Levy 1991.

Methods	Prospective randomised crossover study
Participants	George Washington University Medical Center, Washington, DC As stated in the abstract, 31 IVF‐ET candidates with regular ovulatory menstrual cycles and no male factor have enrolled thus far
Interventions	Natural cycle versus stimulated cycle IVF In the natural cycle, 4000 IU hCG was given in an effort to precede the endogenous LH surge In the stimulated cycle, luteal phase initiated GnRH suppression was followed by human menopausal gonadotropin (10.000 IU) administration
Outcomes	Pregnancy rates, cancellation rates, oocyte retrieval and fertilisation rate
Notes	Stated as ongoing. Attempts to contact any of the authors failed.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Does not state method of randomisation. No further information obtained
Allocation concealment (selection bias)	Unclear risk	Method not stated
Blinding of participants and personnel (performance bias) Objective outcomes	Low risk	The outcomes are not likely to be influenced by any lack of blinding
Blinding of outcome assessment (detection bias) All outcomes	Low risk	The outcome measurement is not likely to be influenced by any lack of blinding
Incomplete outcome data (attrition bias) All outcomes	Low risk	Thirty‐one patients included, 16 patients underwent natural cycle and 13 underwent the stimulated cycle; 2 patients are missing; 94% of participants included in analysis
Selective reporting (reporting bias)	Unclear risk	No protocol available
Other bias	Low risk	No other sources of bias were found