Maternal Thyroid Function

Hyperthyroidism is conventionally associated with SGA rather than LGA, making the elevated LGA risk observed at low TSH counterintuitive. Prior meta-analyses found that each 1 SD increase in maternal TSH is associated with approximately 6 g lower birth weight on average. During early pregnancy, before the fetal thyroid matures, the fetus depends almost entirely on maternal thyroid hormones transferred across the placenta. Thyroid hormone is critical for fetal growth and basal metabolic regulation. Maternal hypothyroidism and subclinical hypothyroidism promote fetal hyperinsulinaemia and excessive intrauterine growth via insulin resistance and dyslipidaemia, resulting in LGA. An estimated 30–50% of newborn thyroxine (T4) concentrations derive from maternal sources alone.