Breech presentation at term is the most common abnormal foetal presentation and is associated with neonatal and maternal morbidity and mortality. The incidence of breech presentation decreases with term of gestation from approximately 16% at 32 weeks to 3-5% at 40 weeks gestation.1 Many aetiological factors associated with breech presentation have been described previously, including, among others, prematurity, low birth weight, primiparity and smoking during pregnancy.2-5 As these factors explain only 15 % of the variance of breech presentation,6,7 further research into the mechanisms of breech presentation is needed.8 Ample evidence suggests a relationship between (sub)clinical thyroid dysfunction during gestation and impaired obstetrical outcome.9-15 However, these studies mostly refer to overall impaired obstetric outcome with little emphasis on specifics including foetal presentation at term. A previous study of our group suggested a relationship between low maternal FT4 at 12 weeks gestation and breech presentation.16 The number of breech cases, however, was very low due to the small sample size, and the sample was highly selected according to first trimester FT4 levels, which prevented us from looking at other thyroid parameters such as TSH. The current study examines whether maternal thyroid hormone function during gestation is related to foetal position in a large sample of pregnant women from the general population, who were prospectively followed during gestation. The primary outcome measure was the relationship between maternal thyroid function and breech presentation at term. Over a period of 2 years, 1507 Dutch Caucasian pregnant women in five community midwifery practices, living in and around the city of Eindhoven (The Netherlands), were invited to participate at the time of their first antenatal visit at 12 weeks gestation. Seventy-nine percent (n = 1190) of the women consented to participate. Non-responders did not differ from the responders with regard to age, parity, or educational level. Women on thyroid medication (n = 10), those with known clinical hyperthyroidism (n = 8) or hypothyroidism (n = 2) at screening, those who became pregnant after hormonal stimulation (n = 8), those with multiple pregnancy (n = 8), as well as women with Type 1 diabetes (n = 5) were excluded. Throughout gestation, a decrease in mean FT4 was paralleled by an increase in mean TSH (table 1). The number of women with elevated TPO-Ab concentrations decreased towards term. Table 2 shows the differences in thyroid parameters between the 58 women (5.5%) who presented in breech position at term versus the remaining 1000 women who presented in cephalic position. At 36 weeks gestation, women with foetuses in breech position had significantly higher TSH concentrations compared to those with foetuses in cephalic position (Mann-Whitney U-test, Z = 2.7, p = 0.007) whereas there were no differences in TSH concentrations at 12 and 24 weeks gestation. FT4 was not significantly related to breech presentation at any trimester.