Some randomized studies have suggested a significant beneficial effect of 17-hydroxyprogesterone caproate in the prevention of preterm labor [1, 2], The therapy in both studies was initiated at approximately 12-16 weeks and continued until labor. We considered the possibility that a deficient proges­ terone effect during the first trimester may lead to a higher rate of preterm deliveries. A retrospective study was thus performed comparing the incidence of preterm versus full-term pregnancies in women with luteal phase defects who were either treated or not treated with proges­ terone supplementation during the first trimester.