Thyroid Hormones in Early Pregnancy and Birth Weight: A Retrospective Study

Background/Objectives: Various factors influence intrauterine growth and birth weight. We investigated the possible association between first-trimester pregnancy thyroid functions and birth weight. Methods: A retrospective observational study was conducted from 31 March 2021 to 30 September 2022. Ninety-eight low-risk pregnant women were evaluated. To obtain a homogeneous population, we included only patients with no maternal or fetal comorbidities. TSH, FT3, and FT4 levels in the first trimester and birth weight were recorded and analyzed. Results: The average maternal age was 33.37 years (IQR = 8.67), with a median BMI of 24.98 kg/m² (IQR = 5.66). The average gestational week of pregnancy was 39.40 weeks (IQR = 2.10). Linear regression for birth weight according to TSH effects adjusted by sociodemographic and clinical factors did not show any associations between birth weight and TSH, age, ethnicity, BMI, smoking, or weight gain. Pregnancy duration was associated with birth weight: β = 172.027, p-value < 0.001. A negative significant correlation between FT3 and birth weight was found (beta = -118.901 95% CI: -222.942 to -14.859, p = 0.026). Age, ethnicity, BMI, smoking, and weight showed non-significant associations with birth weight. The FT3 scatterplot showed a negative correlation of FT3 levels with birth weight. Higher values of FT3 were associated with a low birth weight (R = -0.22, p = 0.029). Conclusions: Our study, focused on the first trimester, found a negative correlation between FT3 variations and birth weight.