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. 2025 Jul 9;14(4):e250111.
doi: 10.1530/ETJ-25-0111. Print 2025 Aug 1.

Birth weight and placental weight in children born to mothers with hypothyroidism

Maja Hjelm Lundgaard, Niels Henrik Bruun, Stig Andersen, Stine Linding Andersen

Birth weight and placental weight in children born to mothers with hypothyroidism

Maja Hjelm Lundgaard et al. Eur Thyroid J. .

Abstract

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.

Keywords: birthweight; gestation; placental; pregnancy; thyroid.

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Conflict of interest statement

The authors declare that they have no conflicts of interest that could be perceived as prejudicing the impartiality of the research reported.

Figures

Figure 1
Figure 1
Flowchart illustrating the inclusion of pregnant women in the nationwide birth weight (bw) cohort, the nationwide placental weight (pw) cohort, the regional bw cohort, and the regional pw cohort.
Figure 2
Figure 2
Adjusted odds ratios with 95% confidence intervals for small for gestational age and large for gestational birth weight of the child and placental weight according to maternal thyroid peroxidase antibody (TPO-Ab) and thyroglobulin autoantibody (Tg-Ab) positivity in the regional cohorts. The adjusted model included pregnancy week of blood sampling, maternal age, pre-pregnancy body mass index, parity, origin, smoking, diabetes, thyroid hormone treatment in pregnancy, thyroid-stimulating hormone in early pregnancy, and birth year of the child.
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