Pregnancy Outcomes in Frozen Embryo Transfers

The primary outcome was clinical pregnancy rate, which was defined as the detection of fetal heart beats using ultrasonographic examinations 35 days after FET. Serum beta subunit of human chorionic gonadotropin (β-hCG) levels were examined using electrochemiluminescence analysis (Cobas e411 System Product, Germany) on day 14 following embryo transfer in all patients. Implantation was defined as a serum quantitative β-hCG ≥ 10 U/L. All pregnancies were followed until 7 weeks after FET. A biochemical loss is defined as a transient but significant increase in serum β-hCG ≥ 10 U/L between days 12 and 20 after embryo transfer without detecting a gestational sac by an ultrasonography. An early abortion is defined as the detection of an empty gestational sac using ultrasonographic examinations before 12 weeks of gestation. Implantation rate is defined as the ration of the number of embryos implanted and the number of transferred embryos.