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Objective  To evaluate the relation between breech position at term (>37 weeks of gestation) and low maternal fT4 levels during gestation in women not suffering from overt thyroid dysfunction.

Design  A prospective cohort study of pregnant women.

Setting  Community-based study.

Population/Sample  At random selected pregnant women of the general population.

Methods  At antenatal booking, based on thyroid function assessed at 12 weeks of gestation in a large cohort of pregnant women, two groups of participants were defined: women with low fT4 levels—below the 10th centile (n= 135) and women with fT4—between the 50th and 90th centiles at 12 weeks of gestation (n= 135). Women with clinical thyroid dysfunction (fT4 and TSH outside reference range) at 12 weeks of gestation were excluded. Maternal thyroid function (fT4 and TSH) was subsequently assessed at 24 and 32 weeks of gestation. Analysis refers to 204 women who met the inclusion and exclusion criteria and in whom all thyroid parameters were assessed.

Main outcome measures  Fetal presentation (cephalic–breech) at delivery in women with term gestation (>37 weeks of gestation) in relation to maternal thyroid function at 12, 24 and 34 weeks of gestation.

Results  Breech presentation at term delivery was independently related to fT4 levels <10th centile at 12 weeks of gestation (OR = 4.7, 95% CI 1.1–19 [but not to an fT4 level below the 10th centile at 24 and 32 weeks of gestation]) as well as primiparity (OR = 4.7, 95% CI 1.3–15).

Conclusions  Women with hypothyroxinaemia (fT4 level at the lowest 10th centile) during early gestation but without overt thyroid function are at risk for fetal breech presentation at term (>37 weeks of gestation).