Previous studies have shown that gastrointestinal disease in women during pregnancy may be a risk factor for low-birthweight infants. In the present study, the prevalence of gastroenteritis during pregnancy and its effect on neonatal outcome was examined for each gestational month in 10,597 single-birth mother-infant pairs in the ABIS Project (All Babies in Southeast Sweden). After exclusion of mothers with inflammatory bowel disease, celiac disease, lactose intolerance, or cow's milk allergy, data on the remaining 10,229 mother-infant pairs were compiled. Overall, 32.5% of the mothers suffered from gastroenteritis during pregnancy (95% confidence interval [CI], 32.5-32.5%). Risk factors included young maternal age (P for trend, <0.001), previous infants (P<0.001), work in a pediatric day-care facility (P=0.004), and experience of a major life event (P=0.027). Binary logistic and multiple linear regression analyses were adopted for the following variables of neonatal outcome: birth week, preterm birth (<37 weeks), birthweight, low birthweight (< or = 2,499 g), birth length, cesarean section, and hospitalization in a neonatal care unit. Maternal gastroenteritis during month 4 (-0.18 week; 95%CI=-0.36, -0.01 week), month 5 (-0.30 week; 95%CI=-0.49, -0.11 week), or month 7 (-0. 18 week; 95%CI=-0.35, -0.01 week) of pregnancy was associated with a shorter pregnancy (adjusted for confounders). Gastroenteritis during part of the pregnancy was associated with a shortened pregnancy, but it had no other adverse effects on neonatal outcome. The reduction in the duration of pregnancy is probably of little clinical relevance. These findings should be confirmed in a prospective study.