Introduction Maternal obesity is a recognised risk factor for gestational diabetes mellitus (GDM). Maidstone and Tunbridge Wells' trust protocol originally screened all women with a body mass index (BMI) ≥35 with an oral glucose tolerance test (OGTT). Recently, guidelines were unified with National Institute for Health and Clinical Excellence recommendations, suggesting testing all women with a BMI ≥30.

Aim To determine if the prevalence of GDM in women with BMI 30–34 justifies screening this subgroup with an OGTT.

Method The OGTT results for 139 women with BMI ≥30 over 6 months were obtained from the diabetic nurse's notes. Patient notes for women with a positive OGTT were obtained to determine additional risk factors for GDM for which they would have been tested with OGTT regardless of their BMI.

Results 56/139 patients had a BMI 30–34 and 83/139 patients had a BMI ≥35. Of patients with BMI 30–34, 5/56 (8.9%) were positive for GDM, of which 4/5 (80%) had additional risk factors for GDM. Of patients with BMI ≥35, 8/83 (9.6%) had GDM, of which 4/7 (57.1%) had additional risk factors for GDM.

Conclusion The change in guideline picked up only 1/56 (1.8%) new cases of GDM, hence screening in the BMI 30–34 subgroup was associated with a low pick up rate in this population. The argument for raising the threshold for screening to BMI ≥35 is reasonable and supports the targeting of limited resources to the more at risk group.