Objective: evaluate the efficacy of micronized progesterone used to prevent very early and early preterm births of very low and low birthweight infants in the Yaroslavl Region. Subjects and methods. A total of 15,219 women aged 28.0±6.3 years with a singleton pregnancy were followed up in 2014. All the pregnant women underwent transvaginal cervicometry during the second screening study (at 18-21 weeks’ gestation). Three subgroups of the examined patients were formed. Subgroup 1 included 30 pregnant women with a history of miscarriage and with a cervical length of less than 25 mm; Subgroup 2 consisted of 270 pregnant women with an asymptomatic shortened cervix measuring less than 25 mm; Subgroup 3 comprised 339 pregnant women with a cervical length of less than 25 mm and with a diagnosis of threatened premature labor. The patients of each subgroup received micronized progesterone 200 mg intravaginally overnight. A control group consisted of women with a singleton pregnancy who did not use micronized progesterone. The investigators estimated preterm labor rates at less than 36 weeks and 6 days of gestation, at less than 33 weeks, and at less than 28 weeks; as well as the rates of birth of babies weighing less than 1500 and 2500 grams; the incidence of intraventricular hemorrhage, and infant mortality rates in the region. Results. During the prophylactic use of micronized progesterone, the rate of very early preterm birth significantly reduced in the examined groups (p = 0.037). There was a significant decline in the rates of birth of infants weighing less than 1500 g (p = 0.013) and a decrease in that of neonatal intraventricular hemorrhage. In 2014, death rates for babies in the first year of life reduced by 30.1% compared with those in 2012, amounting to 5.8‰. At the same time, the rate of preterm labor at less than 33 weeks of gestation and at less than 36 weeks and 6 days and that of birth of babies weighing less than 2500 g were not significantly different in the control and study groups. Conclusion. Micronized progesterone used in the groups at risk for miscarriage is an effective agent to prevent very early preterm labor and birth of very low and low birthweight infants, neonatal intraventricular hemorrhage, and infant mortality rates. The authors declare no conflicts of interest.