According to the patients' age, up to 3 frozen-thawed Embryos are thawed and transferred at cleavage stage. Hormone therapy is continued until pregnancy test (2 weeks after ET) is performed and in case of positive pregnancy, administration of estradiol valerate and progesterone continued by 10 and 12 weeks of gestation. Chemical pregnancy is defined by a rising hCG level in serum without the detection of a gestational sac. Clinical pregnancy is diagnosed by the presence of a gestational sac with fetal heart in vaginal ultrasonography. The miscarriage rate is defined as the loss of pregnancy before 20 weeks' gestational age.