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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