Aflatoonian 2018.

Study characteristics
Methods	Multicentre RCT Conducted: in 3 fertility clinics throughout Iran Enrolment: January 2014‐January 2017 Power calculation: stated Randomisation: computer‐generated random numbers in wrapped, unlabeled envelope each holding a unique number Timing randomisation: at day of oocyte retrieval Nature of intervention: day‐2 embryo cryopreservation by means of vitrification Follow‐up: LBR after the first ET
Participants	240 women (121 freeze‐all, 119 control) Inclusion criteria: women with OHSS risk Age between 20‐40 years A number of 14‐ 25 follicles ≥ 12 mm on the day of trigger BMI > 18 and < 35 kg/m2 Exclusion criteria were: women with < 14 and > 25 follicles ≥ 12 mm on the day of trigger, women with a previous history of OHSS development, endocrine disorders and > 40 years of age
Interventions	In the fresh transfer group, 2 embryos of good or excellent quality were transferred 48‐72 h after oocyte retrieval. In the fresh transfer group, 1500 IU hCG was administered on the day of ET. Moreover, progesterone suppositories 400 mg twice daily were administered vaginally, from the day of oocyte retrieval until the observation of fetal heart activity by ultrasound in the 8th week. In the freeze‐all group embryos were vitrified on day 2 after oocyte collection. The subsequent cycle was considered as a study cycle. The endometrium was artificially prepared prior to transfer in freeze‐all group. ET was performed 3 days after the beginning of progesterone administration.
Outcomes	Primary outcome was clinical pregnancy, defined as observation of fetal heart activity by transvaginal ultrasonography 2‐3 weeks after positive β‐hCG. Secondary outcomes included chemical pregnancy, LBR, OHSS development and perinatal data.
Notes	Funding: by Yazd Reproductive Sciences Institute. 2 of the authors reported conflicts of interest because they received unrestricted research grants from MSD, Merck and Ferring, as well as honoraria for lectures. We requested additional information regarding cumulative data from the study authors by email but we did not receive a response.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Computer‐generated random numbers
Allocation concealment (selection bias)	Low risk	Wrapped, unlabeled envelope each holding a unique number
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Blinding of doctors and participants was not possible due to the nature of the intervention.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Outcome assessor blinding was not reported, however primary outcome is not likely to be influenced by lack of blinding.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Data were analysed for all randomised women.
Selective reporting (reporting bias)	Unclear risk	All registered outcomes were reported. Study was registered in a prospective trials register with the trial number: IRCT2016092224512N4. However, the study was registered while recruiting as it is stated in the trials register.
Other bias	Unclear risk	(Cumulative) data per subsequent menstrual or cryo‐transfer cycle not reported (relevant for time‐to‐pregnancy comparison and the related comparison of results after first transfer in frozen group vs results after first 2 transfers in fresh group).