Gestational diabetes screening

What it is

Gestational diabetes mellitus is one of the four main categories of diabetes. It is diabetes diagnosed in the second or third trimester of pregnancy that was not clearly overt diabetes prior to gestation.

How it is diagnosed

The screening test on the common screening list is the oral glucose tolerance test, offered to pregnant women at 24 to 28 weeks gestation. The U.S. Preventive Services Task Force recommends screening for gestational diabetes mellitus after 24 weeks gestation, and it accepts three ways of doing it: a fasting blood glucose level, a 50-g oral glucose challenge, or an assessment of risk factors. Screening at an earlier date is rated insufficient evidence, and screening at later dates is not recommended. Eye screening runs on a rule of its own. Gestational diabetes requires no ophthalmic screening for diabetic retinopathy, whereas a pregnant woman with pre-existing diabetes should be screened in the first trimester, and if there is no retinopathy she is reviewed after delivery if required. In contrast, in type 1 diabetes the first eye screening comes within 3 to 5 years from diagnosis, and in type 2 diabetes at the time of diagnosis.

Against that, a second timing is set out, and it is the one that answers a case where the woman is sitting in front of you at her first visit of the pregnancy: universal screening is recommended at first prenatal visit, irrespective of the trimester. Screening at the first prenatal visit sets when screening starts, and it does not stand in place of the later test: all pregnant women have an FBS or RBS at registration, and a woman whose FBS is under 5.1 mmol/l or RBS under 7.0 mmol/l and who is low risk for GDM does the OGTT at 22-24 weeks of gestation, while a high risk woman whose OGTT at that visit is negative repeats the OGTT at 22-24 weeks of gestation. The week of that later test itself differs between our pages: 24 to 28 weeks gestation on the screening list, and 22-24 weeks of gestation on the national figure. The risk factors listed beside it are BMI≥30, past history of GDM, a first-degree relative with diabetes, previous macrosomia with baby weight ≥4 kg, previous unexplained IUFD, polycystic ovary syndrome, A1C ≥5.7% or impaired fasting glucose or impaired glucose tolerance on a previous diabetes screening test, and other clinical conditions associated with insulin resistance such as severe obesity and acanthosis nigricans.