Mancozeb and its main metabolite ethylene thiourea (ETU) may alter thyroid function; thyroid hormones are essential for fetal brain development. In Costa Rica, mancozeb is aerially sprayed at large-scale banana plantations on a weekly basis. The objectives of the study was to (1) evaluate urinary ETU concentrations in pregnant women living nearby large-scale banana plantations; (2) compare their estimated daily intake (EDI) with established Reference Doses (RfDs); and (3) identify factors that predict their urinary ETU concentrations.