Objective: Hyperthyroidism in pregnancy occurs with a prevalence of 0.05--0.2% and has been shown to affect neonatal outcomes. Fetal weight increases markedly during the third trimester of pregnancy. This retrospective study was performed to examine the effect of maternal hyperthyroidism during late pregnancy on neonatal birth weight (NBW).
Design: Medical and obstetric records of 293 pregnant women with present and past history of hyperthyroidism were retrospectively reviewed.
Patients: There were 188 records of 181 patients with adequate data for inclusion in the analysis. The patients were divided into two groups according to the maternal thyroid function during the third trimester of pregnancy: hyperthyroidism (HT; 35 cases) and euthyroidism (ET; 153 cases).
Measurements: Maternal thyroid function tests were periodically evaluated before and during the third trimester of pregnancy. Neonatal thyroid function tests and birth weight of the newborn infants were also assessed.
Results: There was no significant difference of maternal age between HT and ET groups mean +/- SD (27.6 +/- 5.5 vs. 29.2 +/- 5.4 years). The NBW of the HT group was not significantly different from that of the ET group (2880 +/- 590 vs. 3019 +/- 426 g). However, the prevalence of infants with low birth weight (LBW) defined as NBW of lower than 2500 g in HT group was 22.9% which was significantly higher than the 9.8% in the ET group (P = 0.039, OR = 2.7, 95%CI = 1.1--7.1) and 9.7% of infants born to healthy mothers at Siriraj Hospital (control group) between 1991 and 1995 (P = 0.01, OR = 2.7, 95%CI = 1.3--6.1). The 90% CI for the true difference between the prevalence of LBW infants born to ET and HT mothers was 0.7--25.4. There was no significant difference in the prevalence of LBW infants in ET and control groups. Multiple logistic regression analyses showed that maternal hyperthyroidism during the third trimester of pregnancy was an independent factor associated with increased prevalence of LBW infants (P = 0.037, OR = 4.1, 95%CI = 1.1--15.0).
Conclusions: Maternal hyperthyroidism during the third trimester of pregnancy independently increases the risk of low birth weight by 4.1-fold. Appropriate management of hyperthyroidism throughout pregnancy is essential in the prevention of this undesirable neonatal outcome.