Assessment, data collection and randomization
Pregnancy was assessed by a blood test to evaluate hCG 12 days after FET and ongoing pregnancy was confirmed by the visualization of a fetal heartbeat during a transvaginal ultrasound at 7 weeks [Randomization took place on day 7 of endometrial preparation with oestradiol for all patients with endometrial thickness above 6.5 mm. The randomization was performed by means of white sealed opaque envelopes with a 1:1 allocation, and the random list was generated with sequential numbers by using STATA version 15.1 (StataCorp, College Station, Texas, USA). The study nurses’ team together with the senior clinicians involved in the study, were in charge of the enrolment, randomization and allocation.