Gestational diabetes

What it is

Gestational diabetes mellitus (GDM) is a pregnancy complication affecting approximately 7–10% of all pregnancies worldwide, in which blood glucose rises because the body cannot produce enough insulin to meet the heightened demands of pregnancy. Also called pregnancy diabetes, GDM typically develops in the second or third trimester in women who had no prior diabetes diagnosis. In Canada, Diabetes Canada estimates the condition affects roughly 8–18% of pregnancies depending on the diagnostic criteria used and the population screened — rates that are rising alongside increasing rates of obesity and older maternal age. With well-managed blood glucose, most women with GDM have healthy pregnancies and healthy babies. Left uncontrolled, however, GDM carries meaningful risks for both: large birth weight, birth complications, neonatal hypoglycaemia, and a 7-fold increased lifetime risk of type 2 diabetes for the mother.

GDM typically resolves after delivery, but it is one of the strongest predictors of future metabolic disease. Approximately 50% of women with a history of GDM will develop type 2 diabetes within 5–10 years of their pregnancy.

Causes and mechanism

GDM results from the intersection of two processes: normal pregnancy-driven insulin resistance and an insufficient compensatory response from the pancreas.

From early pregnancy onward, the placenta secretes hormones — human placental lactogen (hPL), progesterone, cortisol, and estrogen — that progressively blunt insulin sensitivity. This is a deliberate physiological adaptation: mild maternal insulin resistance diverts glucose toward the fetus. In most pregnant women, the pancreas compensates by producing substantially more insulin. In women who develop GDM, the pancreas cannot keep pace, likely because of pre-existing reduced beta-cell reserve or early beta-cell dysfunction that was subclinical before pregnancy.

Key risk factors:

Risk factor	Notes
Pre-pregnancy overweight or obesity	One of the strongest modifiable risk factors
Previous GDM	Risk of recurrence in subsequent pregnancy is high
Family history of type 2 diabetes	First-degree relative significantly raises risk
Age over 35	Risk increases with maternal age
Polycystic ovary syndrome (PCOS)	Underlying insulin resistance amplifies GDM risk
Ethnicity	South Asian, East Asian, Middle Eastern, Indigenous, and Afro-Caribbean women have higher rates in Canadian cohort data
Previous macrosomic baby (>4 kg)	Suggests prior glucose dysregulation

Symptoms and diagnosis

GDM rarely produces obvious symptoms — increased thirst, frequent urination, and fatigue can all be attributed to normal pregnancy. This is why universal screening is standard practice.

In Canada, Diabetes Canada guidelines recommend a 50 g glucose challenge test (GCT) or a 75 g oral glucose tolerance test (OGTT) between 24 and 28 weeks of pregnancy. Women with risk factors should be offered earlier screening at their first prenatal visit. The OGTT is the diagnostic standard: the patient fasts overnight, drinks a glucose solution, and blood glucose is measured fasting, at 1 hour, and at 2 hours. An elevated value at any time point confirms GDM. An HbA1c drawn early in pregnancy can identify pre-existing undiagnosed type 2 diabetes in high-risk women. Testing is available through LifeLabs and Dynacare across most Canadian provinces, and is covered under provincial health insurance as part of routine prenatal care.

Complications of uncontrolled GDM:

• For the baby: macrosomia (large for gestational age), shoulder dystocia, neonatal hypoglycaemia, preterm birth, and elevated long-term risk of obesity and type 2 diabetes
• For the mother: preeclampsia, higher likelihood of caesarean delivery, and a substantially elevated risk of type 2 diabetes after pregnancy

Treatment options

The goal of GDM management is to keep blood glucose within safe targets throughout pregnancy — fasting below 5.3 mmol/L and 2-hour postprandial below 6.7 mmol/L, per Diabetes Canada guidance.

First-line: lifestyle measures

• Medical nutrition therapy (MNT): the cornerstone of GDM management. A registered dietitian experienced in GDM designs a plan that distributes carbohydrates across three meals and two to three snacks, emphasizes low-glycaemic foods, and eliminates sugary drinks and refined carbohydrates.
• Physical activity: moderate activity — particularly walking after meals — meaningfully reduces postprandial glucose and is safe for most pregnancies.
• Blood glucose monitoring: women test four times daily (fasting and after each main meal) to assess whether targets are being met.

Pharmacological treatment when lifestyle is insufficient

• Insulin: the preferred agent in pregnancy. It does not cross the placenta and has the longest safety record in GDM management.
• Metformin: used in some settings as an alternative or adjunct to insulin. It does cross the placenta, and its long-term effects on offspring are still under study (see Limitations section).

After delivery GDM typically resolves once the placenta is delivered. Blood glucose should be formally re-checked with an OGTT 6–12 weeks postpartum to confirm resolution. Women with a history of GDM should be screened for type 2 diabetes at least every 1–3 years thereafter, per Diabetes Canada recommendations.

When to see a clinician in Canada

All pregnant women should attend routine prenatal care, which includes GDM screening between 24 and 28 weeks. Women with any of the risk factors listed above should raise early screening at their first prenatal appointment.

Seek earlier assessment if you notice excessive thirst or urination beyond what is typical for pregnancy, or fatigue that seems disproportionate. After delivery, book a follow-up OGTT at 6–12 weeks postpartum and discuss an ongoing screening schedule with your family physician or midwife. Lifestyle modification after pregnancy — maintaining a healthy weight, eating well, staying active — is the most evidence-supported strategy for reducing long-term type 2 diabetes risk.

For ongoing monitoring between pregnancies or postpartum metabolic support, Canadian telehealth platforms such as Felix, Maple, Cleo, and others can facilitate follow-up where in-person access is limited, though any pharmacological management should involve a physician or nurse practitioner.

Limitations and open questions

Research is still emerging on several aspects of GDM. The optimal diagnostic threshold for GDM remains debated internationally: the IADPSG one-step 75 g OGTT criteria (endorsed by the World Health Organization) and the traditional two-step approach used in North America identify different populations, and there is no consensus on which reduces adverse outcomes more cost-effectively in the Canadian context. Diabetes Canada has acknowledged this ongoing debate in its clinical practice guidelines.

The long-term safety of metformin use in pregnancy is not fully established. While short-term maternal outcomes appear comparable to insulin, studies including the MiG trial have shown that children exposed to metformin in utero may have higher rates of adiposity at follow-up — the clinical significance of this finding is still being evaluated, and Health Canada has not issued updated guidance specifically on metformin in GDM.

It is also unclear why some women with GDM progress to type 2 diabetes quickly while others do not, and which postpartum interventions most effectively prevent that progression in Canadian populations with diverse ethnic risk profiles. Indigenous women in Canada face disproportionately high GDM rates, and culturally appropriate screening and support pathways remain underdeveloped.