Previous studies have shown that gastrointestinal disease in the mother may be a risk factor for low birth weight infants. The author examined the prevalence of gastroenteritis during pregnancy and its effect on neonatal outcome for each gestational month, 10597 singlebirth mother-infant pairs in the ABIS project (All Babies In Southeast Sweden). Mothers with inflammatory bowel disease, celiac disease, lactose intolerance or cow's milk allergy were excluded (remained 10229 mother-infant pairs with data on gastroenteritis). 32.5% of the mothers suffered from gastroentedtis during pregnancy (95% confidence interval 32.5-32.5%), risk factors included young age (P for trend<0,001), previous infants (P<0.001), work in pediatric day-cure (P=0.004) and exposure to life event (P=0.027). Binary logistic and multiple linear regression analyses were adopted for the analyses of neonatal outcome: birth week, preterm birth (<37 weeks), birth weight, Low Birth Weight (≤2499g), birth length, cesarean section and neonatal hospital care. Maternal gastroentetitis during the 4th (-0.18wk; 95% CI=-0.36, -0.01 wk), 5th (-0.30wk; 95% CI= -0.49, -0.11 wk) or 7th (-0.18 wk; 95% Cl= -0.35, -0.01 wk) month of pregnancy was associated with shorter pregnancy duration (adjusted for confounders). Pregnancy gastroenteritis did not affect birth weight or any other neonatal parameter. Gastroentetitis affects a large proportion of pregnant women, with increased dsk for women with frequent child contact. Gastroenteritis duting part of the pregnancy was associated with shortened pregnancy duration, but it had no other adverse effects on neonatal outcome. The reduction in pregnancy duration is probably of little clinical relevance. The findings of this study need to be confirmed in a prospective study.