AbstractBackgroundThere is conflicting information regarding the risk of gestational diabetes mellitus in twin pregnancies. This study was conducted to determine whether the risk of gestational diabetes mellitus is higher in pregnant Japanese women with twin versus singleton pregnancy.MethodsThe prevalence of gestational diabetes mellitus was studied in two different populations: 144 589 women registered with the Japan Society of Obstetrics and Gynecology (JSOG cohort) over 3 years between 2007 and 2009 in which patient selection bias was unavoidable; and 430 Japanese women who gave birth at a single centre over 5 years between 2008 and 2012 (single‐centre cohort), consisting of 86 women with twins and 344 women with singletons matched for maternal age and prepregnancy body mass index. The gestational diabetes mellitus was diagnosed on the basis of the previous criteria in the JSOG cohort. The gestational diabetes mellitus was screened in a stepwise method and diagnosed on the basis of the new criteria in the single‐centre cohort.ResultsIn the single‐centre cohort, neither frequency of random glucose level ≥105 mg/dL in the first trimester [9.0% (31/344) vs 5.8% (5/86)], positive result (≥140 mg/dL) on 50 g glucose challenge test in the second trimester [26.5% (90/339) vs 26.7% (23/86)], nor women diagnosed with GDM [8.4% (29/344) vs 9.3% (8/86)] differed between the two groups. The prevalence of hyperglycaemia was higher in singleton than in twin pregnancies in the JSOG cohort (2.6% vs 1.8%, p < 0.001).ConclusionsThe risk of gestational diabetes mellitus may be similar between Japanese women with singleton and twin pregnancies. Difference in the risk of hyperglycaemia in the JSOG cohort may have been due to selection bias. Copyright © 2014 John Wiley & Sons, Ltd.