Study	Reason for exclusion
Abbott 2012	The comparison was not relevant (progesterone vs cerclage)
ACTRN12616000875404	Appears to be trial registration for a study examining effect of cervical pessary on preterm birth in twins, so not looking at progesterone
Borna 2008	No multiple pregnancies included
Breart 1979	2 different types of progesterone compared
Brenner 1962	Women in this study were already at term (36 ‐ 38 weeks) and no separate data for multiples
Chandiramani 2012	Brief abstract ‐ comparison of progesterone with cerclage
Coomarasamy 2015	Women included in this study were at risk of preterm birth because of recurrent miscarriage and not because of multiple pregnancy
Facchinetti 2006	This study included singletons only
Grobman 2013	This study included singletons only
Gyamfi‐Bannerman 2015	It was not clear that this was a randomised trial
Hauth 1983	This study included singletons only
Hobel 1986	It was not clear that this was a randomised trial
Ionescu 2012	Comparison of progesterone versus cerclage ‐ not a relevant comparison for this review
Johnson 1975	The criteria for high risk of preterm birth in this study did not include multiple pregnancy in current pregnancy
LeVine 1964	This was not a randomised controlled trial. Alternate allocation
Manuck 2008	Secondary analysis
Mardy 2016	Abstract only, secondary analysis
Martinez de Tejada 2014	This study included singletons only
McKay 2014	This is secondary analysis for a trial that excluded multiple pregnancies
Meints 2016	This study examines the effects of type of conception on outcomes in twins
Meis 2003b	This study included singletons only
Moghtadei 2008	This is a study focusing on women at risk of preterm birth because of age
NCT00099164	Insufficient information to assess eligibility for inclusion
NCT02350231	This is a trial registration for a trial that has now been completed. The study examined relevant progesterone versus tocolytics and was aimed at delaying birth in preterm labour
NCT02623881	Trial registration for a study comparing progesterone with pessary device (not relevant comparison)
O'Brien 2009	This study included singletons only
Palacio 2013	This study included singletons only
Papiernik 1970	This was a study of women in labour (or with symptoms of preterm labour)
Rozenberg 2007	This study included singletons only
Rust 2006	Trial of progesterone versus cerclage which is not a relevant comparison for this review
Suvonnakote 1986	It was not clear that this was a randomised controlled trial
Turner 1966	This was not a randomised controlled trial (alternate allocation)
Walch 2005	This study examines the use of progesterone to prevent miscarriage