Study	Reason for exclusion
Ali 2020	Not eligible population.
Beigi 2016	Trial includes both 1st and 2nd trimester treatment and does not subgroup the effects, therefore data cannot be included.
Berle 1977	Combination therapy of progesterone and oestrogen was used in this study.
Berle 1980	Quasi‐randomised trial, study type which is excluded from this review.
Check 1985	No method of randomisation was used for this trial.
Check 1987	It is unclear as to whether there was any randomisation. The authors of this review attempted, but failed, to contact the trial authors.
Check 1987a	It is unclear as to whether there was any randomisation. The authors of this review attempted, but failed, to contact the trial authors.
Check 1987b	It is unclear as to whether there was any randomisation. The authors of this review attempted, but failed, to contact the trial authors.
Check 1995	The intervention considered in this trial is progesterone in association with immunotherapy rather than progesterone alone.
Chi 2019	Progestogen supplementation for luteal phase support.
Clifford 1996	Not an eligible population.
Corrado 2002	Not an eligible population.
Costantino 2016	The intervention is not eligible for inclusion in this review.
Daya 1988	This trial was not a randomised controlled trial.
Devine 2018	Not an eligible population.
El‐Zibdeh 2005	Quasi‐randomised trial, which are excluded from this review.
El‐Zibdeh 2009	Quasi‐randomised trial, which are excluded from this review.
EUCTR2016‐002777‐35‐IT	This trial was terminated early due to difficulty in recruitment. Therefore, no data are available for analysis.
Fuchs 1966	This trial was terminated before the results were of sufficient size to statistically analyse. Therefore, data are incomplete.
Goldzieher 1964	Trials using medroxyprogesterone are excluded from this review.
Govaerts‐Videtzky 1965	No method of randomisation was used for this trial.
Ismail 2018	This publication has been formally withdrawn by the journal.
Johnson 1975	It is not clear from the manuscript when progestogen treatment was commenced. Since the outcome measure reported in this trial was preterm delivery and not miscarriage, treatment was most probably commenced in the 2nd trimester.
Klopper 1965	Trials using progesterone 3‐cyclopentyl enol ether are excluded from this review.
LeVine 1964	Quasi‐randomised trial, which are excluded from this review.
Moller 1966	The intervention is not eligible for inclusion in this review.
Nyboe Andersen 2002	Not an eligible population.
Ozer 2020	Not eligible population.
Porcaro 2015	The intervention is not eligible for inclusion in this review.
Prietl 1992	The active intervention given in this trial was a combination of progestogen and oestrogen.
Pustotina 2018	Progestogen supplementation in 2nd trimester, therefore excluded from this review.
Rehal 2020	Progestogen supplementation in 2nd trimester, therefore excluded from this review.
Reijnders 1988	The trial population is not eligible for inclusion in this review.
Shu 2002	Intervention group administered Chinese herbal medicine (CHM) plus human chorionic gonadotropin and progesterone, compared with multi‐vitamin.
Sondergaard 1985	This trial was conducted to ascertain the efficacy of progesterone to prevent preterm birth rather than miscarriage.
Song 2007	Combination progestogen with vitamin E, therefore excluded from this review.
Souka 1980	Trial includes both 1st and 2nd trimester treatment and does not subgroup the effects, therefore data cannot be included.
Swyer 1953	Quasi‐randomised trial, which are excluded from this review.
Tognoni 1980	No method of randomisation was used for this trial.
Tomic 2015	Progestogen supplementation for luteal phase support, therefore excluded from this review.
Zhang 2000	Combination progestogen with vitamin E, therefore excluded from this review.