Pregnancy Physiology — Bone Wizardry

Up or Down? — Tap your answer, then find out why

Mechanism First — What's driving these changes?

→↑ Plasma volume (40–50%) — estrogen drives renal sodium and water retention → expanded intravascular volume → increased preload → increased stroke volume

→↑ TBG (thyroid-binding globulin) from liver → more bound T4 → total T4 rises, but free T4 stays normal (euthyroid)

→↑ Pro-clotting factors (I, VII, VIII, X, XII, vWF) → hypercoagulable state → risk of DVT/PE during pregnancy and postpartum

→Reduced biliary transport → cholestasis → pruritus, cholelithiasis risk

→Pituitary hyperplasia → normal in pregnancy, but blood supply doesn't increase proportionally → risk of Sheehan syndrome if hemorrhage occurs

→↓ Systemic vascular resistance — progesterone relaxes smooth muscle → vasodilation → decreased afterload → lower BP (most pronounced in 2nd trimester)

→↑ Tidal volume & minute ventilation — progesterone stimulates the respiratory center → hyperventilation → respiratory alkalosis (↓ pCO2), compensated by renal ↓ HCO3

→↓ Gallbladder motility — smooth muscle relaxation → bile stasis → cholelithiasis risk

→↓ GI motility → constipation, GERD common in pregnancy

→↓ Ureteral tone → urinary stasis → increased UTI and pyelonephritis risk

→Nausea & vomiting of pregnancy — peaks when hCG peaks (weeks 8–12), then improves as hCG falls. Hyperemesis gravidarum = severe form

→Stimulates thyroid — hCG has structural homology with TSH → mild thyroid stimulation at peak hCG levels (can cause subclinical hyperthyroidism in 1st trimester)

→Maintains corpus luteum until placenta takes over progesterone production (~10 weeks)

→↑ Insulin resistance — hPL (human placental lactogen) antagonizes insulin signaling → maternal hyperglycemia → more glucose available for fetus. Can cause gestational diabetes if pancreas can't compensate

→↑ Maternal lipolysis — breaks down maternal fat → fatty acids used for maternal energy → glucose spared for fetus

→Marker of placental function — rising hPL throughout pregnancy reflects growing placenta. Low hPL = placental insufficiency

Board Traps — The ones that get people

🩸Hematology

Anemia? But RBCs increase...

Tap to reveal the logic

Plasma volume increases ~40–50%, while RBC mass only increases ~20–30%. The net result is dilution → lower Hgb and Hct despite more total RBCs. It's a dilutional anemia, not a production problem. MCV stays normal.

🦋Thyroid

Total T4 is high — is she hyperthyroid?

Tap to reveal the logic

No. Estrogen raises TBG (thyroid-binding globulin), so more T4 is bound, raising total T4. But free T4 — the active fraction — is normal. Patient is clinically euthyroid. Don't treat a high total T4 in pregnancy without checking free T4.

🧠Endocrine

Sheehan Syndrome — Why?

Tap to reveal the logic

Estrogen causes pituitary hyperplasia during pregnancy — the gland grows, but its blood supply doesn't increase proportionally. If significant hemorrhage occurs during delivery, the enlarged pituitary undergoes ischemic necrosis → panhypopituitarism (failure to lactate is the classic first sign).

🫁Pulmonary

She's hyperventilating — is it anxiety?

Tap to reveal the logic

Progesterone stimulates the respiratory center → increased tidal volume → physiologic hyperventilation → respiratory alkalosis (↓ pCO2). The kidneys compensate by excreting HCO3 (↓ bicarb). Normal in pregnancy. Don't treat the "alkalosis" — it's compensated and expected.

🫘Renal

Creatinine 0.4 — acute kidney injury?

Tap to reveal the logic

In pregnancy, GFR increases ~50% due to increased plasma volume and decreased SVR → more renal plasma flow → more filtration. Serum creatinine drops. A "normal" creatinine of 1.0 mg/dL in a pregnant patient may actually indicate impaired renal function. Baseline is lower.

🩺Cardiovascular

BP is low in 2nd trimester — preeclampsia?

Tap to reveal the logic

The opposite. Progesterone causes vasodilation → SVR drops → BP naturally falls in the 2nd trimester. It rises back toward baseline in the 3rd trimester. Preeclampsia = new-onset HTN ≥140/90 after 20 weeks. A low BP in 2nd trimester is physiologic, not concerning.

Quiz — 4 questions, shuffled each time