OB/GYN

Physiologic Changes of Pregnancy

OB/GYN

Physiologic Changes of Pregnancy

Cardiovascular, renal, hematologic, endocrine, respiratory adaptations.

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◆Cardiovascular

•↑ Plasma volume (50%) + ↑ RBC mass (30%) → DILUTIONAL anemia (↓ hematocrit)
•↑ Preload + ↑ HR → ↑ Cardiac output (~30–50%)
•↑ Progesterone → ↓ SVR → ↓ BP in first 20 weeks
•Supine position + IVC compression → ↓ preload → SUPINE HYPOTENSION SYNDROME → LEFT LATERAL DECUBITUS
•Physiologic S3 (3rd trimester) — benign

◆Renal/urinary

•↑ GFR + ↑ renal blood flow → ↓ BUN and ↓ creatinine (elevated = concerning)
•Progesterone → ureteral dilation + stasis → ↑ risk asymptomatic bacteriuria + pyelonephritis
•Always treat asymptomatic bacteriuria in pregnancy

◆Hematologic

•Hypercoagulable state (↑ clotting factors) → ↑ VTE risk; protective against postpartum hemorrhage
•↑ Fibrinogen, ↑ factors VII, VIII, IX, X
•Decreased protein S

◆Endocrine/respiratory

•Estrogen → ↑ TBG → ↑ TOTAL T4 (free T4 normal)
•Progesterone → ↑ tidal volume → respiratory ALKALOSIS (facilitates CO₂ transfer from fetus)
•Pituitary doubles in size → Sheehan syndrome risk if massive PPH
•Prolactin rises throughout pregnancy

High-yield pearls

◆Hypotension supine → left lateral decubitus
◆Asymptomatic bacteriuria → always treat (nitrofurantoin, amox, cefalexin)
◆Total T4 ↑ but free T4 normal in pregnancy
◆Physiologic anemia from plasma volume expansion (not iron deficiency)

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