To measure the impact of a “Preventive Letter” designed to encourage the return of gestational diabetes mellitus (GDM) mothers to follow up visit after delivery, in the context of a worldwide concern about low return rates after delivery of these patients. Mothers with GDM require medical evaluation and an oral glucose tolerance test (OGTT) 6 weeks after delivery, in order to: [a] confirm remission of GDM and [b] provide advice on the prevention of type 2 diabetes. In the year 2003 we developed a “Preventive Letter”, containing three aspects: [a] current treatment, [b] suggested management during labor, and [c] a stapled laboratory order for OGTT to be performed 6 weeks after delivery. The return rate after delivery was assessed in two groups of GDM mothers: [a] “Without Preventive Letter” (n = 253), and “With Preventive Letter” (n = 215). Both groups, similar with respect to age (33.0 ± 5.4 and 32.3 ± 4.9 years respectively, p = 0.166) and education time (14.9 ± 1.8 and 15.0 ± 1.8 years respectively, p = 0.494), showed a significant difference in the 1-year return rate after delivery, as assessed by the Kaplan–Meier test: 32.0 % for the group “Without Preventive Letter”, and 76.0 % for the group “With Preventive Letter” (p < 0.001). The 1-year return rate after delivery of GDM mothers was 2.4 times higher in the group “With Preventive Letter” than in the group without it. We believe that this low-cost approach could be useful in other institutions caring for pregnant women with diabetes.