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

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