Approach

Screening protocols for GDM vary between countries.[2] Check your local protocol.

UK recommendations for GDM screening and diagnosis

Universal screening of all pregnant women is not recommended by the UK National Institute for Health and Care Excellence (NICE). Individual screening is based on the presence of risk factors. The NICE recommendations are summarised below:[4]

Check for GDM risk factors at the woman’s booking appointment.[4]

Offer a 75-g 2-hour oral glucose tolerance test (OGTT) at 24 to 28 weeks of gestation to any woman with one or more of the following risk factors:[4]

  • BMI >30 kg/m²

  • A previous baby weighing ≥4.5 kg

  • A first-degree relative with diabetes

  • Ethnic origin associated with high prevalence of diabetes. NICE no longer specifies which ethnic origins this includes, leaving this to be decided at a local level or by individual clinicians.

Offer any woman who has previously had GDM:[4]

  • Early self-monitoring of blood glucose as soon as possible after booking, OR

  • A 75-g 2-hour OGTT as soon as possible after booking, with a repeat test at 24 to 28 weeks of gestation if the results of the first OGTT are normal.

A 2-hour OGTT involves taking a fasting plasma glucose sample after an overnight fast, giving the woman a 75-g oral glucose solution to drink and then taking a second plasma glucose level after 2 hours.

Diagnose GDM if a pregnant woman has either:[4]

  • A fasting plasma glucose >5.6 mmol/L (100 mg/dL), OR

  • A 2-hour plasma glucose >7.8 mmol/L (140 mg/dL).

Do not use fasting plasma glucose, random blood glucose, haemoglobin A1c (HbA1c), glucose challenge test, or urinalysis for glucose to assess for the risk of developing GDM.[4]

  • Consider a 75-g 2-hour OGTT to exclude GDM if a pregnant woman has glycosuria of 2+ or above on one occasion or glycosuria of 1+ on two or more occasions.[4]

  • HbA1c, a measure of glycosylated red blood cell haemoglobin, reflects ambient glucose levels over the preceding 2 to 3 months. HbA1c is insufficiently sensitive to substitute for OGTT as a screening test.[40][41] The extended effect of previous glycaemia limits the utility of HbA1c in pregnancy, although it is recommended as a routine test to rule out overt type 2 diabetes after a woman is diagnosed with GDM.[4]

Some UK hospital trusts will consider GDM testing in women whose fetus is found to be large for gestational age (LGA) on ultrasound scan, but this is not a national guideline recommendation.

  • LGA is variously defined as estimated fetal weight (EFW) >90th centile, EFW >95th centile, EFW >4 kg, EFW >4.5 kg.[42]​​

  • A service evaluation of local GDM guidelines in England found that 9 out of 13 responding trusts had high EFW and/or abdominal circumference as an indication for testing in their local guidelines.[43][44]​​

  • If the woman should have received an OGTT on the basis of having risk factors, but for some reason failed to have one, then LGA would certainly be an indication for a later test. However, the utility of testing after 34 weeks is uncertain, as there is limited opportunity for treatment to have any beneficial effect on fetal growth.

NICE does not recommend GDM testing on the basis of maternal age, even though this is a strong risk factor, based on the conclusion that it would result in an excessive burden of testing.[4]

Previous bariatric surgery

Women who have had previous bariatric surgery are at higher risk of GDM than women with a normal BMI, but may struggle to tolerate the 75-g 2-hour OGTT.[45] The OGTT may also cause reactive hypoglycaemia at 2 hours in women who have had previous bariatric surgery, especially gastric bypass.[46] Therefore, alternative approaches to GDM testing have been suggested in these women, although these are not part of UK national guidance:[47][46][48]

First trimester

  • Fasting plasma glucose or HbA1c[46][47]

Between 24 to 28 weeks of gestation

  • OGTT for women with an adjustable gastric band, and also consider for women with a gastric sleeve[46][47]

  • One week of blood glucose monitoring or continuous glucose monitoring for women who have had other bariatric procedures or who cannot tolerate the OGTT.[46][47][48]

Symptomatic presentation

Occasionally, women present with overt signs or symptoms of hyperglycaemia such as polyuria or polydipsia. This is more likely in women with undiagnosed pre-existing diabetes than in those with true GDM. In such a scenario, tests of fasting and/or random glucose levels are appropriate. The World Health Organization (WHO) and the International Association of Diabetes and Pregnancy Study Groups (IADPSG) recommends that high-risk women with confirmed fasting glucose levels of ≥7.0 mmol/L (≥126 mg/dL) or random glucose levels ≥11.1 mmol/L (≥200 mg/dL) in the first trimester receive a diagnosis of overt (rather than gestational) diabetes.[1][2]

Other screening and diagnosis recommendations

Globally there is some variation in guideline recommendations for screening and diagnosis of GDM. For example in the US, a two-step method is recommended by the American College of Obstetricians and Gynecologists and the US National Institutes of Health, using a 1-hour 50-g (non-fasting) glucose load test to screen, followed by a 3-hour 100-g OGTT for those who screen positive.[49][50] ​The IADPSG criteria recommend an alternative one-step approach using a 75-g 2-hour OGTT, with a post-load, 2-hour plasma glucose ≥8.5 mmol/L (153 mg/dL) or a 1-hour level ≥10.0 mmol/L (180 mg/dL) diagnostic for GDM.[1] The American Diabetes Association and the US Preventive Services Task Force guidelines recommend screening for GDM with either the two-step or one-step method at 24 weeks’ gestation or after.[3][51] If women enter antenatal care after 28 weeks’, screening should be performed as soon as possible.[51]

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 to 28 weeks’ gestation as possible approaches.[2] Its guideline stresses the importance of the distinction between GDM (hyperglycaemia in pregnancy that fails to meet the diagnostic thresholds for diabetes) and diabetes in pregnancy (where the non-pregnant adult thresholds for diagnosing diabetes are met). The WHO recommends making a diagnosis of GDM at any time during pregnancy if any one or more of the following criteria are met:[2]

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

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

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

The International Federation of Gynecology and Obstetrics recommends universal screening using a one-step approach with either the WHO or the IADPSG cut-offs to establish a diagnosis of GDM.[52]

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