<b>Association Between </b><b>Maternal Thyroid Function in Early Pregnanc</b><b> and </b><b>Stillbirth: A Multicenter Prospective Cohort Study</b> <b>in China</b>

<b>Abstract</b><b>Context:</b> The association between maternal levels of thyroid stimulating hormone (TSH), free thyroxine (FT4) and thyroid peroxidase antibody (TPOAb) status and stillbirth has not been clearly established.<b>Objective:</b> We aimed to confirm the association of thyroid function in early pregnancy with stillbirth.<b>Methods:</b> The study was designed as a multicenter prospective cohort study, nested within the China Birth Cohort Study (CBCS). A total of 140,245 pregnant women were enrolled from 9 tertiary maternal and child health care hospitals across China between February 12, 2018 and December 31, 2022.<b>Results:</b> The final analysis included 91,673 pregnant women, and the overall incidence of stillbirth was 0.45%. The results showed that FT4 exhibited a significant positive association with stillbirth risk (aOR = 1.04, 95%CI: 1.01–1.07, <i>P</i> = 0.003), with significant linearity (overall <i>P</i> = 0.025, nonlinear <i>P</i> = 0.397). TSH, TPOAb positivity (&gt;60 IU/L), free triiodothyronine (FT3), total triiodothyronine (TT3), total thyroxine (TT4) showed no significant associations with stillbirth. Overt hyperthyroidism was associated with a 2.13-fold higher stillbirth risk compared to euthyroidism (aOR = 2.13, 95%CI: 1.09–4.14, P = 0.026), and the results revealed significant associations between early pregnancy FT4 levels and stillbirths attributed to umbilical cord and maternal causes using the ReCode classification.<b>Conclusions: </b>Maternal FT4 level in early pregnancy is significantly associated with stillbirth risk, even within the clinically defined normal range. Additionally, overt hyperthyroidism is associated with a 2.13-fold increased risk of stillbirth.