Clinical implications of the 100-g oral glucose tolerance test in the third trimester

The clinical implications of gestational diabetes mellitus (GDM) diagnosed in the third trimester are not well established and controversy continues regarding the performance of diagnostic tests beyond 28-week gestation. This study aimed to evaluate the incidence of abnormal third trimester oral glucose tolerance test (OGTT) results in women at high risk and to compare the obstetric and neonatal outcomes with those of women with normal OGTT results.

The study included 372 women who completed late (>29 weeks) 100-g OGTT due to suspected fetal macrosomia, polyhydramnios or a personal risk factor for GDM, diagnosed according to the Carpenter & Coustan criteria. Women with only one abnormal OGTT value were diagnosed with GDM by abnormal glucose follow-up and analyzed separately. Obstetric and neonatal outcomes were compared between the GDM and the non-GDM groups.

GDM was diagnosed in 85/372 (22%) women, including 35 (59.3%) women with one abnormal OGTT value who were later diagnosed with GDM. Of 200 women who had a normal 1-h 50-g glucose challenge test at 24-28 weeks, late GDM was diagnosed in 33 (16.5%). Seventy-six (89.5%) of those with GDM were treated by dietary therapy and 9 (10.5%) by pharmacological therapy. Among women with GDM, large-for-gestational-age fetuses, labor induction and elective cesarean section were more prevalent than for those without GDM. Significant differences were not found between the groups in macrosomia and neonatal outcomes.

The performance of OGTT in women with risk factors during the third trimester should be considered following further prospective trials.