Introduction Nanoscale particles (NPs) are sized between 1 and 100nm. Based on experimental animal studies on NPs and epidemiological studies on air pollution particles, maternal inhalation of NPs may constitute a hazard to fetal development. It is plausible that NPs may translocate from the respiratory tract to the placenta and fetus, but also adverse effects may occur following a maternal systemic inflammatory response. The aim of this study was to investigate the association between maternal occupational exposure to unintentional nanoscale particles (UNP) during pregnancy and small for gestational age (SGA) within the ELFE cohort. Methods Analyzes were based on data from ELFE cohort that included 18,329 infants born at 33 weeks of amenorrhea or more in France in 2011. Sociodemographic characteristics, detailed information on jobs held by mothers during pregnancy, lifestyle and clinical data were collected through a standardized questionnaire administered at birth. Medical files were consulted to supplement relevant maternal and newborn medical information. Mothers who did not work during pregnancy were excluded from analyses. Maternal occupational exposure to UNP was characterized with a job-exposure matrix by frequency and probability of exposure. Mothers who, during pregnancy, held jobs with a probability of exposure greater than 50% were considered to be exposed. Odd ratios (OR) and 95% confidents intervals (CI) were estimated from logistic regression models adjusted for smoking, marital status, maternal education, monthly household income, high blood pressure during pregnancy, gestational diabetes and alcohol consumption during pregnancy. Results The study sample included 11,224 mother-child pairs. Among mothers who worked during pregnancy, 5.07% were exposed to UNP. Maternal occupational exposure to UNP was significantly associated with SGA (OR=1.63, 95% CI: 1.22, 2.18) in adjusted analyzes. In a sub-analysis conducted in exposed mothers group (n=569), the total duration of work during pregnancy weighted by the frequency of exposure was not associated with SGA (OR=1.02, 95% CI: 0.97, 1.08) in adjusted analyzes. Conclusion This is the first epidemiological study showing a significant association between maternal occupational exposure to UNP and SGA. These results are consistent with those of the toxicological studies that demonstrated the toxicity of NPs on pregnancy. This preliminary research needs to be deepened by further epidemiological studies.Read less <