Gestational diabetes

Hyperglycaemia in pregnancy becomes more common as the second trimester progresses, as placental hormones and adipokines act on maternal metabolism to induce insulin resistance. Poorly controlled GDM is associated with increased perinatal morbidity — the same pattern seen with infants of mothers with overt diabetes: macrosomia, shoulder dystocia, neonatal hypoglycaemia, hyperbilirubinaemia, respiratory distress syndrome and stillbirth.

Maternal risks include caesarean delivery (~25%), hypertensive disorders of pregnancy (~20%) and polyhydramnios (~20%).

“Overt diabetes in pregnancy” (overt DIP) is the term used where blood glucose is in the diabetic range during pregnancy. Type 2 diabetes cannot be diagnosed during pregnancy — it needs postpartum confirmation.

Risk factors for hyperglycaemia in pregnancy (including GDM)

Previous hyperglycaemia in pregnancy (overt or gestational); previously elevated blood glucose; history of impaired fasting glucose/glucose tolerance; age over 35 (HNE HealthPathways; some lists use 40); ethnicity (Aboriginal and Torres Strait Islander, Māori and Pacific Islander, Indian subcontinent, Asian, Middle Eastern, non-white African); family history of diabetes (a first-degree relative, or a sister with GDM); pre-pregnancy obesity (BMI >30); fetal macrosomia in a previous pregnancy (birthweight >4500 g or >90th centile); PCOS; drugs causing hyperglycaemia (glucocorticoids, immunosuppressants, cytotoxics, antipsychotics, antiretrovirals, statins, thiazide diuretics).

Screening — ADIPS 2025

Early pregnancy, women with risk factors: test HbA1c in the first trimester alongside the other initial antenatal bloods. This screens for undiagnosed pre-existing diabetes — it is not a screening test for GDM. Only do it if HbA1c screening for type 2 diabetes hasn’t already been done in the last 12 months.

Early pregnancy OGTT is now routinely recommended only for a woman with prior GDM, or where the HbA1c is intermediately elevated at 6.0–6.4%. Consider it otherwise if you are worried about a woman with multiple risk factors, or where local guidelines direct early screening.

All women without known pre-existing diabetes or already-diagnosed hyperglycaemia in pregnancy: 75 g OGTT at 24–28 weeks.

When not to do an OGTT

An OGTT in pregnancy is hazardous or uninformative in four situations — check before you order it:

• gestation under 10 weeks
• a screening HbA1c ≥6.5% (already diagnostic of overt diabetes in pregnancy)
• previously diagnosed diabetes
• previous metabolic or bariatric surgery (unreliable, and can provoke significant hypoglycaemia)

A high-carbohydrate diet beforehand is not required, but carbohydrate should not be restricted either.

If she cannot tolerate an OGTT, do a fasting plasma glucose instead — and tell her GDM cannot be excluded without an OGTT. A fasting glucose ≥5.3 mmol/L as her only test at any point after 10 weeks is managed as GDM. Between 10 and 20 weeks a fasting glucose under 5.3 mmol/L can wait for routine 24–28 week screening. At 24–28 weeks a fasting glucose of 4.7–5.2 mmol/L should prompt a strong recommendation to attempt the OGTT. HbA1c is not a screening test later in gestation, but a first-trimester HbA1c of 6.0–6.4% in a woman who has had no OGTT should be managed as GDM for monitoring and dietary purposes.

Two adapted algorithms

Previous bariatric or metabolic surgery — substitute one week of capillary glucose monitoring (fasting and 2 hours after meals) for the OGTT at each testing point. Diagnose GDM on more than one fasting glucose ≥5.3 mmol/L, or more than two postprandial glucose readings ≥6.7 mmol/L. Screen early (before 10 weeks) with HbA1c if there are risk factors, again before 20 weeks (ideally 10–16 weeks) if previous GDM or HbA1c 6.0–6.4%, and universally at 24–28 weeks. Refer urgently if she has symptomatic hyper- or hypoglycaemia. If an OGTT is done inadvertently, interpret only the fasting (≥5.3 mmol/L) and 2-hour (≥9 mmol/L) values.

PCOS on metformin at conception — stopping metformin can cause significant hyperglycaemia. With no other risk factors, cease it for one week before the OGTT. With one or more other risk factors, continue it and do one week of capillary glucose monitoring instead; if more than 20–30% of readings are above threshold (or fewer, if the clinical picture is concerning), diagnose presumed GDM.

Diagnosis — ADIPS 2025 thresholds

GDM — any one of the following on a 75 g two-hour pregnancy OGTT:

• Fasting plasma glucose (FPG) of 5.3–6.9 mmol/L on the pregnancy OGTT
• 1-hour plasma glucose (1hPG) of ≥10.6 mmol/L on the pregnancy OGTT
• 2-hour plasma glucose (2hPG) of 9.0–11.0 mmol/L on the pregnancy OGTT

Overt diabetes in pregnancy — HbA1c ≥6.5%, FPG ≥7.0 mmol/L, or 2hPG ≥11.1 mmol/L.

A fasting plasma glucose ≥5.3 mmol/L alone is diagnostic, but the full OGTT remains the recommended test unless it isn’t tolerated — all three cut-offs are equally predictive of adverse obstetric outcomes. Interpret an isolated fasting result in early pregnancy with caution.

Laboratories were told they could start reporting against the new criteria from 1 July 2025, though the timing is decided locally by the supervising chemical pathologist.

