MacDonald 1972.

Methods	Unit of randomization: pregnancy Method of randomization: unclear Timing of randomization: after they were found to have evidence of hormonal imbalance on cervical mucus smears Power calculation: no Blinding: yes (double) Number of centers: 1 specialized antenatal center 40 women randomized, 0 women excluded, 40 women analyzed Source of funding: grant from N.V. Philips‐Duphar
Participants	Women with ≥ 2 consecutive proven abortions and then subsequent pregnancy with cervical mucus ferning present
Interventions	2 x 5 mg tablets of dydrogesterone tablets 3 times daily, increased to 4 tablets 3 times daily if ferning persisted, no duration specified Control: placebo
Outcomes	Miscarriage
Notes	Dates of study: not stated Funding sources: grant from N.V. Philips‐Duphar Declarations of interest: not stated
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Not stated
Allocation concealment (selection bias)	Unclear risk	Not stated
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Stated double‐blind
Blinding of outcome assessment (detection bias) All outcomes	Low risk	Not stated 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