Ferraretti 1999

Methods	Single‐centre randomised controlled trial Conducted: in Italy, from January 1996 until July 1997 Power calculation: not reported Randomisation: allocation was performed with sealed envelopes, timing of randomisation was not reported. Nature of intervention: slow freezing Follow‐up: until no cryopreserved embryos were left or delivery of child
Participants	125 women (58 freeze‐all, 67 control) Inclusion criteria: all women with a high level of oestradiol the day of hCG administration (oestradiol ≥ 1500 pg/mL or ≥ 5.500 mmol/mL (conversion factor to SI unit 53.671)) and a high number of retrieved eggs (≥ 15 oocytes)
Interventions	Intervention: zygotes were cryopreserved, 3 or 4 zygotes were thawed and cultured for 36 to 40 h before embryo transfer. If 2 or more zygotes did not cleave 24 h after being cultured, 1 or 2 additional zygotes were thawed. Control: zygotes were cultured for a subsequent 48 h, 3 or 4 fresh embryos were transferred, surplus embryos were cryopreserved.
Outcomes	Clinical pregnancies: gestational sac and foetal heartbeat by ultrasound
Notes	Funding was not reported. Additional information was obtained from the authors by email.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Random sequence was used, but it is unclear whether envelopes were opaque and sequentially numbered.
Allocation concealment (selection bias)	Low risk	Allocation concealment was performed with sealed envelopes.
Blinding of participants and personnel (performance bias) All outcomes	Unclear risk	Blinding of doctors and participants was not possible due to the nature of the intervention. Blinding of doctors to interim analyses of outcomes of the study was not reported. Blinding of investigators was not reported (which is relevant for determining end of study), therefore judged to be unclear risk.
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	No protocol available.
Other bias	High risk	No power calculation reported. Unclear what determined the end of study. (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).