Background. While monitoring birth defects in a registry, statistically significant increases in prevalence occasionally occur. In the European Registration 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 provinces of The Netherlands. Methods. A stepwise enquiry into this increase, which included checking for misclassification and change in coding and ascertainment when necessary, was done. All cases of omphalocele and associated or similar birth defects registered at the EUROCAT registry were retrieved and if necessary recoded. Results. This study showed that the increase reported previously was not a true time trend. A few cases of e.g. diastasis recti and trisomy 18 were misclassified. 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 isolated omphalocele, but the numbers are small. Conclusions. The stepwise enquiry described should be a standard procedure after 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 signals.