Context
It remains unclear if migrants have different odds for adverse outcomes associated with gestational diabetes mellitus (GDM).

Objective
We investigated if the associations between GDM and adverse pregnancy outcomes are modified by country of origin and examined the odds of these outcomes according to GDM status and country of origin.

Methods
Data were extracted from a nationwide register-based study of singleton deliveries in Denmark, 2004-2015. We used logistic regression models and tested for interaction.

Results
Among the 710 413 singleton deliveries, 2.6% had GDM and 14.4% were immigrants. Country of origin modified the association between GDM and pre-eclampsia, large for gestational age (LGA), and small for gestational age (SGA) but not between GDM and planned or emergency cesarean section and preterm delivery. GDM increased the risk of pre-eclampsia among women from Denmark (OR 1.28; 95% CI, 1.18-1.39), Lebanon (OR 3.34; 95% CI, 1.35-8.26), and Morocco (OR 2.28; 95% CI, 1.16-6.88). GDM was associated with increased odds of LGA among women from most countries, particularly women from Sri Lanka (OR 4.20; 95% CI, 2.67-6.61), and was associated with reduced odds of SGA in some countries. Compared with Danish-born women with GDM, the odds of LGA were significantly lower and the odds of SGA higher among women with GDM from India, Lebanon, Pakistan, Iraq, and Somalia.

Conclusions
Our study documents that different immigrant groups have higher odds of different GDM-associated adverse pregnancy outcomes and also among countries of origin often grouped together. This highlights the importance of increased awareness to both immigrant background and GDM status in the clinical assessment.