The study compared two methods of preparing the uterus before frozen embryo transfer in naturally ovulating women: the natural mode (following the natural cycle) and the programmed mode (administration of hormones). The study involved 2,185 women in the natural regimen group and 2,191 women in the programmed regimen group. Both groups achieved a similar number of healthy births – 41.6% in both cases. However, the natural regimen resulted in a lower risk of preeclampsia (2.9% vs. 4.6%), fewer early pregnancy losses (12.1% vs. 15.2%), fewer caesarean sections (69.5% vs. 75.6%), and less postpartum hemorrhage (2.0% vs. 6.1%). There were no between-group differences in birth weight or neonatal complications. The disadvantage of the natural regimen was a higher cycle cancellation rate (16.2% versus 11.5%). The conclusion of the study is that the natural regimen is as effective as the programmed regimen in achieving a healthy birth, but with a lower risk of complications for the mother during pregnancy.