Selection bias in teratology information service pregnancy outcome studies

Background: Pregnancy outcome studies conducted through Teratology Informat ion Services (TIS) rely on volunteer subjects, If these subjects tend to ha ve different risk profiles than the population from which they are drawn, t he results of TIS studies may have limited generalizability. Methods: We selected all subjects who enrolled in the California Teratogen Information Service (TIS) pregnancy outcome study for prenatal exposure to carbamazepine or valproic acid between 1990 and 1997 and who received prena tal care through Kaiser Permanente of Southern California (n = 13). We comp ared these subjects to Kaiser patients identified through the Maternal Seru m Alpha Fetoprotein Program with exposure to carbamazepine or valproic acid but who had not enrolled in the CTIS project. The controls were matched by Kaiser location and pregnancy year using a 2:1 ratio (n = 26). Medical rec ords were reviewed and the prevalence of 14 pregnancy risk factors was comp ared between the two groups. Results: There were no significant differences between the groups on any on e risk factor; however, a notably higher proportion of women who did not en roll in the CTIS study used tobacco or had a positive family history of con genital anomalies. Conclusions: Although the sample was small, and results may not apply to ot her exposures in different health care environments, these data provide som e evidence that women who enroll in TIS pregnancy outcome studies do not ha ve a substantially different pregnancy risk profile than women who do not. Efforts to address possible selection bias should be incorporated in future TIS study design. (C) 2001 Wiley-Liss, Inc.