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The aim of this study was to assess the association between maternal employment and preterm delivery. A nationwide case–control study was conducted in 25 Portuguese public maternities. During a 4-month period, 769 consecutive single spontaneous preterm (<37 gestation weeks) live births and 1,053 term singleton babies born immediately after each preterm, were evaluated. Information was obtained by attending physicians using a questionnaire, with special emphasis on maternal work characterization. Logistic regression odds ratios (OR) were adjusted for maternal age, marital status, education and obstetrical characteristics. Women entering pregnancy while unemployed presented a significantly increased risk of spontaneous preterm delivery (OR = 1.5; 95% confidence interval (CI) 1.18–1.88). Weekly duration of work (<40 versus ≥40 h) had no significant effect on the occurrence of spontaneous preterm (OR = 1.2; 95% CI 0.88–1.54). Unemployed women presented a significant increase in the risk of preterm delivery.