Background and AimsPreterm birth (PTB) is the leading cause of perinatal mortality and morbidity, progesterone is one of the main hormones to maintain a normal pregnancy. However, there are still conflicting results regarding using progesterone supplementation to prevent PTB and improve neonatal outcomes. The length of treatment with progesterone supplementation is also one of the challenges ahead, so the present study was conducted to investigate the relationship between the duration of progesterone supplementation treatment and neonatal outcomes.MethodsThis retrospective cohort study was conducted on pregnant women at risk of PTB and who have taken progesterone supplementation. They were asked about the length of treatment with progesterone supplements and finally, the neonatal outcomes of these women were measured.ResultsA total of 265 pregnant women who met the inclusion criteria were included in this study and the subjects were divided into two groups that received progesterone <12 weeks and received progesterone ≥12. In the group of women receiving progesterone with a treatment duration of ≥12 weeks, the rate of preterm labor, respiratory distress syndrome, low birth weight, and the need for hospitalization were significantly lower than in the group receiving progesterone with a treatment duration of ≥12 weeks.ConclusionProgesterone administration for longer than 12 weeks in women at risk of PTB can improve neonatal outcomes.