Maternal thyroid hormone concentration during late gestation is associated with foetal position at birth

Objective To evaluate whether there is an association between maternal thyroid hormone and foetal cephalic head position at term gestation. Context Rotation and flexion of the head enables the foetus to negotiate the birth canal. Low-normal range thyroid hormone concentrations in euthyroid pregnant women constitute a risk of infant motor abnormality. We hypothesized that low normal maternal thyroid hormone levels are associated with increased risk of abnormal foetal position at delivery. Design In 960 healthy Dutch women with term gestation and cephalic foetal presentation, thyroid parameters [foetal T4 (FT4), TSH and thyroid peroxidase antibody] were assessed at 36 weeks of gestation, and related to foetal head position (anterior cephalic vs. abnormal cephalic) and delivery mode (spontaneous vs. assisted delivery). Results Women presenting in anterior position (n = 891) had significantly higher FT4 levels at 36 weeks of gestation than those with abnormal cephalic presentation (n = 69). There were no between-group differences for TSH. Regression analyses indicated that the risk of abnormal head position decreased as a function of increasing FT4 [single odds ratio (OR) = 0·87, 95% confidence intervals (CI) 0·77-0·98; multivariate OR = 0·88, 95% CI 0·72-0·99)]. A similar inverse relationship between maternal FT4 and risk of assisted delivery was obtained (OR = 0·86, 95% CI 0·79-0·95; OR = 0·91, 95% CI 0·84-0·98). Conclusion The lower the maternal FT4 concentration at 36 weeks of gestation, the higher the risk of abnormal cephalic foetal presentation and assisted delivery.