The reproductive histories of 177 women with Crohn's disease and 84 women with ulcerative colitis were compared with those of healthy control subjects. Inflammatory bowel disease was associated with reduced fertility as measured by number of pregnancies, but measures of childlessness, infertility, fecundability, and methods of birth control suggested that this reduced fertility was the patients' choice rather than a consequence of disease-mediated biological impairments. There was no evidence of increased risk of pregnancy loss, but the risk of preterm birth was significantly elevated for patients with Crohn's disease (odds ratio, 3.1; 95% confidence interval, 1.8-5.4) and for those with ulcerative colitis (odds ratio, 2.7; 95% confidence interval, 1.8-5.4). Several plausible biological mechanisms that could explain the link between preterm birth and inflammatory bowel disease are suggested. These results may be useful when counseling patients who desire pregnancy. Physicians can reassure patients about potential fertility problems but should also encourage close obstetrical monitoring during the third trimester of pregnancy.