Introduction Moderate and severe obesity is associated with a high risk of developing gestational diabetes mellitus (GDM). We evaluated screening with a diagnostic oral glucose tolerance test (OGTT) earlier than 28 weeks gestation in women with a Body Mass Index (BMI) >34.9kg/m2.

Methods Women were recruited in the first trimester. Height and weight were measured. Women were booked for a 100g OGTT before 20 weeks, and, if normal, another test at 28 weeks gestation. A postpartum GTT was offered to women with an abnormal test during pregnancy. Clinical and sociodemographic details were collected prospectively.

Results Of the 100 booked for a OGTT before 20 weeks gestation, 92 attended. Of these, 10 women (10.8%) had an abnormal result before 20 weeks with impaired glucose tolerance in 5 cases (5.4%) and GDM in 5 cases (5.4%). Of those with a normal GTT at 20 weeks, 81 attended for a GTT at 28 weeks gestation. A further 4 (4.9%) had impaired glucose tolerance and 4 (4.9%) had GDM. A total of 18 (20.5%) out of the 88 who complied with the screening had an abnormal OGTT.

Conclusion One in five of women screened, who had moderate/severe obesity, had an abnormal OGTT. Earlier screening during pregnancy facilitates earlier interventions to improve carbohydrate balance, which may improve clinical outcomes. However, unless it is repeated at 28 weeks, the diagnosis of GDM may be missed. We suggest that women with a BMI >34.9 kg/m2 should be screened early in pregnancy and, if the test is normal, again at 28 weeks gestation.