The association between maternal hydronephrosis and acute flank pain during pregnancy: a prospective pilot-study. 
Maternal hydronephrosis may cause flank pain during pregnancy.  We aimed to investigate the association between maternal hydronephrosis and flank pain intensity.  From 2014 to 2015, all consecutive women with singleton pregnancies, who presented at our tertiary center due to acute flank pain, were prospectively evaluated by renal ultrasonography and pain questionnaires.  A visual analogue scale was used to assess pain intensity.  The study had 90% power to detect a significant correlation between hydronephrosis and flank pain (Spearman's test).  A total of 51 consecutive women with left-sided (13.7%), right-sided (64.7%) or bilateral (21.6%) pain were enrolled.  The mean gestational age of these women, who presented due to their pain, was 27.5 ± 6.8 weeks at the time of consultation.  The mean VAS score was 7.6 ± 2.2.  In 43/51 (84.3%) women, hydronephrosis was found on renal sonograms.  No correlation was found between the grade of hydronephrosis and pain intensity (p = 0.466; r= -0.28).  Women delivered at a mean gestational age of 38.1 ± 2.4 weeks and their infants had a mean birthweight of 3138 ± 677 g. Hydronephrosis is a common finding among pregnant women with acute flank pain.  The grade of hydronephrosis does not affect pain intensity.  This study suggests normal pregnancy outcomes in these women.