Swyer 1953.

Methods	Unit of randomization: pregnancy Method of randomization of allocation: there were 2 centers in this study. 1 allocated by alternation. It was stated in the paper that the other center used "randomisation". However, method of randomization was not given. Timing of randomization: before 12 weeks' gestation Blinding: unclear Power calculation: no Number of centers: 2 113 women were enrolled, 0 women were excluded, 113 women were analyzed Source of funding: not stated
Participants	Women having had ≥ 2 consecutive miscarriages before 12 weeks' gestation Exclusions: women with any other known complicating factor (positive Wassermann reaction, extensive cervical tear, uterine malformation, associated medical disease, etc) Age: not stated Location: London, UK Timing and duration: not stated
Interventions	6 x 25 mg progesterone pellets inserted within the gluteal muscle either: as soon as pregnancy was confirmed; or not later than 10th week of gestation; or not later than the earliest previous miscarriage. Control: no treatment Duration: unclear
Outcomes	Miscarriage Preterm delivery Stillbirth Live birth rate
Notes	Dates of study: not stated Funding sources: stated that an author was grateful to Organon Laboratories, Ltd for "generous supplies of progesterone implants" Declarations of interest: not stated but it did state that "one of us (D.D.) is grateful" to Dr. Tindall of Organon but no details of why only that author was grateful were given.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Not stated
Allocation concealment (selection bias)	High risk	There were 2 centers in this study. 1 allocated by alternation. It was stated in the paper that the other center used "randomisation"
Blinding of participants and personnel (performance bias) All outcomes	Low risk	Not stated but unlikely that knowledge of assignment would influence outcomes
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 noted

hCG: human chorionic gonadotropin;IM: intramuscular; IU: international unit: LBW: low birthweight; NIHR: National Institute for Health Research; PPROM: preterm premature rupture of the membranes; SGA: small‐for‐gestational age