Gestational Diabetes

Gestational diabetes mellitus (GDM) is glucose intolerance with onset or first recognition during pregnancy — a definition that deliberately excludes diabetes already present before conception but only discovered once prenatal screening began, which is instead classified and managed as pre-existing (pregestational) diabetes in pregnancy on a separate pathway.^[1] GDM is common, and it is deliberately given its own chapter in this library rather than folded into the general Diabetes Mellitus chapter: pregnancy changes essentially every part of the management hierarchy, from diagnostic thresholds and glycemic targets — both genuinely tighter than the non-pregnancy targets used elsewhere — to the preferred pharmacologic agents themselves, where insulin displaces the oral agents that dominate first-line non-pregnancy type 2 diabetes therapy.^[1]

Untreated or poorly controlled GDM raises the risk of fetal macrosomia, birth trauma (including shoulder dystocia), neonatal hypoglycemia, and maternal preeclampsia, while even modest glycemic improvement through lifestyle measures alone meaningfully reduces these outcomes.^[1] For a community or prenatal pharmacist, this chapter's highest-yield territory is the pharmacotherapy decision itself: insulin remains the traditional and still frequently preferred first-line agent once lifestyle measures are insufficient, since it does not cross the placenta in clinically significant amounts, but current Diabetes Canada guidance also supports metformin as a reasonable evidence-based alternative in appropriate patients — while glyburide, once commonly used, now carries a genuinely diminished role given evidence of less favourable outcomes than either. Insulin dose counselling deserves particular attention: requirements rise predictably and substantially as pregnancy progresses, and a patient who is not warned of this in advance can easily mistake a normal, expected dose increase for a sign that treatment has failed.

Placental hormones unmasking a limited beta-cell reserve

Pregnancy is a state of progressively increasing physiologic insulin resistance, driven mainly by placental hormones — human placental lactogen (hPL) chief among them, alongside progesterone, cortisol, prolactin, and placental growth hormone — that begin to meaningfully antagonize insulin action around 20-24 weeks' gestation and continue to intensify through the third trimester.^[1] This is an adaptive mechanism present in every pregnancy: by making maternal tissues comparatively insulin-resistant, more circulating glucose is made available to cross the placenta and support fetal growth. In a woman with adequate pancreatic beta-cell reserve, the beta cells compensate by increasing insulin secretion enough to maintain normal maternal glycemia despite the rising resistance.

GDM develops when that compensatory capacity is insufficient — the same fundamental beta-cell-reserve limitation that underlies type 2 diabetes outside pregnancy, simply unmasked earlier by the added metabolic stress of pregnancy. This shared mechanism is exactly why a GDM diagnosis is not just a pregnancy-limited event: it identifies a woman whose beta-cell reserve is already borderline, which is why she carries a substantially elevated lifetime risk of progressing to type 2 diabetes after the pregnancy ends (see Special Populations and Monitoring & Follow-Up).^[2]

Clinical presentation — usually silent

The great majority of GDM is entirely asymptomatic and is found only because of routine, universal screening — this is precisely why screening is universal rather than symptom-triggered (see Assessment & Diagnosis). When symptoms do occur, they mirror hyperglycemia symptoms seen outside pregnancy — polyuria, polydipsia, unexplained fatigue — but polyuria in particular is a notoriously unreliable sign in pregnancy given how common it already is from gestational bladder pressure, so a symptom-based approach would miss most cases.^[1] A well-appearing, asymptomatic patient with no personal risk factors is not a reason to skip or defer screening.

Diabetes Canada recommends universal screening between 24-28 weeks for essentially every pregnant patient without known pre-existing diabetes (see Assessment & Diagnosis), but the following factors specifically identify patients at high enough risk of undiagnosed pre-existing (pregestational) diabetes to warrant earlier testing, in addition to being long-recognized GDM risk factors in their own right. Any one factor is enough to prompt early risk-based screening, well before the standard 24-28-week window — this is a single-factor threshold, not a count-based one:^[1]

• Age 35 years or older.
• Member of a higher-risk ethnic population — African, Arab, Asian, Hispanic, Indigenous, or South Asian descent.
• Obesity — pre-pregnancy body mass index of 30 kg/m² or higher.
• Current or recent corticosteroid use.
• A personal history of prediabetes (impaired fasting glucose, impaired glucose tolerance, or an elevated A1C).
• A previous pregnancy affected by gestational diabetes.
• A previous delivery of an infant weighing more than 4 kg (macrosomia).
• A first-degree relative with type 2 diabetes.
• Polycystic ovary syndrome (PCOS) or acanthosis nigricans.