Management

Urgent referral to the maternity unit (endocrinologist, obstetrician, diabetes educator, dietitian). Review BGLs every 2 weeks (shared care): test before breakfast and 2 hours after each meal.

Glycaemic targets: the 2025 ADIPS recommendations covered screening and diagnosis only, not treatment targets — an update on management is in development. ADIPS notes that some services and jurisdictional guidelines now use a fasting target ≤5.2 mmol/L with post-meal targets ≤7.4 mmol/L at one hour and ≤6.7 mmol/L at two hours, and regards that as a reasonable approach in the meantime. Otherwise follow local guidance. HNE HealthPathways sets a fasting capillary glucose of 3.8–5.2 mmol/L and 3.8–6.7 mmol/L at 2 hours after starting a main meal — note the lower bound, which the ADIPS figures omit. Self-monitor four times a day: fasting, then 2 hours after breakfast, lunch and dinner. Tell her to seek urgent medical advice if a glucose reads ≥10 mmol/L or she feels unwell.

Ketone monitoring

Fasting urinary ketones are worth checking in GDM. Positive fasting ketones point to carbohydrate restriction and should prompt dietary review — excessive caloric restriction causes ketonuria and small-for-gestational-age babies. Persistent high urinary ketones (>1+) warrant prompt capillary ketone testing and obstetric/endocrinology referral, because the question is undiagnosed type 1 diabetes. Any pregnant woman with diabetes who is hyperglycaemic (≥10 mmol/L) or unwell needs urgent ketone testing to exclude DKA; capillary ketones ≥0.6 mmol/L → emergency department.

Diet and physical activity

Refer to a dietitian. Lifestyle modification alone controls hyperglycaemia in pregnancy in more than half of women — adequate, preferably low-GI carbohydrate spread through the day, and 20–30 minutes of moderate exercise on most days, which also improves cardiovascular health and reduces the risk of pre-eclampsia and caesarean delivery.

Psychosocial impact

A GDM diagnosis can be stressful, given the demanding management regimen and concern about the baby’s health.

Antihyperglycaemic agents

Insulin remains first line where lifestyle change has not reached target. A multiple daily injection (basal-bolus) regimen allows flexible, tailored dosing. For women reluctant or unable to use basal-bolus dosing, a fixed-dose biphasic premixed regimen (2–3 times daily) may be appropriate, though it is more likely to cause both hyper- and hypoglycaemia.

Metformin is an option, specialist-guided. In the MiG trial, 46% of women randomised to metformin still needed supplemental insulin to reach target. Metformin crosses the placenta readily, but recent data give some reassurance about a lack of teratogenicity with first-trimester exposure. Avoid it in intrauterine growth restriction, pre-eclampsia, or inadequate weight gain (especially in slim women). Consider it where there is needle phobia, refusal of insulin, poor insulin adherence, or excessive weight gain on insulin.

Sulfonylureas, thiazolidinediones, meglitinides, DPP-4 inhibitors, GLP-1 agonists and SGLT-2 inhibitors are not recommended in pregnancy.

Escalation and registration

Urgent maternity or private endocrinology referral, to be seen within one week, for newly diagnosed overt diabetes in pregnancy, pre-existing type 1 or type 2 diabetes, anyone meeting the GDM diagnostic criteria, or previous bariatric surgery with symptomatic hypoglycaemia. If she is systemically unwell and unstable, phone 000; if stable but needing immediate review, over 18 weeks go to the birth suite, under 18 weeks to the emergency department.

Register her with the NDSS National Diabetes Register — it gives subsidised supplies, written information, and reminders for postnatal testing and long-term follow-up.

Timing of birth

• Type 1 or type 2 diabetes — 37 to 38+6 weeks
• GDM treated with insulin or metformin — 39 to 39+6 weeks
• GDM treated with diet alone — 40 to 40+6 weeks

Postnatal management

Insulin is ceased immediately after delivery. Monitor BGL twice daily (fasting or postprandial) for at least 48 hours, stopping once the fasting BGL is under 6 mmol/L and the 2-hour postprandial under 8 mmol/L. Not every woman diagnosed with overt diabetes in pregnancy has diabetes postpartum — all need reassessment.

Arrange a 75 g OGTT at 6–12 weeks postpartum (can be delayed to 4–6 months if impractical). If normal, recommend a fasting BSL and HbA1c annually, or an OGTT every 2–3 years.

Women with a history of GDM have close to a 10-fold higher risk of progressing to type 2 diabetes, and up to 70% will eventually develop it. A diet-and-exercise weight loss program improves insulin sensitivity and reduces that risk.

Women diagnosed under the previous ADIPS criteria should still be offered type 2 diabetes screening and prevention advice — observational data show they remain at increased risk.

Breastfeeding on metformin is fine. Only small amounts pass into breast milk and there is no risk of neonatal hypoglycaemia, unlike the sulfonylureas.

Discuss contraception and pregnancy spacing — outcomes are best when delivery and the next conception are at least 18 months apart.

Source (local): Gestational Diabetes Mellitus (GDM), Community HealthPathways Hunter New England, last updated 11 May 2026 (registry: hne-healthpathways-gestational-diabetes; verified 2026-08-31). OGTT contraindications, the bariatric-surgery and PCOS-on-metformin algorithms, local glycaemic targets, ketone monitoring, timing of birth and postnatal BGL monitoring come from this pathway.