Maternal obesity and the incidence of large-for-gestational-age newborns in isolated hypothyroxinemia pregnancies: a comparative cohort study.

BACKGROUND

The synergistic impact of isolate maternal hypothyroxinaemia (IMH) and other modulators on fetal growth outcomes is unknown. This study was aimed to determine whether third trimester IMH [free thyroxine level (FT4) below the the 5th percentile and thyroid stimulating hormone (TSH) between the 5th and 95th percentiles] and prenatal body mass index (BMI) jointly increase the risk of large for gestational age (LGA) deliveries.

METHODS

A retrospective analysis was conducted on 11,478 Chinese pregnant women with laboratory data (including thyroid hormone levels and routine biochemical tests) and hospitalization records from a specialized hospital.

RESULTS

The prevalence of obesity (BMI ≥ 30 kg/m) and IMH was 20.1% (2312/11478) and 4.5% (519/11478), retrospectively. Women with obesity had a 6.96-fold greater risk of IMH (95% CI: 4.58, 10.58) and a 5.88-fold increased risk of LGA (95% CI: 4.87, 7.11) than those with normal weight (BMI < 25 kg/m), while women with IMH had a 1.32-fold increased risk of LGA (95% CI: 1.05, 1.65) than euthyroid women. The positive associations of LGA risk with obesity and IMH remained robust in sensitivity analyses conducted among women aged < 35 years, primiparas, and those without pregnancy complications. Compared to euthyroid women with normal weight, women with obesity and IMH had a 7.60-fold higher risk of LGA (95% CI: 5.26, 10.97). Additionally, a significant interaction between BMI categories and IMH on LGA was observed (P < 0.013). Subgroup analyses validated this interaction among women with aged < 35 years, multiparity, and non-pregnancy complications.

CONCLUSIONS

Obesity and IMH in late pregnancy are both associated with an increased risk of LGA newborns, and their coexistence may further amplifies this risk; prenatal BMI and thyroid hormone levels could serve as potential indicators for identifying individuals at elevated LGA risk.