Environmental factors, improved prenatal screening, or poor data collection may explain changing incidence Accurate epidemiological data about the incidence of congenital malformations is essential for studying underlying risk factors, implementing possible preventative measures, and providing services. The data reported by Tan et al in this week's BMJ (p 903) suggest that there have been substantial changes in the incidence of anterior abdominal wall defects in Britain, in particular an increase in gastroschisis (a paraumbilical defect with evisceration of abdominal contents) and a decline in exomphalos (a ventral defect with herniation of the intra-abdominal contents into the base of the umbilical cord and a covering peritoneal membrane).1 Certainly, in our unit over the past few years there has been an increase in the number of fetuses with gastroschisis referred for prenatal diagnosis and delivery, from 24 in the six years to 19912 to 28 in the past three years.3 Some, but not all, other reports have confirmed an increase in incidence of gastroschisis, with a small decline in birth prevalence of exomphalos,4 but the substantial difference in incidence of the two types of defect reported by Tan et al is not supported by …