Background: The socioeconomic conditions have made more job opportunities available to women. This has created interest to conduct studies on the effect of working lifestyle on pregnancy outcomes. Aim: This study was conducted with the aim to assess the relationship between mothers’ working status as a social determinant and the incidence of low birth weight (LBW) of the newborn. Subjects and Methods: This case–control study was conducted on 500 women with normal weight infants (control group) and 250 women with LBW infants (case group) in selected hospitals in Tehran. Data were collected using a researcherâmade questionnaire, designed to assess the effect of mothers’ prenatal lifestyle, as a social determinant, on LBW of the newborn. A section of the questionnaire involved assessment of mother’s working condition in terms of the work environment, activities, and job satisfaction. Data were analyzed using Chiâsquare and logistic regression tests. Results: LBW among employed mothers was 5 times more likely than unemployed ones (odds ratio = 5.35, P < 0.001). Unfavorable work conditions such as humid environment, contact with detergents, and being in one standing or sitting position for long hours were significantly associated with LBW (P < 0.001). Conclusion: The present study showed that unfavorable work conditions were associated with LBW; therefore, they need special attention.