Objective: The aim of the present study was to estimate the effect of viral infections on the development of pregnancy complications and on birth outcome. Design: A population-based retrospective study. Setting and Population: 57,231 control pregnancies (without any birth-defects) were analysed in The Hungarian Case-Control Surveillance of Congenital Abnormalities (HCCSCA). Methods: Associations between viral infection exposures in the 1st trimester of pregnancy and pregnancy complications and birth outcomes were analysed using the non-exposure group as reference, adjusting for maternal age, highest education, and maternal tobacco use. Main Outcome Measures: Quantitative variables such as mean maternal age, birth weight and gestational age and categorical variables like pregnancy complications were evaluated in the group of viral infections and control mothers. Results: In total, 2,238 cases with maternal viral infections during pregnancy were identified in the HCCSCA (influenza: 2,016, enterovirus: 48, herpes simplex: 28, hepatitis B: 22, varicella-zoster: 14, respiratory syncytial virus: 11 and unspecified virus infections: 104). The incidences of threatened abortion (OR: 1.3, 95% CI: 1.2-1.5), threatened preterm birth (OR: 1.4, 95% CI: 1.1-1.7) and anaemia (OR: 1.4, 95% CI: 1.3-1.6) were higher in the mothers of cases. The risk of gestational diabetes was lower in the group of viral infections (OR: 0.4, 95% CI: 0.23-0.9). No significant differences have been detected in preterm birth, birth weight or IUGR between the infected and the control groups. Conclusions: The findings of this study suggest that viral infections during pregnancy do not exert a deleterious effect on birth outcomes.