Objective Small for gestational age (SGA) newborns constitute still a major cause of perinatal morbidity and mortality. Overt thyroid disease is a known cause of preterm birth and low birthweight but in its untreated condition it is rare today. In this study, we investigated the possible relation between maternal thyroid function assessed in euthyroid women at each trimester and the incidence of term born SGA neonates. Design A prospective cohort study was performed. Patients Thyroid function was assessed at 12, 24 and 36 weeks gestation in 1051 healthy Caucasian women who delivered at ≥37 weeks gestation. Measurements One-way anova was used to compare mean TSH and FT4 levels between women with SGA neonates and controls. Multiple logistic regression analysis was performed to adjust for known risk factors of SGA. Results Seventy (6·7%) SGA neonates were identified and they were significantly more often born to women with a TSH ≥ 97·5th at first and third trimester. Multiple logistic regression analysis showed that smoking (OR: 4·4, 95% CI: 2·49-7·64), pre-eclampsia (OR: 2·8, 95% CI: 1·19-6·78) and TSH ≥ 97·5th percentile (OR 3·3, 95% CI 1·39-7·53) were significantly related to SGA. Maternal FT4 levels and TPO-Ab status were not associated with SGA offspring. Conclusions Our data show that TSH levels in the upper range of the reference interval at different trimesters (3·0-3·29 mIU/l) are independently related to an increased risk of delivering SGA neonates at term.