Objectives To examine the incidence of fetal congenital abnormalities in cases of isolated polyhydramnious after exclusion of the common causes of polyhydramnious.

Methods Retrospective analysis of all cases of polyhydramnious over a 7-year period in a busy fetal medicine unit. Cases with structural fetal abnormalities at the initial scan, abnormal glucose tolerance tests, multiple pregnancies, red cell alloimmunisation or fetal infections were excluded from further analysis. Polyhydramnious was defined as amniotic fluid index >95th centile for gestational age.

Results A total of 280 patients had polyhydramnious, of which, 136 met the inclusion criteria of being an isolated finding at the initial scan and negative for the routine tests. 133 babies were live born while there were three intrauterine deaths. There were two neonatal deaths, one as a result of prematurity. A total of 13 babies (9.5%) had congenital abnormalities. These included cases of Down's syndrome (four), cystic fibrosis (five), tracheo-oesophageal fistulae (two), microcephaly (one) and one case of neurofibromatosis. None of these abnormalities were obvious at the first ultrasonographic assessment but most were diagnosed or suspected at follow-up scans. Fetal abnormalities were not related to the severity of the polyhydramnious.

Conclusion Isolated and unexplained polyhydramnious, even if mild or moderate is an independent risk factor for fetal congenital abnormalities. Karyotyping is indicated and follow-up scans are necessary even if the initial scan did not reveal obvious structural abnormalities, as these may become clearer at a later stage of the pregnancy. Postnatal examination of the baby is also recommended.