Live Birth After Letrozole-stimulated Cycles Versus Hormone Replacement Treatment Cycles for the First Frozen Embryo Transfer in Women With PCOS

Inclusion Criteria: * 1\. women diagnosed with PCOS according to modified Rotterdam criteria; * 2\. Women who are participating in their first cycle of IVF or ICSI. * 3\. Women whose IVF/ICSI ovarian stimulation protocol was either GnRH antagonist protocol or long agonist protocol. * 4\. Women with whole embryos freezing. * 5\. Women aged 20 to 38 years old; * 6\. Women BMI 18 kg/m2 to 30 kg/m2 * 7\. Women with at least one good-quality embryo suitable for transfer, including day 3 cleavage stage embryo with grade 7CI /8CI and day 5 or day 6 blastocyst with grade 4BB or higher. Exclusion Criteria: * 1\. Women who has a history of recurrent spontaneous abortion. * 2\. Women with unilateral/bilateral oophorectomy. * 3\. Women with untreated Hydrosalpinx. * 4\. Women with a uterine cavity abnormality, such as a uterine congenital malformation, untreated uterine septum (except shallow uterine septum), adenomyosis, submucous myoma, or moderate to severe intrauterine adhesions. * 5\. Women with uncontrolled diabetes mellitus, thyroid disease and hypertension. * 6\. Women who are indicated and planned to undergo preimplantation genetic test (PGT).