Gestational diabetes mellitus (GDM) is any degree of glucose intolerance with onset or first recognition during pregnancy.
Hyperglycemia in pregnancy may be suggested by the presence of glycosuria, a fetus that is large for date, or polyhydramnios.
Overt diabetes mellitus may be found in women presenting with risk factors for type 2 diabetes during the first prenatal visit (before 13 weeks of gestation).


Overt Diabetes Mellitus (DM)

  • Diabetes diagnosed in the 1st trimester of pregnancy is classified as pre-existing pregestational diabetes 
  • Overt DM may be found in women presenting with risk factors for type 2 diabetes during the 1st prenatal visit (before 13 weeks of gestation) 
  • Risk factors for type 2 DM include:
    • Overweight
    • Physically inactive
    • History of gestational diabetes mellitus (GDM)
    • With hypertension, history of CVD, or polycystic ovary syndrome, acanthosis nigricans
    • HDL cholesterol level 0.90 mmol/L (<35 mg/dL) and/or triglyceride level 2.82 mmol/L (>250 mg/dL)
    • With previous test result that showed HbA1c ≥5.7%, impaired glucose tolerance or impaired fasting glucose
    • With DM in 1st-degree relative
    • Glycosuria at 1st prenatal visit
  • Diagnostic criteria for overt DM and GDM at the 1st prenatal visit in women not known to have diabetes include a fasting plasma glucose (FPG) of ≥7 mmol/L (≥126 mg/dL) for overt DM and 5.1-6.9 mmol/L (92-125 mg/dL)  for GDM  
    • Other criteria for overt DM include an HbA1c level of ≥6.5% and a random plasma glucose of ≥11.1 mmol/L (≥200 mg/dL)  
    • Women who may be at risk of pre-pregnancy DM should be offered a 75-g oral glucose tolerance test (OGTT) at 24-28 weeks of gestation if initial screening test is negative 

Screening/Diagnosis of Gestational Diabetes Mellitus (GDM)

  • Uses either the “one-step” or “two-step” approach 
  • Performed at 24-28 weeks of gestation in women without previous diagnosis of overt diabetes
    • Consider offering the 75-g OGTT to women with clinical features of maternal hyperglycemia or those with risk factor but presented after 28 weeks of gestation

“One-step” Approach

  • Diagnostic criterion recommended by WHO 
  • A 75-g oral glucose tolerance test (OGTT) is performed after an overnight fasting of 8-14 hours
  • Perform OGTT in the morning after an overnight fasting of at least 8 hours
  • Plasma glucose measurement at fasting and at 1 and 2 hours
  • Diagnosis of GDM1 is made when any 1 of the following is present:
    • FPG 5.1-6.9 mmol/L (92-125 mg/dL)
    • 1-hour plasma glucose ≥10 mmol/L (≥180 mg/dL)
    • 2-hour plasma glucose 8.5-11 mmol/L (153-199 mg/dL)
  • OGTT should be repeated 4-6 weeks later in patients with initial normal results
  • May also be offered at 24-28 weeks in women with previous GDM
  • Diagnostic criteria for overt DM using a 75-g 2-hour OGTT at 24-28 weeks of gestation are an FPG of ≥7 mmol/L (126 mg/dL) and a 2-hour plasma glucose of ≥11.1 mmol/L (200 mg/dL) 
  • 1National Institute of Health and Care Excellence 2015 guideline value for diagnosis of GDM with a 75-g 2-hour OGTT includes an FPG of ≥5.6 mmol/L or a 2-hour plasma glucose of ≥7.8 mmol/L

“Two-step” Approach

  • Diagnostic criterion recommended by the American Congress of Obstetricians and Gynecologists (ACOG)  
  • A 50-g glucose 1-hour challenge test (non-fasting) followed by a 100-g 3-hour OGTT (fasting) for those with positive screen
    • Perform 100-g OGTT if the 1-hour plasma glucose level is ≥7.2, 7.5 or 7.8 mmol/L (≥130, 135, or 140 mg/dL)  
  • Diagnosis of GDM is made when at least 2 of the following are present:
    • FPG ≥5.3 mmol/L (≥95 mg/dL)
    • 1-hour plasma glucose ≥10 mmol/L (≥180 mg/dL)
    • 2-hour plasma glucose ≥8.6 mmol/L (≥155 mg/dL)
    • 3-hour plasma glucose ≥7.8 mmol/L (≥140 mg/dL)

Postpartum Screening

  • All women who will be identified to have overt DM during pregnancy or have GDM should have postpartum glucose testing
    • Women with GDM should be screened for persistent DM 4-12 weeks postpartum as the risk of developing maternal type 2 diabetes is increased after pregnancy and also because GDM may be an undiagnosed pre-existing type 1 or 2 diabetes
      • Probability of progression to DM is high with the following postnatal test results: Impaired glucose tolerance with 75-g OGTT, HbA1c 5.7-6.4%, FPG 5.7-6.9 mmol/L 
    • Uses the fasting plasma glucose test and nonpregnancy diagnostic criteria
    • A 75-g 2-hour OGTT is not routinely offered in women with normal postpartum blood glucose levels
    • A1c is not advised for diagnosis of postpartum-persistent diabetes; may be offered at 1 year postpartum in women with GDM who have negative postnatal test for diabetes
    • Lifelong screening for development of DM or prediabetes at least every 3 years is recommended to all women with history of GDM with screening frequency depending on other risk factors


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