Introduction
With the introduction of more efficient cryopreservation techniques, and an increasing number of studies claim better results from frozen-thawed embryo transfer (FET) compared to fresh embryo transfer1-3. As a consequence, FET cycles are on the increase worldwide.
Embryo implantation is a critical step affecting clinical outcomes of in vitro fertilization (IVF). The optimal endometrial preparation for FET is still under discussion. Endometrial preparation can be done in the natural cycles (NC) or with artificial cycles (AC).
A Cochrane review published in 2017 suggested a comparable clinical pregnancy rate (OR 1.06; 95% CI, 0.40-2.80) of different types of endometrial preparation. The quality of the underlying studies was rated as very low4, while live birth and miscarriage rates were not reported. Some studies have shown similar live birth rates and miscarriage rates between NC and AC5,6. In contrast, other studies have reported increased miscarriage rates and lower live birth rates in AC than NC7. In view of these conflicting data, we compared live birth rates and other clinical outcomes following FET conducted using natural cycle to those in which endometrial preparation was achieved via artificial cycles.