M. Phoojaroenchanachai et al., Effect of maternal hyperthyroidism during late pregnancy on the risk of neonatal low birth weight, CLIN ENDOCR, 54(3), 2001, pp. 365-370
OBJECTIVE Hyperthyroidism in pregnancy occurs with a prevalence of 0.05-0.2
% and has been shown to affect neonatal outcomes. Fetal weight increases ma
rkedly during the third trimester of pregnancy. This retrospective study wa
s performed to examine the effect of maternal hyperthyroidism during late p
regnancy 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 incl
usion in the analysis. The patients were divided into two groups according
to the maternal thyroid function during the third trimester of pregnancy: h
yperthyroidism (HT; 35 cases) and euthyroidism (ET; 153 cases).
MEASUREMENTS Maternal thyroid function tests were periodically evaluated be
fore 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 bir
th weight (LBW) defined as NBW of lower than 2500 g in HT group was 22.9% w
hich 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 Siri
raj 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 significa
nt difference in the prevalence of LBW infants in ET and control groups. Mu
ltiple logistic regression analyses showed that maternal hyperthyroidism du
ring 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 pregnanc
y independently increases the risk of low birth weight by 4.1-fold, Appropr
iate management of hyperthyroidism throughout pregnancy is essential in the
prevention of this undesirable neonatal outcome.