Through an individual participant data meta-analysis of 19 prospective cohort studies with 47,045 pregnant women, experts examined whether maternal thyroid function test abnormalities and thyroid autoimmunity were risk factors for preterm birth. For women with subclinical hypothyroidism vs euthyroid women, the risk of preterm birth was greater. The risk of preterm birth was 7.1% in women with isolated hypothyroxinemia vs 5.0% in euthyroid women. Each 1-SD higher maternal thyrotropin concentration was related to a greater risk of preterm birth. Thyroid peroxidase antibody-positive women had a greater risk of preterm birth compared to thyroid peroxidase (TPO) antibody-negative women. Subclinical hypothyroidism, isolated hypothyroxinemia, and TPO antibody positivity were significantly linked to a greater risk of preterm birth in pregnant women without overt thyroid disease. Clinical decision-making approaches should acknowledge the potential impairments and advantages of screening programs and levothyroxine treatment during pregnancy.