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.