Investigating the Relationship between the Day of Embryo Freezing and Pregnancy Rates

In the last decades, infertility incidence is continuously increasing, reachingto about 10–15% of the couples of reproductive age. As more patients turnto assisted reproductive technologies, there is an ongoing search for the development of new techniques and improvement of the existing ones in the field. Frozen embryo transfer is one of the approaches that are widely used in recent years, because of the advance in the cryopreservation procedures. Still, there is an ongoing debate whether the embryos should be frozen on day 3, 4 or 5, before culture and transfer. The present study is a retrospective analysis of the clinical pregnancy rates in the period of 2018–2021, in “In vitro OB Medical Centre Dimitrov”, depending on which stage the embryo is cryopreserved. Our results point out that there are significantly higher pregnancy rates of day 3 and day 4 frozen embryos, compared to day 5.