Sufficient iodine intake is needed during pregnancy to ensure proper fetal development. The iodine levels of women in their first trimester of pregnancy in Catalonia are currently unknown. This data would help to determine whether our public health services should establish recommendations or interventions in this line. The aim of this study was to investigate the iodine nutritional status, prevalence of urinary iodine <150 μg/L, and tobacco use in the first trimester of pregnancy in our setting. Cross-sectional study. Data were collected during 2008-2009 from women in their first trimester at the primary care centers of the province of Barcelona (Spain). Pregnant women included in the study completed a questionnaire on eating habits and underwent urinary iodine concentration (UIC) assessment. Nine hundred forty five women completed the dietary questionnaire and urinary iodine testing. Median UIC was 172 μg/L, with 407 participants (43.1%) showing levels <150 μg/L. On multivariate logistic regression analysis, intake of 1-2 glasses of milk per day, OR = 0.636 95% CI (0.45-0.90) or >2 glasses, OR = 0.593 95% CI (0.37-0.95); iodized salt consumption, OR = 0.678 95% CI (0.51-0. 90); and use of iodine supplementation, OR = 0.410 95% CI (0.31-0.54), protected against the risk of UIC <150 μg/L. Simultaneous consumption of iodized salt and milk (≥1 glass/day) showed a larger protective effect: OR = 0.427, 95% CI (0.31-0.54). The median UIC of the pregnant women surveyed indicated an acceptable iodine nutritional status according to the criteria established by the WHO and ICCIDD. The risk of urinary iodine <150 μg/L decreased with simultaneous consumption of milk and iodized salt, similar to the decrease seen with iodine supplementation. The online version of this article (doi:10.1186/s12884-017-1423-4) contains supplementary material, which is available to authorized users.