FETAL MILD HYDRONEPHROSIS AND CHROMOSOMAL DEFECTS - RELATION TO MATERNAL AGE AND GESTATION

The presence of multiple ultrasonographic abnormalities is associated with a significantly increased risk of chromosomal defects, while for isolated abnormalities, the association is less clear. In a study of 1 ,177 fetuses with mild hydronephrosis at 16-26 weeks of gestation, the fetal karyotype was abnormal in 86 (7.3%) of the cases and the most c ommon chromosomal defects were trisomies 21, 18 and 13. The frequency of chromosomal defects increased with the number of additional abnorma lities and for each chromosomal defect there was a characteristic patt ern of associated abnormalities. However, in the 805 fetuses with appa rently isolated hydronephrosis there were 5 (0.62%) with trisomy 21. O n the basis of the maternal age and gestational age distribution of th e population the expected frequency of trisomy 21 was 0.40%, which was not significantly different from the observed (0.62%). To demonstrate that such a difference is significant, it would be necessary to inves tigate at least 1 million pregnancies. In the meantime, parents could be counselled that the presence of mild hydronephrosis does not increa se significantly the risk that the fetus has trisomy 21. Alternatively , the risk is 1.6 times higher than the maternal age and gestational a ge-related risk.