OBJECTIVE: To study the relationship between maternal thyrotrophin (TSH) and breech presentation at term. DESIGN: Combined data sets of two prospective studies to obtain adequate epidemiological power. PATIENTS: One thousand and fifty-eight healthy pregnant women (58 breech, 1000 cephalic) and 131 women who presented in breech at an obstetrical outpatient clinic. MEASUREMENTS: Maternal thyroid parameters [TSH, free thyroid hormone (FT4), thyroid peroxidase antibody (TPO-Ab)] and foetal presentation were assessed in both groups between 35 and 38 weeks gestation. Power calculations suggested that at least 148 breech cases were required. RESULTS: The characteristics of the women in breech in both samples were similar. Women in breech (n = 58 + 131) had significantly higher TSH (but not FT4) than those (n = 1000) with cephalic presentation (Mann-Whitney U-test, P = 0·003). Different cut-offs were used to define high TSH in the 916 TPO-Ab-negative women with cephalic presentation: the 90th, 95th and 97·5th percentiles were 2·4 mIU/l (n = 149), 2·7 mIU/l (n = 77) and 3·2 mIU/l (n = 37). The prevalence rates of breech presentation in these women were all higher compared to the prevalence of breech in women below these cut-offs (df = 1, P <0·01). The relative risk of the 149 women with a TSH >90th percentile (>2·4 mIU/l) to present in breech was 1·82 (95% CI: 1·30-2·56). CONCLUSIONS: Women with high TSH at end term are at risk for breech presentation. Substantial evidence for a relation between breech presentation and neurodevelopmental delay exists. As high TSH during gestation has also been linked to poor neurodevelopment, the relation between breech presentation and poor neurodevelopment might be thyroid-related.