Screening

Screening determines who should undergo an oral glucose tolerance test, the definitive diagnostic test. Screening may be accomplished using clinical risk factors, biochemical testing with a glucose challenge test (“two-step testing”), or a combination of both. Positive screening is followed by a diagnostic oral glucose tolerance test.[44]​ Guidelines and expert consensus variously recommend screening for gestational diabetes mellitus (GDM) in pregnant women at both high and usual risk; however, screening strategies have not been compared in randomized trials.[61]

The American Diabetes Association (ADA) and American Association of Clinical Endocrinology (AACE) both endorse universal screening for GDM in later pregnancy.[1][42]​ This means that at 24-28 weeks' gestation, all women not known to have diabetes (including high-risk women if earlier testing was normal) should undergo screening with a glucose tolerance test.[1][42]​ US Preventive Services Task Force guidelines note that testing can occur later if women enter prenatal care after 28 weeks’ gestation.[43] Other guidelines use risk factor-based screening to inform the need for oral glucose tolerance testing at 24-28 weeks’ gestation.[44]​ Guidelines concur in their recommendation that screening should be undertaken from 24 weeks; this is because treatment of GDM at or after 24 weeks of gestation has been shown to be significantly associated with improved health outcomes (decreased risk of primary cesarean deliveries, shoulder dystocia, macrosomia, large for gestational age, birth injuries, and neonatal intensive care unit admissions).[45] Due to the lack of consistent evidence to indicate neonatal and maternal benefit from early diagnosis and treatment of GDM, the American College of Obstetricians and Gynecologists (ACOG) does not recommend routine screening for GDM before 24 weeks of gestation.[48]​​

The criteria and approach for diagnosis of GDM are not universally accepted. Testing strategies adopt either a one-step method using the 75-g oral glucose tolerance test or a two-step method using a 50-g (nonfasting) glucose load to screen, followed by a 100-g oral glucose tolerance test for those who screen positive.[1] See Diagnosis approach for further information on testing strategies.

Other screening guidelines for GDM

World Health Organisation (WHO)

The WHO leaves the decision on universal or targeted screening to local health authorities according to local burden of GDM, resource availability, and priorities, although it suggests first-trimester screening of high-risk women and universal screening at 24-28 weeks’ gestation as possible approaches.[41] It uses glucose thresholds to distinguish between GDM and diabetes mellitus in pregnancy; GDM is defined as hyperglycemia at any point in pregnancy that fails to meet the nonpregnant adult diagnostic thresholds for diabetes, whereas diabetes in pregnancy is diagnosed when these thresholds are met. Based on this definition, WHO guidelines advise that diagnosis of GDM should be made at any time in pregnancy if one or more of the following criteria are met:[41]

  • Fasting plasma glucose 92-125 mg/dL (5.1 to 6.9 mmol/L)

  • 1-hour plasma glucose ≥180 mg/dL (10.0 mmol/L) following a 75 g oral glucose load

  • 2-hour plasma glucose 153-199 mg/dL (8.5 to 11.0 mmol/L) following a 75 g oral glucose load.

Meanwhile, if one or more of the following criteria (the 2006 WHO criteria for diabetes in non-pregnant adults) are met, a diagnosis of diabetes in pregnancy should instead be made:[41]

  • Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L)

  • 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) following a 75 g oral glucose load

  • Random plasma glucose ≥200 mg/dL (11.1 mmol/L) in the presence of diabetes symptoms

International Federation of Gynecology and Obstetrics (FIGO)

FIGO recommends universal screening, utilizing a one-step approach with either the WHO or IADPSG cut-offs to establish a diagnosis of GDM.[62]

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