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 of pyelonephritis. Finally, there may be complications unique to pregnancy such as preeclampsia. With good prenatal care, most women with these disorders will likely develop no long-term serious consequences.
Significant changes in both structure and function that take place in the urinary tract during normal pregnancy are discussed in Chapter 5, Urinary System. The kidneys become larger, and as shown in Figure 48-1, 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 muscular layers. More marked dilatation is apparent beginning in midpregnancy because of ureteral compression, especially on the right side (Faúndes and associates, 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. (Reprinted from American Journal of Obstetrics & Gynecology, Vol. 178, No. 5, A Faúndes, M Bricola-Filho, JC Pinto e Silva, Dilatation of the urinary tract during pregnancy: Proposal of a curve of maximal caliceal diameter by gestational age, pp. 1082–1086, Copyright 1998, with permission from Elsevier.)
Evidence of functional hypertrophy becomes apparent very soon after conception. Glomeruli are larger, although cell numbers do not increase (Strevens and colleagues, 2003). Pregnancy-induced intrarenal vasodilatation increases effective renal plasma flow and glomerular filtration. By 12 weeks' gestation, the glomerular filtration rate is already increased 20 percent above nonpregnant values (Hladunewich and colleagues, 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 in pregnancy (see Appendix). 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. Higby and colleagues (1994) reported 24-hour protein excretion to be 115 mg with a 95-percent confidence level at 260 mg/day (see Appendix). 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 ...