['The present study aimed to provide normal reference ranges for decidual thickness measured ultrasonographically in healthy first-trimester pregnancies and to evaluate whether there was a relationship between decidual thickness and serum progesterone levels.', 'Decidual thickness of 173 women with a healthy gestation between 6+0 and 9+6 weeks was measured ultrasonographically and serum progesterone concentration was determined. Distribution of decidual thickness at each gestational week and its relation with serum progesterone levels was evaluated.', 'Mean decidual thickness was 6.8, 5.7, 5.5, and 6.1 mm at 6th, 7th, 8th, and 9th gestational weeks, respectively. Decidual thickness showed a mild negative correlation with gestational week (ρ=-0.207, P=0.006) and a mild positive correlation with serum progesterone concentrations (ρ=0.191, P=0.021).'] Does first-trimester reference range for decidual thickness and its relation to progesterone levels?