If your wife was my patient, and if she had any significant risk factors for
diabetes, I would check a blood sugar early in
pregnancy and then at about 24-28 weeks as you have read. Most of the research done on patients with gestational diabetes show no difference in outcomes for mothers and babies with gdm as compared to mothers with gdm. So IF your wife had GDM it is unlikely that testing a few weeks late is going to effect the outcome of her pregnancy in any significant way.