Associations of early-pregnancy maternal thyroid hormones with fetal growth trajectories

While overt thyroid dysfunction is a known risk factor for fetal growth restriction, the impact of subtle variations in early-pregnancy thyroid hormone levels remains poorly understood. This study investigated whether such variations in women without diagnosed thyroid disease affect fetal growth trajectories. Using a prospective birth cohort at Chongqing Maternal and Child Health Hospital, we constructed three analytical sets based on early-pregnancy measurements of free thyroxine (FT4), thyroid-stimulating hormone (TSH), and thyroid peroxidase antibody (TPOAb). Fetal growth parameters, including biparietal diameter, head circumference, abdominal circumference, femur length, and estimated fetal weight, were repeatedly measured by trained sonographers. Linear mixed models were applied to examine the associations. Among 556 – 601 mother – infant pairs, each interquartile range increase in maternal FT4 was nominally associated with a change in fetal abdominal circumference of − 0.91 mm at 30 weeks and + 0.09 mm at 37 weeks. Higher TSH was nominally associated with a change in biparietal diameter of − 0.08 mm at 30 weeks and − 0.29 mm at 37 weeks. Neither primary association remained statistically significant after false discovery rate correction. In sensitivity analyses, the TSH–BPD association was not observed after restricting TSH concentrations to the cohort-specific 2.5th–97.5th percentile range. No associations were observed for TPOAb positivity. Early-pregnancy FT4 and TSH showed nominal gestational age-dependent associations with selected fetal biometric measures among women without diagnosed thyroid disease; however, none remained significant after false discovery rate correction. These findings are exploratory and require replication to evaluate their potential implications for fetal growth assessment.