The objective of this study was to explore the association between occupational factors and pregnancy outcomes in a prospective cohort of Irish pregnant women.This study has a prospective design.The Lifeways cohort included 1124 pregnant women, 676 of whom delivered a single baby and were working at their first prenatal care visit when they filled in a self-administered questionnaire.Occupational factors were measured using this questionnaire and included eight factors describing job and working conditions. Data including pregnancy outcomes were also obtained from clinical hospital records. Logistic regression analysis was used to adjust for well-known risk factors.Birthweight (<= 3000 g and <= 2500 g), preterm delivery (< 37 gestation weeks) and small-for-gestational-age.Significant associations were found between physical work demands and low birthweight (<= 2500 g) and working with between a temporary contract and preterm delivery. Trends were also observed between working 40 hours or more a week and shift work, and birthweight of 3000 g or less. The study of a cumulative index showed that being exposed to at least two of these occupational factors significantly predicted birthweight of <= 3000 g (OR = 2.44, 95% CI: 1.17-5.08) and of <= 2500 g (OR = 4.65, 95% CI: 1.08-20.07) and preterm delivery (OR = 5.18, 95% CI: 1.00-27.01).Our findings suggest that occupational factors may predict birthweight through their predictive effects on preterm delivery. This is one of the few prospective studies on pregnancy outcomes that include working conditions. As they may be modifiable, occupational factors deserve more attention in relation to birth outcomes.