A Multicenter Prospective Randomized Controlled Clinical Trial Comparing Constant (5%) Versus Sequential (5%-2%) Oxygen Concentration Embryo Culture Protocols in Assisted Reproductive Technology

Inclusion Criteria: 1. Female: age ≥20 and ≤40 years, male: age ≥20 and ≤50 years; 2. Proposed 1st or 2nd cycle of IVF or ICSI fertilization; 3. ≥ 4 transferable embryos at oocyte stage. Exclusion Criteria: 1. Diagnosis of abnormal uterine cavity morphology (confirmed by 3D ultrasound or hysteroscopy), including uterine malformations (mediastinal uterus, unicornuate uterus, bicornuate uterus), submucosal uterine fibroids, or uterine adhesions; 2. Patients who are proposed to undergo IVM; 3. Patients who are proposed to undergo PGD/ PGS; 4. Patients with untreated severe hydrosalpinx (confirmed by ultrasound or HSG); 5. Patients with a history of recurrent miscarriage (2 or more previous pregnancy losses, excluding biochemical pregnancies); 6. Patients who plan to freeze whole embryos and are unable to complete a single embryo transfer within six months; 7. Patients with contraindications to assisted reproductive technology and pregnancy, or suffering from diseases that have a definite effect on pregnancy: including uncontrolled hypertension, heart disease with definite symptoms, uncontrolled diabetes, undefined diagnosis of liver or kidney disease or liver or kidney insufficiency, severe anemia, history of previous venous thrombosis, pulmonary embolism or cerebrovascular events, history of malignant tumors, suspected malignant tumors, and undiagnosed abnormal uterine bleeding.