Objective-We evaluated the risk of first-trimester exposures among nurses and the risk of preterm birth among participants of the Nurses' Health Study II.Study Design-Log binomial regression was used to estimate the relative risk (RR) for preterm birth in relation to occupational risk factors, adjusting for age, parity, work schedule, physical factors, and exposures to chemicals and x-rays.Results-Part-time work (<= 20 hours a week) was associated with a lower risk for preterm birth [RR=0.7, 95% confidence interval (CI) = 0.6-0.9]. Self-reported exposure to sterilizing agents was associated with an increased risk (RR=1.9, 95% CI = 1.1-3.4). Other exposures, including shift work, physical factors, anesthetic gases, antineoplastic drugs, antiviral drugs, and x-ray radiation were not associated with risk of preterm birth. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.