Gestational diabetes mellitus (GDM) has important maternal and fetal implications. In 2010, the Health Service Executive published guidelines on GDM. We examined the impact of the new guidelines in a large maternity unit. In January 2011, the hospital replaced the 100 g Oral Glucose Tolerance Test (OGTT) with the new 75 g OGTT. We compared the first 6 months of 2011 with the first 6 months of 2010. The new guidelines were associated with a 22% increase in women screened from 1375 in 2010 to 1679 in 2011 (p < 0.001). Of the women screened, the number diagnosed with GDM increased from 10.1% (n=139) to 13.2% (n=221) (p<0.001).The combination of increased screening and a more sensitive OGTT resulted in the number of women diagnosed with GDM increasing 59% from 139 to 221 (p = 0.02).This large increase has important resource implications but, if clinical outcomes are improved, there should be a decrease in long-term costs.