Universal screening — timing

Diabetes Canada recommends screening essentially every pregnant woman without known pre-existing diabetes between 24 and 28 weeks' gestation. In women at high risk of undiagnosed type 2 diabetes (see Risk Factors), earlier screening — before 20 weeks — with an A1C is recommended in addition to, not instead of, standard screening later in pregnancy: an early A1C under 6.5% does not rule out GDM developing later, so a patient who screens negative early should still be screened again at the standard 24-28-week window.^[1] An early A1C of 6.5% or higher, or an early fasting plasma glucose of 7.0 mmol/L or higher, instead identifies likely pre-existing (pregestational) diabetes and is managed on that separate pathway (see Recognize the Condition and this platform's general Diabetes Mellitus chapter).^[1]

Diabetes Canada's preferred two-step approach

The preferred strategy is a two-step approach:^[1]

1. Step 1 — 50 g glucose challenge test (GCT), non-fasting. Measure plasma glucose 1 hour later.
   • 1-hour PG under 7.8 mmol/L: screen negative, no further testing needed.
   • 1-hour PG 7.8-11.0 mmol/L: proceed to Step 2.
   • 1-hour PG 11.1 mmol/L or higher: diagnostic of GDM on its own — no confirmatory OGTT is required.
2. Step 2 — 75 g oral glucose tolerance test (OGTT), fasting, performed only when the GCT falls in the 7.8-11.0 mmol/L range. GDM is diagnosed if any one of the following is met or exceeded: fasting ≥5.3 mmol/L, 1-hour ≥10.6 mmol/L, or 2-hour ≥9.0 mmol/L.

The alternative one-step approach

Diabetes Canada also accepts a one-step alternative: a single fasting 75 g OGTT for every patient at 24-28 weeks, with GDM diagnosed if any one of the following is met or exceeded: fasting ≥5.1 mmol/L, 1-hour ≥10.0 mmol/L, or 2-hour ≥8.5 mmol/L.^[1] These lower thresholds originate from the International Association of Diabetes and Pregnancy Study Groups (IADPSG) consensus, itself built on the Hyperglycemia and Adverse Pregnancy Outcomes (HAPO) study — a large, multinational, blinded observational study of over 23,000 women showing a continuous, graded relationship between maternal glucose and adverse perinatal outcomes (macrosomia, cord-blood hyperinsulinemia, primary cesarean delivery, neonatal hypoglycemia), with no single obvious inflection point of the kind a diagnostic cut-off ordinarily implies.^[3][4] Because the one-step approach uses lower cut-offs and needs only a single abnormal value, it labels meaningfully more women with GDM than the two-step approach — Diabetes Canada frames it as an acceptable alternative rather than the preferred strategy, reflecting genuine ongoing debate about the added resource burden of diagnosing (and treating) a substantially larger number of women without conclusively proven outcome benefit over the two-step approach.^[1] A local hospital or laboratory protocol may specify one approach or the other — confirm which pathway is in local use before interpreting a borderline result.

Investigations

• Confirm any borderline or technically difficult result with repeat testing rather than accepting a single ambiguous value.
• No additional baseline investigations (e.g., a dilated retinal exam or urine albumin-to-creatinine ratio) are routinely required for GDM specifically at diagnosis, unlike pre-existing diabetes — these reflect longer-standing hyperglycemia exposure that GDM, by definition, has not had.
• Once GDM is confirmed, self-monitored blood glucose becomes the primary ongoing monitoring tool rather than repeat OGTT or routine A1C (see Monitoring & Follow-Up) — A1C is a poor short-interval monitoring tool in pregnancy given physiologically altered red-cell turnover and the need for tighter, faster feedback than a 2-3-month average can provide.

Glycemic targets in GDM are genuinely tighter than the targets used for non-pregnant type 2 diabetes, reflecting how sensitive the fetus is to even modest maternal hyperglycemia:

• Fasting and pre-meal: under 5.3 mmol/L.
• 1-hour post-meal: under 7.8 mmol/L.
• 2-hour post-meal: under 6.7 mmol/L.

^[1] A patient may check either the 1-hour or 2-hour postprandial value (not necessarily both, at every meal) once a consistent pattern is established, per the treating provider's monitoring plan (see Monitoring & Follow-Up).

Beyond the numeric targets themselves, the goals of therapy in GDM are to:

• Reduce the risk of fetal macrosomia, birth trauma (including shoulder dystocia), and neonatal hypoglycemia — all of which rise with the degree of maternal hyperglycemia.^[1]
• Reduce the risk of maternal preeclampsia and cesarean delivery.^[1]
• Achieve these targets using the lowest-risk effective approach for both mother and fetus — lifestyle measures first, escalating to pharmacotherapy only when targets are not met (see Non-Pharmacologic Management and Choose Initial Treatment) — while avoiding maternal hypoglycemia from over-aggressive treatment.
• Set the patient up for accurate postpartum risk stratification and follow-up, since GDM meaningfully changes her own long-term diabetes risk and that of her child (see Special Populations).

