This study is a single-center, randomized, controlled prospective study. Those who will recieve hormone replacement therapy-frozen thawed embryo transfer (HRT-FET) are enrolled in the study. To determine the effect of oestrogen dosage reducion on maternal and fetal complications in HRT-FET cycles while maintaining the similar clinical pregnancy outcome in HRT-FET cycles with regular oestrogen dosage.

According to the enrollment and exclusion criteria, the patients were enrolled, and the subjects were randomly divided into two groups by computer randomization. Group A was the reduced oestrogen dosage group (test group). Group B was the regular hormone replacement group (control group).

Inclusion Criteria:

1. Female;
2. Aged between 20 and 40 years old;
3. Frozen thawed embryo transfer is proposed, and the type of transferred embryo is blastocyst (the number of transferred embryos is 1);
4. Body mass index (BMI) ≤ 28 kg/m2, ≥ 18.5 kg/m2;
5. The total number of transfer cycles was < 3;
6. Volunteer to participate in the study and sign the informed consent.

Exclusion Criteria:

1. Patients with chromosomal abnormalities;
2. Contraindications to hormone replacement therapy;
3. Patients with uterine myoma, severe adenomyosis, endometriosis, congenital uterine malformation, endometrial tuberculosis, intrauterine adhesions and other diseases that significantly affect embryo implantation;
4. Participating in other clinical studies.