Klopper 1965.

Methods	Unit of randomization: pregnancy Method of randomization: random schedules generated by a statistician Timing of randomization: before 10 weeks' gestation Power calculation: no Blinding: yes (double) Number of centers: 2 33 women randomized, 33 women analyzed Source of funding: not stated
Participants	Women who had ≥ 2 miscarriages, no pregnancy beyond 28 weeks' gestation, were < 10 weeks into the current pregnancy and with no other obvious causes of miscarriage
Interventions	50 mg twice daily of oral cyclopentyl enol ether of progesterone Control: placebo Duration: not stated
Outcomes	Miscarriage
Notes	Dates of study: not stated Funding sources: somewhat unclear although the authors acknowledge "the firm of Vister who supplied the steroid and allowed their product to be subjected to a much more rigorous test than is usual in this field." Declarations of interest: not stated
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Random schedules generated by statistician
Allocation concealment (selection bias)	Unclear risk	Given according to a random schedule
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Double‐blinded design ‐ neither participants nor physician knew which of the 2 they were getting.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Not stated but likely blinded, but unlikely that knowledge of assignment would influence outcomes.
Incomplete outcome data (attrition bias) All outcomes	Low risk	None
Selective reporting (reporting bias)	Low risk	None
Other bias	Low risk	None