Renal and urinary tract disorders are commonly 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. Finally, there may be renal pathology 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 that take place in the urinary tract during normal pregnancy are discussed in Chapter 4 (Urinary System). The kidneys become larger, and as shown in Figure 53-1, dilatation of the renal calyces and ureters can be striking. Some dilatation develops before 14 weeks and likely is due to progesterone-induced relaxation of the muscularis. More marked dilatation is apparent beginning in midpregnancy because of ureteral compression, especially on the right side (Faúndes, 1998). There is also some vesicoureteral reflux during pregnancy. An important consequence of these physiological changes is an increased risk of upper urinary infection, and occasionally erroneous interpretation of studies done to evaluate obstruction.
The 50th, 75th, and 90th percentiles for maternal renal caliceal diameters measured using sonography in 1395 pregnant women from 4 to 42 weeks. (Redrawn from Faúndes, 1998.)
Evidence of functional renal hypertrophy becomes apparent very soon after conception. Glomeruli are larger, although cell numbers do not increase (Strevens, 2003). Pregnancy-induced intrarenal vasodilatation—both afferent and efferent resistance decreases—leads to increased effective renal plasma flow and glomerular filtration (Helal, 2012; Hussein, 2014). By 12 weeks’ gestation, the glomerular filtration rate is already increased 20 percent above nonpregnant values (Hladunewich, 2004). Ultimately, plasma flow and glomerular filtration increase by 40 and 65 percent, respectively. Consequently, serum concentrations of creatinine and urea decrease substantively across pregnancy, and values within a nonpregnant normal range may be abnormal for pregnancy (Appendix, Serum and Blood Constituents). Other alterations include those related to maintaining normal acid-base homeostasis, osmoregulation, and fluid and electrolyte retention.
Assessment of Renal Function During Pregnancy
The urinalysis is essentially unchanged during pregnancy, except for occasional glucosuria. Although protein excretion normally is increased, it seldom reaches levels that are detected by usual screening methods. As discussed in Chapter 4 (Urinalysis), Higby and colleagues (1994) reported 24-hour protein excretion to be 115 mg with a 95-percent confidence level of 260 mg/day. There were no significant differences by trimester. Albumin constitutes only a small part of total protein excretion and ranges from 5 to 30 mg/day. From their review, Airoldi and Weinstein (2007) concluded that proteinuria must exceed 300 mg/day to be considered abnormal. Many consider 500 mg/day to be important with gestational hypertension. ...