Mild hydronephrosis may be present in upto 90% of pregnancies. The degree of hydronephrosis was determined by maximal calyceal diameter (MCD). The aim of this study is to investigate whether there is a relationship between grade of maternal hydronephrosis and birth weight of the babies. Subjects were examined in three groups: group 1 MCD of 5-10 mm (grade I), group 2 10-15 mm (grade II) and group 3 patients >15 mm (grade III). There were 45, 30, 13 patients in the groups, respectively. Estimated fetal weight (EFW) at the time that hydronephrosis was diagnosed, birth weight and duration of pregnancy were compared. The average birth weight of the babies was not statistically different in the three groups (p > 0.05), but there was a statistically significant difference in fetal weights at the time of diagnosis (p = 0.02). The grade of maternal hydronephrosis does not affect the duration of pregnancy.