Table 1. Characteristics of the included studies (N = 8).

Author and year	Country	Study design	Sample size in singleton GDM	Sample size in twin GDM	Study participants	Mean age
Ashwal 2021	Canada	Retrospective cohort	1893	180	All women with a singleton or twin pregnancy (with outcome reported separately for GDM patients with singleton or twin pregnancy) who were followed up at a single tertiary referral center (Sunnybrook Health Sciences Center, Toronto, Ontario, Canada) between January 1, 2011, and April 31, 2020	S = 34.6 T = 35.1
Guillén-Sacoto 2017	Spain	Retrospective observational	122	62	GDM pregnancies, women were followed throughout their pregnancy, then underwent postpartum examination between January 1991 and December 2015.	S = 35.5 T = 35.8
Guillén-Sacoto 2018	Spain	Retrospective observational	240	120	Pregnant women with GDM: 120 pregnancies twins and 240 singleton pregnancies as controls	S = 35.1 T = 35.3
Hiersch 2019	Canada	Retrospective cohort	16731	649	All twin and singleton live births in Ontario, Canada, 2012–2016 with pregnancy outcomes compared between women with vs without gestational diabetes mellitus, analysed separately for twin and singleton births	S = 33.1 T = 34
Lai 2015	Canada	Retrospective analysis	18137	805	Singleton and twin GDM pregnancies resulting in live/still births in Alberta	NR
Sheehan 2019	Australia	Retrospective cohort	1370	39	All women who delivered at the Royal Women’s Hospital, Melbourne, between January 2016 and December 2017 with outcome reported separately between GDM singleton and GDM twin pregnancy	NR
Weiner 2018	Israel	Retrospective analysis	228	114	All deliveries of singleton and twin pregnancies complicated by GDM at a single university-affiliated hospital between 1/2008-10/2016	S = 32.3 T = 31.9
Xue 2019	China	Retrospective analysis	572	286	Twin and singleton pregnancies complicated by GDM, which delivered in Peking University First Hospital from January 1st, 2012—December 31st, 2017	S = 32 T = 33

S-Singleton pregnancy; T-Twin pregnancy; NR-Not reported