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Anatomic Changes in Pregnancy
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Kidney size increases by approximately 1 cm during pregnancy. The urinary collecting system (renal calyces, pelvis, and ureters) dilates. Hormonal and mechanical forces are thought to account for ureteral dilation as early as 6 weeks gestation. In the later stages of pregnancy, mechanical compression of the ureter against the pelvic brim may lead to hydroureter and hydronephrosis. Hydronephrosis occurs on the right in 90% of cases due to dextrorotation of the uterus by the sigmoid colon.
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In rare instances this becomes a clinically significant cause of obstructive uropathy. The dilated collecting systems can hold up to 300 mL of urine and hence serve as a reservoir for bacteria. The dilated urinary tract also allows for urinary stasis and increases the risk of pyelonephritis in pregnant women with asymptomatic bacteriuria.
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Physiologic Changes in Pregnancy
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Renal physiologic changes are characterized by marked vasodilation, which leads to increases in glomerular filtration rate (GFR) and renal plasma flow (RPF). These changes occur early in the first trimester and peak increases in GFR and RPF to 50% above baseline are seen by the end of the first trimester. The filtration fraction (GFR/RPF) falls significantly, indicating a greater rise in effective RPF. Creatinine production is unchanged in pregnancy but creatinine clearance is increased, resulting in lower levels of serum creatinine; the normal creatinine value during pregnancy is <0.8 mg/dL (see Table 55–1).
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Increased urinary excretion of protein, amino acids, uric acid, glucose, and calcium occurs as a result of the elevated GFR. Hence, proteinuria in pregnancy is considered abnormal when it exceeds 300 mg/day compared with an upper limit of normal of 150 mg/day in the nonpregnant population. Uric acid clearance also increases in pregnancy and serum uric acid levels in pregnant women usually do not exceed 4.5 mg/dL by the third trimester.
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Electrolytes & Acid Base Changes
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Pregnancy is associated with significant changes in water metabolism. Serum osmolality falls by 5–10 mOsmol/kg as a result of several forces. The serum osmostat is reset as suggested by normal responses to water loading and water deprivation despite a lower serum osmolality. There is a decrease in the osmotic thresholds for thirst and arginine vasopressin (AVP) release. Enhanced catabolism of AVP by release of placental vasopressinases leads to transient diabetes insipidus and in some women is severe enough to warrant treatment. Total body water increases by 6–8 L, most of which is extracellular. Plasma volume increases throughout pregnancy ...