• Medical nutrition therapy (MNT): refer every patient to a registered dietitian for an individualized plan — a consistent, moderate distribution of carbohydrate across 3 meals and 2-3 snacks daily, favouring lower-glycemic-index and higher-fibre carbohydrate sources, generally works better for postprandial control than a small number of large meals. Avoid excessive calorie or carbohydrate restriction — the goal is glycemic control appropriate to a still-growing fetus, not maternal weight loss.^[1]
• Structured physical activity: in the absence of an obstetric contraindication, recommend roughly 30 minutes of moderate-intensity activity on most days of the week; a short walk after meals is a particularly practical, genuinely effective way to blunt postprandial glucose excursions and is an easy, concrete instruction to give.^[1]
• Self-monitored blood glucose (SMBG): teach correct technique and timing — typically fasting plus either 1-hour or 2-hour post-meal readings after each of the 3 main meals (roughly 4 checks daily). Have the patient bring a log (paper or app-based) to every visit — the pattern in that log, more than any single reading, is what actually drives pharmacotherapy decisions (see Choose Initial Treatment).
• Weight gain: counsel toward standard pregnancy weight-gain targets based on pre-pregnancy BMI category, per routine prenatal care — GDM does not change these targets on its own.
• Patient education on what lifestyle measures typically achieve: nutrition therapy, activity, and SMBG together bring the majority of women with GDM to target without needing medication — set this expectation honestly with patients at diagnosis, while also being clear that needing medication later is a function of physiology, not a personal shortfall.^[1]

Obesity

Patients with obesity often need higher initial insulin doses given greater baseline insulin resistance; metformin may be a particularly practical alternative in this group given its more favourable weight profile relative to insulin, though the same insulin-supplementation caveat above still applies.^[1]

Multiple gestation

Twin and higher-order pregnancies are associated with a greater degree of insulin resistance and often need more frequent dose reassessment and higher total insulin requirements than a singleton pregnancy with GDM.

Breastfeeding

Insulin, metformin, and glyburide are all considered compatible with breastfeeding. Actively encourage breastfeeding in every patient with a history of GDM — beyond its general infant-feeding benefits, it is associated with more favourable maternal glucose metabolism in the postpartum period.^[1]

Postpartum — glucose normalizes, pharmacotherapy is stopped, but the risk does not end

For the great majority of women, glucose tolerance returns to normal essentially immediately after delivery, once the placenta — the source of the insulin-antagonist hormones described in Recognize the Condition — is delivered. GDM-specific pharmacotherapy (insulin, metformin, or glyburide) is generally discontinued at delivery; confirm this is appropriate with early postpartum SMBG before assuming normalization in an individual patient, particularly one who needed high insulin doses antepartum.^[1]

Discontinuing medication is not the end of the GDM story. Every woman with GDM requires postpartum screening with a 75 g OGTT between 6 weeks and 6 months postpartum, interpreted against standard non-pregnancy diagnostic criteria for diabetes and prediabetes — this is a distinct, separately scheduled test from anything performed antepartum, and it is easy for it to fall through the cracks once the pregnancy itself has ended.^[1] A history of GDM confers a substantially elevated lifetime risk of progressing to type 2 diabetes — a systematic review found the cumulative incidence of diabetes after a GDM-affected pregnancy ranging from roughly 2.6% to over 70% depending on the population studied and length of follow-up, with the steepest rise typically occurring within the first 5 years postpartum.^[2] Even with a normal postpartum OGTT, Diabetes Canada recommends ongoing periodic screening at least every 3 years thereafter, or more often if additional risk factors are present.^[1]

Future pregnancies

A history of GDM meaningfully raises the risk of GDM recurring in a subsequent pregnancy. Counsel patients to raise this history proactively at the start of any future pregnancy, so risk-based early screening (see Assessment & Diagnosis) is applied rather than waiting for the standard 24-28-week window.

Antepartum

Continue SMBG (fasting plus post-meal readings, see Non-Pharmacologic Management) throughout pregnancy for every patient with GDM, whether managed with lifestyle measures alone or with pharmacotherapy — this is what actually drives ongoing titration decisions (see Choose Initial Treatment), not periodic in-clinic testing alone. Reassess the treatment plan, including insulin dose, regularly — commonly every 1-2 weeks — given the predictable, ongoing rise in insulin resistance through the second and third trimesters.^[1]

Fetal surveillance (growth ultrasounds, and antenatal fetal testing where indicated) and delivery-timing decisions follow standard obstetric protocols, individualized to glycemic control and any additional risk factors present — coordinate this directly with the patient's obstetric provider rather than through a fixed rule in this chapter.

