In Belgium, 8% of the newborns are born preterm. There are about 2000 deliveries each year in the University Hospitals Leuven. Between 2 and 5% of all pregnant women are considered to have subclinical hypothyroidism and preterm birth is almost 2-fold higher in women with subclinical hypothyroidism.The incidence of THOP is 30 %. We performed a statistical power calculation with a confidence level of 95%. Given the above data and with the assumption that 50% of the premature babies born to a hypothyroid mothers will develop THOP, we need to include 320 patients who give premature birth to obtain a statistical power of 80%. Therefore, the inclusion time will be 2 years.