Study	Reason for exclusion
Berle 1977	This study was about treating established threatened miscarriage, rather than the prevention of miscarriage.
Crowder 1950	This study was about treating established threatened miscarriage, rather than the prevention of miscarriage. This study randomised before accurate eligibility assessment, leading to the exclusion of over 20% of randomised participants. This study compared oestrogen and standard treatment versus standard treatment only. Although some in the oestrogen group also received progesterone, the criteria of selection for such added progesterone was not mentioned. Progesterone dosage was low (30 mg/day) and duration was short (hospitalisation period) whereas oestrogen use continued until the 28th week, hence the authors considered the progesterone component negligible.
Lightman 1999	This study introduced intervention prior to established pregnancy. This study did not have a placebo/no treatment group. This study compared intramuscular progesterone and oestrogen versus vaginal progesterone and oestrogen.
Sathanandan 1991	This study introduced intervention prior to established pregnancy. This study was semi‐randomised and did not assess any of our specified outcomes.