Postpartum

Reassess glucose status with a 75 g OGTT between 6 weeks and 6 months postpartum in every patient (see Special Populations), and continue periodic screening at least every 3 years thereafter, more often if additional risk factors for type 2 diabetes are present.^[1] Use this visit as an opportunity for broader cardiometabolic counselling — weight, physical activity, and cardiovascular risk-factor management — given the shared underlying risk between GDM, future type 2 diabetes, and cardiovascular disease.

1. Targets not met after a genuine 1-2-week trial of lifestyle measures: start pharmacotherapy (see Choose Initial Treatment) — do not extend the lifestyle-only trial indefinitely once a consistent SMBG pattern shows targets are not being met.
2. Fasting hyperglycemia persists on lifestyle measures, or on a rapid-acting-only insulin regimen: add or increase basal insulin, rather than continuing to increase mealtime doses alone.
3. Postprandial hyperglycemia persists after a specific meal: add or increase the rapid-acting insulin dose given before that specific meal.
4. Targets not met on an adequate trial of metformin: add supplemental insulin rather than pushing metformin beyond its usual maximum dose — remember that roughly one-quarter to one-half of patients started on metformin need this step, so it should be framed to the patient as an expected possibility, not a failure of the metformin trial.^[1][5]
5. Targets still not met despite a fully titrated insulin regimen (or metformin plus supplemental insulin): refer to endocrinology or maternal-fetal medicine for regimen review (see Red Flags & Refer) rather than continuing to escalate doses in isolation.
6. Recurrent maternal hypoglycemia on insulin or a sulfonylurea: reduce the implicated dose and reassess timing/distribution rather than accepting recurrent hypoglycemia as the cost of tight control.
7. New hypertension, headache, visual change, or right-upper-quadrant pain at any point: evaluate for preeclampsia promptly and independently of the current glycemic control status — good glucose control does not rule out a coexisting hypertensive complication (see Red Flags & Refer).

• Treating this chapter's insulin-first-line framing as "insulin only" — Diabetes Canada explicitly supports metformin as a reasonable evidence-based alternative in appropriate patients; the accurate message is a genuine choice between two effective agents, not a single mandated pathway.
• Presenting metformin as a way to avoid insulin altogether, without mentioning that a substantial proportion of patients started on it will still need supplemental insulin.
• Continuing to treat glyburide as an equivalent, interchangeable oral alternative to metformin — current evidence shows it crosses the placenta more than once assumed and is associated with less favourable outcomes than either insulin or metformin.
• Not warning a patient starting insulin that her dose is very likely to increase, sometimes substantially, over the following weeks — an unwarned dose increase can look to a patient like the treatment (or her own adherence) is failing, when it is simply tracking normal, expected physiology.
• Applying non-pregnancy glycemic targets to a pregnant patient with GDM — the pregnancy-specific targets in this chapter are genuinely tighter and are what should drive titration.
• Relying on symptoms to decide whether to screen for GDM — the great majority of cases are asymptomatic, which is exactly why universal screening at 24-28 weeks exists.
• Skipping or forgetting the postpartum 75 g OGTT once the pregnancy itself has ended and spot glucose checks look normal — this is a distinct, separately scheduled test, not something inferred from an antepartum result.
• Assuming a patient's future diabetes risk ends with a normal postpartum OGTT — ongoing screening at least every 3 years is still recommended, given the substantially elevated lifetime risk of progression to type 2 diabetes.
• Treating good glycemic control as if it rules out preeclampsia — the two are related (GDM raises preeclampsia risk) but are distinct processes, and hypertensive red flags need their own independent evaluation.

How to use this table: doses are adult obstetric doses for gestational diabetes specifically and require patient-specific confirmation (renal function, weight, glycemic pattern). Drug pricing is intentionally omitted. Select any drug name to open its complete Pharmasist Drug Database page.

First-Line Pharmacotherapy — Insulin

Preferred once lifestyle measures alone do not achieve target — does not cross the placenta in clinically significant amounts, allowing precise titration against the maternal glucose pattern without direct fetal drug exposure.^[1]

Alternative Pharmacotherapy — Metformin

A reasonable, evidence-based oral alternative to insulin, or added to lifestyle measures, when glycemic targets are not met — genuinely supported by current Diabetes Canada guidance, not a fallback agent.^[1][5]

Diminished Role — Glyburide

Evidence note: drug inventory, dosing, and comments reflect the current Diabetes Canada Chapter 36 guidance, cross-checked against the SMFM statement and primary evidence on glyburide's placental transfer.^[1][5][6]