilt because of aortocaval compression. It stresses application of knowledge and organization; a system-by-system list without consequences scores poorly.
The 60-second answer
I organize by what changes my anesthetic. Airway: edematous and friable mucosa, enlarged breasts and weight gain, so I expect a harder intubation (failed intubation is roughly seven times more common at term) and use a smaller tube and video laryngoscope.1 Respiratory: FRC falls about 20% and oxygen consumption rises, so she desaturates quickly;1 minute ventilation rises up to 50%,2 so normal PaCO2 is about 30 mmHg.1 Cardiovascular: cardiac output rises about 50% and higher still in labor and at delivery, systemic vascular resistance falls, and aortocaval compression after 20 weeks drops venous return.1 Hematologic: dilutional anemia, a hypercoagulable state with high fibrinogen. Gastrointestinal: reduced lower esophageal sphincter tone, so I treat her as a full stomach in later pregnancy and in labor. Neuraxial: engorged epidural veins and increased nerve sensitivity, so I reduce local anesthetic doses.
Decision points and the "why"
- Left uterine displacement after 20 weeks — supine compression of the vena cava cuts preload and of the aorta cuts uterine flow. Every positioning answer includes it.
- Preoxygenation and apneic oxygenation — the reduced FRC and increased consumption mean apnea time to desaturation is roughly halved; I preoxygenate to an end-tidal oxygen above 0.9 and use nasal oxygen.
- Full-stomach precautions — progesterone relaxes the lower esophageal sphincter and the uterus displaces the stomach; up to 80% of term parturients have reflux;1 I give sodium citrate and use rapid sequence induction for general anesthesia from mid-pregnancy onward.
- Interpret the blood gas — a PaCO2 of 40 mmHg in a pregnant patient is relative hypercarbia and a warning sign of respiratory fatigue.
- Drug doses — MAC falls by about 30–40%, and less local anesthetic is needed per dermatome for neuraxial block;2 plasma cholinesterase falls about 25%,1 but succinylcholine duration is barely prolonged because volume of distribution rises.
- Cardiac output peaks immediately after delivery — autotransfusion of 500 mL as the uterus contracts and relief of caval compression; this is the dangerous moment for stenotic valve lesions and pulmonary hypertension.
The complication they will inject
A 28-week pregnant patient for appendectomy desaturates to 85% on induction despite good preoxygenation, and the first look shows a swollen airway with bleeding. I declare the difficulty, place her in a ramped position with left tilt, and mask ventilate with two hands and an oral airway while preparing a supraglottic airway. Her small FRC and high consumption explain the rapid fall. I keep the fetus in mind: maternal oxygenation is fetal oxygenation. Once the airway is secured, I avoid hyperventilation because maternal hypocapnia reduces uterine flow, and I keep her mean pressure within 20% of baseline. Post-op, I involve obstetrics for fetal monitoring and tocolysis decisions.
Pitfalls that cost points
- Reciting a list of numbers without saying what each changes in the plan.
- Forgetting aortocaval compression when discussing positioning or arrest.
- Calling a PaCO2 of 40 mmHg normal in a term patient.
- Assuming she has a normal coagulation reserve because the INR is normal; it is the fibrinogen that matters in hemorrhage.
- Stating that succinylcholine is dangerously prolonged in pregnancy.
Rapid-why
Q: Why is the normal hemoglobin lower? A: Plasma volume rises about 45–50% and red cell mass only about 20–30%, a physiologic dilutional anemia.1
Q: Why does the bicarbonate fall? A: Renal compensation for the chronic respiratory alkalosis from progesterone-driven hyperventilation.
Q: Why is GFR higher? A: Renal blood flow rises about 50%, so a creatinine of 1.0 mg/dL is abnormal in pregnancy.
Q: When do these changes resolve? A: Hematologic changes have mostly reverted by 2 weeks postpartum;1 full return of all systems takes up to 6 months.
Numbers to know
| Item | Value |
|---|---|
| Cardiac output | ~+50% at term; higher in labor and immediately after delivery1 |
| Plasma volume / red cell mass | +40–50% / +20–30%2,1 |
| FRC | −20% |
| Minute ventilation | +45–50%; PaCO2 ~30 mmHg |
| MAC | −30–40%2 |
| Fibrinogen | 400–600 mg/dL at term |
| Normal creatinine | ~0.5–0.8 mg/dL (50–73 µmol/L)1 |
Links
- General anesthesia for cesarean delivery, Non-obstetric surgery during pregnancy (coming), Maternal cardiac arrest and resuscitative hysterotomy (coming), aspiration, Cardiac disease in pregnancy (coming), The obese parturient (coming)
- ASA Practice Guidelines for Obstetric Anesthesia; ACOG Committee Opinion on nonobstetric surgery during pregnancy
Sources
- 1.Changes in Maternal Physiology During Pregnancy (Update in Anaesthesia 21) · WFSA · resources.wfsahq.org
- 2.Obstetric Regional Anesthesia · NYSORA · nysora.com
Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.