In rare instances in patients suffering from pyelitis, the pregnant uterus may so compress the ureter as to cause a damming back of the purulent discharge, and thus give rise to a pyelonephritis.
—J. Whitridge Williams (1903)
Renal and urinary tract disorders are frequently encountered in pregnancy. Some precede pregnancy—one example being nephrolithiasis. In some women, pregnancy-induced changes may predispose to development or worsening of urinary tract disorders—an example is the markedly increased risk for pyelonephritis, described above by Williams. Finally, some renal pathology is unique to pregnancy, such as preeclampsia. With good prenatal care, however, most women with these disorders will likely have no long-term sequelae.
PREGNANCY-INDUCED URINARY TRACT CHANGES
Significant changes in both structure and function within the urinary tract during normal pregnancy are discussed in Chapter 4. The kidneys become larger, and dilatation of the right renal calyces and ureters can be striking (Fig. 53-1). Some dilatation develops before 14 weeks and likely stems from progesterone-induced relaxation of the muscularis. More marked dilatation is apparent beginning in midpregnancy because of more distal ureteral compression, especially on the right side (Faúndes, 1998). There is also some vesicoureteral reflux during pregnancy. Because of these physiological changes, the risk of upper urinary infection rises. Also, imaging studies done to evaluate urinary tract obstruction may occasionally be erroneously interpreted.
The 50th, 75th, and 90th percentiles for maternal renal caliceal diameters measured using sonography in 1395 pregnant women from 4 to 42 weeks’ gestation. (Redrawn and modified from Faúndes A, Bricola-Filho M, Pinto e Silva JC: Dilatation of the urinary tract during pregnancy: proposal of a curve of maximal caliceal diameter by gestational age. Am J Obstet Gynecol 178:1082, 1998.)
Evidence of functional renal hypertrophy becomes apparent very soon after conception. Glomeruli are larger, although cell numbers do not grow (Strevens, 2003). Pregnancy-induced intrarenal vasodilatation develops, and both afferent and efferent resistances decline. This leads to greater effective renal plasma flow and glomerular filtration (Helal, 2012; Hussein, 2014). By 12 weeks’ gestation, the glomerular filtration rate (GFR) is already augmented 20 percent above nonpregnant values (Hladunewich, 2004). Ultimately, plasma flow and GFR rise by 40 and 65 percent, respectively. Consequently, serum concentrations of creatinine and urea drop substantively across pregnancy, and values within a nonpregnant normal range may be abnormal for pregnancy (Appendix). Other alterations include those related to maintaining normal acid-base homeostasis, osmoregulation, and fluid and electrolyte retention.
Assessment of Renal Function During Pregnancy
Urinalysis results are essentially unchanged during pregnancy, except for occasional glucosuria. Although protein excretion normally rises, it seldom reaches levels that are detected by usual screening methods. Higby and colleagues (1994) reported 24-hour protein excretion in pregnancy to be 115 mg/d with ...