Regional variations in the prevalence of gestational diabetes mellitus (GDM) have been found across Denmark. The objectives of this exploratory survey were to evaluate adherence to the national guideline for screening and diagnosing GDM and to identify variations in pre-analytical or analytical factors, which could potentially contribute to variations in GDM prevalence across regions. In a national interview-based survey, obstetric departments and laboratories throughout Denmark handling GDM screening or diagnostic testing were invited to participate. Survey questionnaires were completed through personal interviews. In total, 21 of 22 identified obstetric departments and 44 of 45 identified laboratories participated. Adherence to guideline among obstetric departments ranged 67–100% and uniformity in laboratory procedures was high. However, the gestational age at the time of late diagnostic testing with oral glucose tolerance test (OGTT) varied considerably, with 48% (10/21) of departments testing outside the recommended 24–28 weeks’ gestation. Procedural heterogeneity was most pronounced for the parts not described in current guidelines, with choice of laboratory equipment being the most diverse factor ranging 3–39% nationally. In conclusion, the overall adherence to the national guidelines was high across regions, and obstetric departments and laboratories had high uniformity in the procedures for screening and diagnosing GDM. Uniformity was generally high for procedures included in the guideline and low if not included. However, a high proportion of GDM testing was performed outside the recommended gestational window in late pregnancy, which may be a pre-analytical contributor to regional differences in GDM prevalence.