In Scotland, standard practice for diagnosis of GDM involves offering a 75 g OGTT at 24–28 weeks’ gestation and reviewing postload glucose levels against thresholds in those with any of the following risk factors:

• BMI ≥30 kg/m² (restricted to ≥35 kg/m² in some areas)
• Previous macrosomia (baby with birth weight ≥4,500 g)
• Previous GDM
• Family history of diabetes (T1DM or T2DM in first degree relative, ie child, parent, brother, sister)
• Family origin with a high prevalence of diabetes (South Asia, Middle Eastern, or Black African/Caribbean).

The following additional risk factors are recommended for inclusion in the diagnostic pathway.

Pregnant women with a history of polycystic ovary syndrome (PCOS) should be considered for screening for GDM (odds ratio for GDM diagnosis 2–3).

Pregnant women over 40 years should be screened for GDM (odds ratio for GDM diagnosis 4.86).

Pregnant women aged 35–40 years should be considered for screening for GDM (odds ratio for GDM diagnosis 2.85).

Selected diagnostic and screening criteria for GDM

The guideline development group notes:

• the existence of observational evidence suggesting that women with fasting glucose levels which lead to diagnosis using IADPSG criteria but not using NICE criteria are at increased obstetric risk compared with women who do not have GDM, however acknowledges the absence of high-quality RCT evidence comparing these approaches.
• that recent large RCTs comparing lower and higher diagnostic criteria did not display improved outcomes associated with lower criteria at population- or whole study level, although a subgroup analysis was supportive of lower criteria. Furthermore, an RCT of early treatment suggested benefit predominately in women with diagnostic fasting glucose levels ≥3 mmol/L, one-hour glucose levels ≥10.6 mmol/L or two-hour glucose levels ≥9.0 mmol/L (HAPO 2.0 criteria) but not in those with fasting glucose levels 5.1–5.2 mmol/L, one-hour glucose levels 10–10.5 mmol/L or two-hour glucose levels 8.5–8.9 mmol/L (IADPSG / HAPO 1.75 criteria but below HAPO 2.0 criteria) who were also at increased risk of small for gestational age (SGA) infants.
• that while the majority of OGTT in Scotland are currently performed at 24–28 weeks’ gestation, women with previous GDM routinely have an OGTT at 14–16 weeks and are diagnosed using current (IADPSG) criteria.

In forming a recommendation, the guideline development group considered a number of practical issues, including that:

• as implemented, very few or no centres in Scotland were measuring a one-hour glucose value, but were relying on fasting and two-hour glucose values
• due to the large numbers of women with risk factors requiring OGTT, not all centres in Scotland had managed to implement OGTT testing in all women eligible for testing.

Therefore, the guideline development group sought to set a minimal reasonable standard where evidence of benefit appears clear. In doing so they considered diagnostic levels early and later in pregnancy and whether there was sufficient evidence to recommend early testing in all women with risk factors. They also considered whether adoption of different diagnostic criteria in early and late pregnancy might potentially lead to confusion – with a preference to a single set of criteria unless strong evidence existed that two criteria were appropriate.

It was concluded that:

• when an OGTT is performed at less than 20 weeks gestation, there is sufficient evidence to diagnose GDM in women with glucose levels which exceed HAPO 2.0 thresholds.
• there is developing, but not yet definitive, evidence examining higher and lower diagnostic thresholds at 24–28 weeks’ gestation. However, at this time, the guideline development group considered there to be a significant potential for confusion if more than one set of diagnostic criteria is used between early and later pregnancy and therefore supports using the same criteria (HAPO 2.0) for later pregnancy.
• OGTT in early and late (if first test is negative) pregnancy is offered to women with previous GDM at present. There would be a considerable resource implication if this were extended to all women and it was considered that further, high-quality studies are required to ascertain in which groups these extra tests might be most effectively targeted.

The diagnosis of GDM is made using a single-step 75 g OGTT when one or more of the following results are recorded in those with risk factors during routine testing:

• fasting plasma glucose ≥5.3 mmol/L
• (one-hour post 75 g oral glucose load ≥10.6 mmol/L, where used)
• two-hour post 75 g oral glucose load ≥9.0 mmol/L.

In light of developing evidence that earlier treatment of GDM may be beneficial, amendment of the current testing windows to the earlier points of 10–14 weeks (for women with prior GDM) and the earlier part of the current testing window (24–26 weeks rather than 24–28 weeks) is suggested

Pooled sensitivity and specificity of HbA1c and cut-offs in the diagnosis of GDM

HbA1c in early pregnancy (first trimester) should be considered to detect overt diabetes in pregnancy and to identify a cohort at risk of GDM.

• Women with HbA1c ≥48 mmol/mol should be diagnosed with overt diabetes and managed as such.
• Women with HbA1c 42–47 mmol/mol are at high risk of GDM. Glucose monitoring and dietary management is recommended from the second trimester.

Women with HbA1c 42–47 mmol/mol and who have glucose levels above treatment thresholds (see SIGN 171, section 4.2.1) should be considered as having GDM.

Diet

All women with gestational diabetes should be provided with lifestyle advice, including a dietary strategy with carbohydrate awareness and support to increase physical activity, by a suitably trained healthcare professional.

All women with gestational diabetes should have access to a registered dietitian for general nutritional advice irrespective of whether or not they are following specific dietary interventions. Dietary advice may be provided individually or in group settings depending on the needs of women with GDM.

Exercise

The combined Chief Medical Officers in the UK have recommended 150 minutes of moderate exercise per week for women in pregnancy for reasons not related to diabetes. 

All women should be encouraged to achieve 150 minutes of moderate physical activity per week during pregnancy. This may be particularly important in women with gestational diabetes.

Multidisciplinary diabetes teams should strongly encourage women with GDM to participate in individualised forms of physical activity.

Myo-inisitol

Myo‐inositol is a naturally occurring sugar found in cereals, corn, green vegetables, and meat, that has a role in the body's sensitivity to insulin in type 2 diabetes. Consumption in dietary form or as supplements has been associated with improvement in a range of conditions including premenstrual dysphoric disorder, symptoms of PCOS and insulin sensitivity, and ovulatory function.

There are insufficient data to evaluate the effect of myo-inositol for the treatment of GDM, and inconsistent results on its effects on prevention.

Probiotics

Probiotics are micro-organisms that naturally occur in foods and when consumed in adequate amounts may confer health benefits for the individual.

The evidence presented on the effects of probiotics in women with GDM describes mainly secondary outcomes and biomarkers, and there is a lack of evidence of benefit in clinically relevant fetal and maternal outcomes to support their use.