Objective:To evaluate the effect of matal employment on birth weight and gestational age. Methods:In this project,1272 pregnant women were recruited from whom referred toTehran hospitals during1 year via randomized sampling.Data were gathered through history taking and medical records.In this study,564 employed women were classified as exposure group and708 housekeepers were as the control group.Chisquare test,t-test,One-wayANOVA and logistic regression were used to analyze data. Results:In this study, mean mother age was25.01 years and mean birth weight was(2884±684) g.The mean birth weight in housekeepers was significantly higher (2991±457 g) than employees(2726±476 g)(P=0.002).The infants’ mean birth weights in farmer were the lowest and in office workers(2831±526 g) was the highest(P<0.01).Gestational age less than37 weeks was more common among employees.The mean birth weight decreased when the working hours increased(P<0.01).In those mothers who were working less than3 months there were a higher mean birth weight(P<0.01).Also the mean birth weight decreased due to the hours of standing during a day.Conclusion:Women employment during pregnancy has an important role in low birth weight and preterm labor compared to those who do not work.