OBJECTIVE

Hypothyroidism in pregnant women has been linked to deviations in birth weight, but associations are not consistent and the role of confounding factors, including maternal body mass index (BMI), is not clear. This study aimed to evaluate the association between maternal hypothyroidism, birth weight of the child, and placental weight.

METHODS

This was a retrospective register-based study of singleton live births in Denmark from 2004 to 2015 (n = 694,734). Small for gestational age (SGA) (<10th percentile), large for gestational age (LGA) (>90th percentile), and placental weight Z-scores were defined according to gestational week at birth and sex of the child. Associations were evaluated using logistic and linear regression, adjusting for potential confounders, which included maternal BMI.

RESULTS

Altogether, 9.8 and 9.9% of pregnant women with no diagnosis of hypothyroidism gave birth to SGA and LGA children. The frequency of SGA was higher among children whose mothers were newly diagnosed with hypothyroidism in pregnancy (12.4%; adjusted odds ratio (aOR) = 1.16 (95% confidence interval (CI): 1.00-1.34)), but not LGA (9.3%; aOR = 1.03 (95% CI: 0.87-1.21)). Children born to mothers with treated hypothyroidism in pregnancy did not have higher frequencies of SGA (aOR = 0.94 (95% CI: 0.86-1.03)) or LGA (aOR = 1.06 (95% CI: 0.98-1.14)). No association between maternal hypothyroidism and placental weight Z-score was found (adjusted beta coefficient: 0.004 (95% CI: -0.016; 0.024)).

CONCLUSIONS

The findings from a large Danish cohort point toward an association between hypothyroidism in pregnancy and lower birth weight of the child, whereas no association with placental weight was found.