This study aims to evaluate the efficacy of atosiban in patients with previous pregnancy failure and abnormal uterine contractions during the peri-embryo transfer period in assisted reproductive technology. The main questions it aims to answer are:

• Does the use of atosiban in patients with a history of implantation failure and abnormal uterine contractions affect the live birth rate in single blastocyst transfer cycles (fresh or frozen-thawed embryo transfer)?
• Evaluate the differences in efficacy of atosiban regarding perinatal complications and neonatal outcomes, as well as differences in safety concerning miscarriage rates and ectopic pregnancy rates.

Researchers will compare atosiban with placebo (a similar substance containing no active medication) to determine whether atosiban is effective in improving live birth rates from single blastocyst transfer cycles (fresh or frozen-thawed embryo transfer) in patients with a history of implantation failure and abnormal uterine contractions.

• Participants allocated to the Atosiban group will be administered Atosiban (37.5 mg/5 mL, Tractocile®, Ferring Pharma, Geneva, Switzerland) as an intravenous (IV) bolus of 6·75 mg/0.9 mL in 1 minute at 30 minutes prior to the ET procedure, followed by an IV infusion at a rate of 18 mg/h for 1 hour; after 1 hour, the dose of Atosiban will be reduced to 6 mg/h, with a total dose of 37.5 mg. Participants allocated to the placebo group will receive identical-looking saline infusions for the same duration.
• One hour post-embryo transfer, all randomized participants will be invited to have a reevaluation of uterine wave patterns via ultrasound, performed by the same examiner.
• Women who had a clinical pregnancy will be subsequently contacted by nurses to document pregnancy events and outcomes at 12 and 24 weeks of gestation, completion of pregnancy, with a follow-up approximately 6 weeks post-delivery. Those participants who had a negative pregnancy test will not be followed up any further as part of this trial.

Participants/eligibility criteria

Infertile women who fulfill all of the following inclusion criteria, based on the results and findings of the Atosiban 1 study are eligible for participation:

1. Scheduled for single blastocyst-stage embryo transfer, either in a fresh or frozen transfer cycle;
2. Have a history of one or more episodes of implantation failures in previous embryo transfer cycles;
3. Age between 20 and 40 years (inclusive);
4. Exhibit abnormal uterine peristalsis on transvaginal ultrasound performed on the morning of the scheduled blastocyst transfer, defined as either: high-frequency peristalsis (>4 contractions per minute), or a negative wave pattern (fundus-to-cervix direction) at any frequency.
5. Each woman could only participate in one transfer cycle.

Exclusion criteria

1. Immunologic or endocrine disorders including thyroid dysfunction, antiphospholipid syndrome, hyperprolactinemia and other severe systematic diseases (e.g., hypertension, diabetes, etc.)
2. Uterine malformations (e.g., intrauterine adhesions; unicornuate, bicornuate or septate uterus)；
3. Untreated hydrosalpinx or endometrial polyp；
4. Cycles that included in-vitro maturation or oocyte donation；
5. Prior participation in Atosiban 1 study；
6. Current participation in any other clinical trial；
7. Inability to provide informed consent to the study.