Updated: 5/22/2020

Physiologic Changes in Pregnancy

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  • Introduction
    • Overview
      • many physiologic changes occur in pregnancy
      • help protect mother from hemorrhage
      • help fetus develop properly
        • ensure proper delivery of nutrients to fetus
        • facilitates waste removal from fetus
        • ensures adequate oxygenation of fetus
  • Hematologic
    • Hypercoagulable state
      • synthesis of clotting factors ↑
      • result of venous stasis secondary to uterine pressure on great veins of lower extremity
      • reduced fibrinolysis
    • Plasma volume ↑ by 30-50%
      • 6-12 weeks of gestation
        • 10-15%
      • 30-34 weeks
        • plateaus or decreases
    • RBC mass ↑ by 30%
      • the increase in plasma volume is larger than the increase in RBC mass
        • this relative difference leads to a dilutional gap of 15-20% which causes physiologic anemia
      • anemia in pregnant women - hemoglobin < 11 g/dL in trimesters 1 and 3
        • hemoglobin < 10.5 g/dL in trimester 2
        • lower by 0.8 g/dL in African Americans
      • increased demand for iron and folate
    • Mild neutrophilia
      • result of granulocyte demargination
        • no absolute increase in leukocyte count
    • Slight thrombocytopenia (still within normal limits)
    • Returns to normal 6-8 weeks postpartum
  • Cardiac
    • 1st trimester
      • systemic vascular resistance (SVR) beginning at 5 weeks
        • total drop 35-40%
        • due to progesterone mediated smooth muscle relaxation
      • heart rate (HR) ↑
      • cardiac output (CO) ↑
        • CO = HR x SV (stroke volume)
    • 2nd trimester
      • SVR plateaus
      • HR rises
      • CO ↑
    • 3rd trimester
      • SVR returns to normal
      • HR peaks
      • CO peaks
      • change in position alters cardiac parameters
        • supine positioning
          • CO ↓
          • SV ↓
          • HR ↑
          • changes due to compression of aorta/vena cava by gravid uterus
          • left lateral decubitus position alleviates these stresses
    • Flow changes
      • ↑ S2 split with inspiration
      • distended neck veins
      • systolic ejection murmor
      • diastolic murmor is NOT a normal finding during pregnancy
      • S3 gallop
    • Complications
      • dramatic shifts in cardiac physiology result in high pregnancy mortality in mothers with cyanotic heart disease
        • can result in Eisenmenger syndrome
  • Pulmonary
    • Mucosal hyperemia
      • nasal stuffiness
      • increased nasal secretions
    • Tidal volume ↑ → resting minute ventilation ↑
      • driven by progesterone stimulation of respiratory drive centers
      • results in chronic respiratory alkalosis with renal compensation
        • ↑ PO2 and ↓ CO2
        • arterial pH 7.40-7.45
      • increases 50% by term
    • Functional residual capacity (FRC) ↓ by 20%
      • residual volume (RV) ↓
      • expiratory reserve volume (ERV) ↓
    • Diaphragm is elevated by expanding uterus
    • Vital capacity, inspiratory reserve, forced expiratory volume (FEV1), and maternal oxygenation do not change
    • Dyspnea of pregnancy
      • accompanies normal pregnancy in majority of women
      • initiated first or second trimester
  • Gastrointestinal
    • Progesterone leads to
      • ↓ GI tract motility
        • ↓ lower esophageal tone → gastroesophageal reflux disease (GERD)
        • constipation
      • ↓ bile acid secretion → ↑ smooth muscle relaxation → slowed gallbladder emptying
        • risk of cholelithiasis ↑
    • Cholesterol secretion
      • due to estrogen ↑
      • risk of cholelithiasis ↑
    • Hemorrhoids due to
      • constipation
      • increased venous pressure
        • uterus compressing inferior vena cava
  • Renal
    • Kidney size ↑
    • Progesterone leading to
      • bladder tone ↓
      • dilation of ureters, renal pelvices, and calyces
      • urinary stasis predisposes to urinary tract infection (UTI)/pyelonephritis
    • Glomerular filtration rate (GFR) ↑
      • within 1 month of conception
      • by second trimester increases 40-50%
    • Glucosuria
      • urine dipstick positive
        • NOT useful in managing diabetes
    • Proteinuria
      • up to 150-200 mg/day in third trimester
      • positive urine dipstick may occur
        • ≥ 300 mg/day warrants further investigation
    • Serum creatinine (Cr) and blood urea nitrogen (BUN) ↓
      • therefore serum Cr of 1.0 gm/dL suggests renal impairment in pregnant people
      • small increases in Cr can indicate reduced renal function
    • Hyponatremia of pregnancy
      • not considered pathologic until below sodium concentration < 130 mEq/L
  • Endocrine
    • Pituitary gland size and vascularity ↑
      • susceptibility to Sheehan syndrome ↑
    • Fasting glucose ↓
    • Post-prandial glucose ↑
      • fetus produces own insulin at weeks 9-12
    • Thyroid binding globulin (TBG) ↑
      • due to ↑ estrogen
      • leading to ↑ total T3 and T4
      • free T3 and T4 remain same (pregnant women are euthyroid)
    • Cortisol and cortisol binding protein ↑
  • Skin
    • Normal skin changes during pregnancy mimic liver disease
      • due to ↑ estrogen
        • can see spider angiomas
        • can see palmar erythema
    • Hyperpigmentation due to
      • ↑ melanocyte stimulating hormone, estrogen, and progesterone
        • umbilicus
        • perineum
        • face
        • nipples
        • areola
        • genital areas
        • linea nigra
        • melasma
    • Acne
    • Vericose veins
    • Striae

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