Artefactual increasing frequency of omphaloceles in the northern Netherlands: lessons for systematic analysis of apparent epidemics

Background While monitoring birth defects in a registry, statistically sign ificant increases in prevalence occasionally occur. In the European Registr ation Of Congenital Anomalies (EUROCAT) in the Northern Netherlands 20 000 births are monitored every year. For omphaloceles, a steady increase in the prevalence from 0.86 per 10 000 live- and stillbirths in 1981-1983 to 3.11 per 10 000 live- and stillbirths in 1994 was seen in the three northern pr ovinces of The Netherlands. Methods A stepwise enquiry into this increase, which included checking for misclassification and change in coding and ascertainment when necessary, wa s done. All cases of omphalocele and associated or similar birth defects re gistered at the EUROCAT registry were retrieved and if necessary recoded. Results This study showed that the increase reported previously was not a t rue time trend. A few cases of e.g. diastasis recti and trisomy 18 were mis classified. The prevalence in more recent years is comparable with that in the rest of Europe, whereas it used to be lower. There was an increase in i solated omphalocele, but the numbers are small. Conclusions The stepwise enquiry described should be a standard procedure a fter noticing an increasing prevalence in a registry. A better subdivision, e.g. in isolated cases versus children with multiple congenital anomalies, before monitoring can contribute to a lower number of false positive signa ls.