Background Thyroid hormones play an essential role in placentation and fetal growth and development. The aim of this study was to examine associations of maternal thyroid function with birth weight.

Methods We performed a systematic literature search to identify potential cohorts, and subsequently collaborated with the primary authors to collect individual-participant data. Eligible studies were prospective studies with data available on maternal thyroid function during pregnancy and birth weight. Main outcomes were small or large for gestational age (SGA or LGA, respectively) and birth weight (grams). Mixed-effects models were used to study predefined associations to account for between-study heterogeneity.

Results The study population comprised 46,599 mother-child pairs from 19 cohorts with a median gestational age at blood sampling of 13 weeks (95% range: 7 to 39·7). Maternal subclinical hypothyroidism was associated with a higher risk of SGA compared to euthyroidism (odds ratio (OR) 1·20 [95% CI 1·00-1.44], P=0·04) and lower mean birth weight (-34·5g [95% CI -58·6 to -10·4], P=0·005). Isolated hypothyroxinaemia was associated with a lower risk of SGA (OR, 0·71 [95% CI 0·55 to 0·92], P=0·01) and higher mean birth weight (difference, 42g [95% CI 14 to 70], P=0·003). Each 1-SD higher maternal TSH concentration was associated with lower birth weight (-5·4g [-9·4 to -1·4] per SD, P=0·008). Each 1-SD higher FT4 concentration (-21g [95% CI -25 to -17] per SD, P<0·001), with a higher effect estimate for measurement in the 3^rd trimester compared with the 1^st or 2^nd trimester. There was no association of thyroid function with LGA.

Conclusion These results advance our understanding of the complex relationships between maternal thyroid function and foetal outcomes, and should prompt careful consideration of potential risks as well as benefits of levothyroxine therapy during pregnancy.