Antenatal screening for thyroid dysfunction: pre-term birth, low birth-weight, and growth restriction.

Objective: To assess pre-term birth, low birth-weight and growth restriction according to maternal thyroid screening results and subsequent treatment.

Methods: This is a nonintervention nested case-control study derived from 10,052 asymptomatic women previously screened during the first trimester marker with anti-thyroid peroxidase antibodies, serum thyroid stimulating hormone, and free thyroxine. Screening results had been classified as positive with one or more markers outside the normal range and referred to an endocrinologist. Cases were 512 women with positive results and information on recommended treatment: 204 thyroxine, propylthiouracil or surgery, and 308 no treatment or only iodine. Controls were a sequential sample of 1292 women with negative results. All cases and controls had information on gestation at delivery or birth-weight. Outcome measures were pre-term birth (<37 weeks), low birth-weight (<2.5 kg) and, for singletons, small for gestational age (SGA; <10th percentile).

Results: Among singleton pregnancies, there was a higher prevalence of both pre-term birth (risk ratio (RR) 1.69, 95% confidence interval (CI) 1.21-2.36,  < .002) and low birth-weight (RR 1.72, 95% CI 1.13-2.62,  < .02) in cases compared with controls. An increase in low birth-weight was also present in term pregnancies, but not significant (RR 1.80, 95% CI 0.78-4.14,  = .16), and there was no difference in SGA prevalence (1.24, 95% CI 0.93-1.65,  = .14). Among cases there was no significant difference in these rates according to treatment even after logistic regression, allowing for the individual screening marker levels and maternal weight.

Conclusions: Women with positive thyroid screening results are at increased risk of pre-term birth regardless of thyroid dysfunction or subsequent treatment. An association with low birth-weight is probably secondary to early delivery.