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KEY POINTS
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Key Points

  • Minimal fetal hydronephrosis is defined as anterior posterior renal pelvic diameter >4mm and <9 mm.

  • Minimal fetal hydronephrosis is associated with a slightly increased risk of aneuploidy.

  • 90% of cases will resolve on their own.

  • Newborns with persistent hydronephrosis should be treated with prophylactic antibiotics until postnatal urologic evaluation.

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CONDITION
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Hydronephrosis is the most common abnormality reported on prenatal sonographic screening (Thomas, 1990; Blyth et al., 1993). The vast majority of cases are mild, so-called physiologic hydronephrosis, which are of no clinical significance. Numerous theories have been proposed to try to account for this common finding. In the past, one popular theory was that mild fetal hydronephrosis resulted from changes in maternal hydration. However, Hoddick et al. (1985) demonstrated that the degree of maternal hydration had no significant influence on fetal urinary tract dilation.

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These findings were subsequently confirmed by Allen et al. (1987). Other potential causes suggested for mild dilation of the fetal urinary tract include transient obstruction, compression by fetal vessels crossing the ureter, vesicoureteral reflux, and natural kinks and folds in the ureter that may occur during development, hyperfiltration of fetal kidneys, or the influence of metabolic or hormonal factors (Homsy et al., 1986; Najmaldin et al., 1990; Zerrin et al., 1993). The hormonal milieu of the fetus may influence the renal pelvic diameter. Maternal hydronephrosis commonly occurs during pregnancy because of the influence of progesterone, a known smooth muscle relaxant. It has been suggested that maternal progesterone may also be responsible for mild fetal upper urinary tract dilation (Cendron et al., 1994).

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Distinguishing physiologic fetal renal pelvic distention from significant or pathologic hydronephrosis is a challenge that requires accurate prenatal sonography and follow-up evaluation. Renal pelvic distention may range in anterior/posterior (A-P) diameter from 3 to 11 mm in up to 18% of normal fetuses studied after 24 weeks of gestation (Hoddick et al., 1985). Because fetal hydronephrosis is so common, Arger et al. (1985) proposed criteria to help distinguish abnormal renal pelvic dilation. They suggested that a pelvic diameter of > 10 mm or a ratio of the A-P pelvic diameter to the A-P renal diameter > 0.5 indicated significant fetal hydronephrosis (Figure 80-1A and B). These criteria were subsequently modified by addition of caliectasis as an additional indicator of significant hydronephrosis (Kleiner et al., 1987). This study suggested that caliectasis might be an even more sensitive and reliable indicator for predicting pathologic hydronephrosis than simple pelviectasis (Figure 80-1A). Renal pelvic dilation less than these criteria for pathologic hydronephrosis is considered minimal fetal hydronephrosis. Morin et al. (1996) defined minimal hydronephrosis as renal pelvic dilation > 4 mm but < 10 mm in a fetus that was less than 24 weeks of gestation.

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Figure 80-1

A. Prenatal sonographic image demonstrating minimal fetal hydronephrosis on the left with an A-P pelvic diameter of 7 mm. The right kidney demonstrates caliectasis even though ...

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