Background and epidemiology: When the Canadian Congenital Anomalies Surveillance Network holds its second annual scientific meeting in Edmonton,

© 2003 Canadian Medical Association or its licensors Background and epidemiology: When the Canadian Congenital Anomalies Surveillance Network holds its second annual scientific meeting in Edmonton, Alta., next week (www.hc-sc.gc.ca/pphbdgspsp/ccasn-rcsac/ccaswkshop_e.html), one of the main agenda items will be abdominal wall defects. This term refers primarily to gastroschisis (characterized by an intact umbilical cord and evisceration of the bowel through a defect in the abdominal wall, generally to the right of the cord, with no membrane covering) and omphalocele (characterized by herniation of the bowel, liver and other organs into the intact umbilical cord, the tissues being covered by membranes unless the latter are ruptured). Although both are congenital defects of the anterior abdominal wall, they differ in several respects. In cases of gastroschisis, a sac is usually absent, associated anomalies (other than intestinal atresia in 25% of cases and cryptorchism in 31%) are rare, the defect occurs to the right of the umbilicus, and the mother is young (less than 25 years). In contrast, in cases of omphalocele, a sac is present, associated anomalies resulting from chromosomal abnormalities (most commonly trisomy 18) are common, the defect occurs within the umbilicus, and the mother is generally older. Obtaining accurate data on the frequency of abdominal wall defects is complicated by the fact that, in a certain proportion of cases, the pregnancy is ended by elective termination. Estimates of the birth prevalence of gastroschisis (1 in 10 000 births) and omphalocele (2.5 in 10 000) in Western countries are comparable to those of Down’s syndrome (14 in 10 000), neural tube defects (6 in 10 000), congenital heart defects (5 in 10 000), orafacial clefts (11 in 10 000) and limb reduction defects (3 to 8 in 10 000). Although the birth prevelance of omphalocele has remained generally stable over the past 20 years, reports from Europe, the United States and Japan suggest that the rate of gastroschisis has increased as much as 10-fold over the past decade. This increase may be partly due to increased detection and ascertainment resulting from increased use of prenatal ultrasonography; however, if this were the only factor, a similar rise in omphalocele rates would be expected. Retrospective analyses of case series of gastroschisis indicate that the risk is greatest for low-income, young mothers who are heavy smokers, who are undernourished and who use overthe-counter medications with vasoactive properties (e.g., pseudoephedrine, phenylpropanolamine, ephedrine, methylenedioxymethamphetamine) in early pregnancy. The cause of gastroschisis is multifactorial and seems to involve vascular disruption of the fetal mesenteric vessels.