Reproductive · PANCE / PANRE

Gestational Diabetes Mellitus (GDM)

Written & clinically reviewed by Ryan Busch, PA-C · Updated August 2026 · Report an error

Glucose intolerance first identified during pregnancy — screen at 24-28 weeks; manage with diet, exercise, insulin.

Also known as: gestational diabetes, GDM, diabetes in pregnancy, pregnancy-related diabetes

Overview

Glucose intolerance with onset or first recognition during pregnancy that does not clearly meet criteria for overt diabetes. Distinguished from pregestational (type 1 or 2) diabetes diagnosed before pregnancy.

Epidemiology

Affects ~6-9% of US pregnancies; higher rates with rising obesity. Disproportionately affects Hispanic, Asian, Native American, and Black populations.

Try two board-style Gestational Diabetes Mellitus questions

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Question 1ReproductiveMedium
A pregnant patient at 28 weeks has abnormal oral glucose tolerance testing consistent with gestational diabetes. Which of the following is the most appropriate initial management?
  • AStart metformin therapy at diagnosis
  • BNutritional therapy and glucose monitoring
  • CWeekly antenatal fetal surveillance testing
  • DStart basal-bolus insulin at diagnosis
Reveal answer & full explanation
Correct answer: B — Nutritional therapy and glucose monitoring
  • AStart metformin therapy at diagnosis
  • BNutritional therapy and glucose monitoring✓
  • CWeekly antenatal fetal surveillance testing
  • DStart basal-bolus insulin at diagnosis

Why Nutritional therapy and glucose monitoring is correct

  • First-line management of gestational diabetes is medical nutrition therapy, appropriate exercise, and home glucose monitoring.
  • Pharmacotherapy is added only when diet and activity fail to meet glycemic targets after one to two weeks.
  • Achieving control reduces macrosomia, shoulder dystocia, neonatal hypoglycemia, and preeclampsia.

Why the others are wrong

  • Start metformin therapy at diagnosis — Oral agents are reserved for patients who miss glycemic targets after a one- to two-week trial of nutrition therapy, and insulin remains the preferred agent when medication is required (right-step-wrong-timing trap).
  • Start basal-bolus insulin at diagnosis — Insulin is the preferred drug WHEN medication is needed, but it is not started before a trial of nutrition therapy (right-drug-wrong-step).
  • Weekly antenatal fetal surveillance testing — Antenatal testing is added for diabetes requiring medication or with poor glycemic control, not for a newly diagnosed diet-controlled patient at 28 weeks (premature-surveillance trap).
Question 2ReproductiveMedium
A 4350 g (9 lb 9 oz) male infant is born at 38 weeks to a 32-year-old woman whose pregnancy was complicated by gestational diabetes managed with insulin in the third trimester. At 1 hour of life the infant is jittery and lethargic; a heel-stick glucose is 28 mg/dL. Physical exam shows a plethoric, large-for-gestational-age infant with no dysmorphic features. The hypoglycemia resolves over the following 48 hours with feeding and intravenous dextrose. Which of the following best explains the findings?
  • AFetal hyperinsulinemia driven by chronic intrauterine hyperglycemia
  • BFetal pancreatic beta-cell destruction from maternal autoantibodies
  • CGlycogen storage deficiency from impaired fetal gluconeogenesis
  • DTransplacental passage of maternal insulin suppressing fetal glucose
Reveal answer & full explanation
Correct answer: A — Fetal hyperinsulinemia driven by chronic intrauterine hyperglycemia
  • AFetal hyperinsulinemia driven by chronic intrauterine hyperglycemia✓
  • BFetal pancreatic beta-cell destruction from maternal autoantibodies
  • CGlycogen storage deficiency from impaired fetal gluconeogenesis
  • DTransplacental passage of maternal insulin suppressing fetal glucose

Why Fetal hyperinsulinemia driven by chronic intrauterine hyperglycemia is correct

  • Maternal glucose crosses the placenta freely, so maternal hyperglycemia produces sustained fetal hyperglycemia. The fetal pancreas responds with beta-cell hyperplasia and hyperinsulinemia.
  • Insulin is the major fetal anabolic/growth hormone, driving fat and glycogen deposition and organomegaly, producing macrosomia (>4000 g) and a plethoric infant.
  • After cord clamping the maternal glucose supply abruptly stops, but the infant's elevated insulin level persists, causing transient neonatal hypoglycemia that resolves over 24-48 hours with feeding and dextrose. This is the classic Pedersen hyperglycemia-hyperinsulinemia hypothesis.

Why the others are wrong

  • Transplacental passage of maternal insulin suppressing fetal glucose — insulin is a large polypeptide that does not cross the placenta; only glucose (and maternal antibodies) crosses. Maternal insulin therapy does not directly lower fetal glucose.
  • Fetal pancreatic beta-cell destruction from maternal autoantibodies — this describes autoimmune (type 1) diabetes pathophysiology and would cause hyperglycemia, not hyperinsulinemic hypoglycemia; it is not the mechanism in GDM.
  • Glycogen storage deficiency from impaired fetal gluconeogenesis — an inborn metabolic defect would cause persistent, recurrent hypoglycemia and hepatomegaly, not transient hypoglycemia with macrosomia that resolves in 48 hours.
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Risk factors

  • Obesity (BMI ≥30)
  • Advanced maternal age (≥25, increased risk >35)
  • Family history of diabetes (first-degree relative)
  • Prior GDM, prior macrosomic infant (>4000 g), prior stillbirth
  • PCOS
  • Ethnicity (Hispanic, Asian, Native American, Black)
  • Hypertension
  • Glycosuria or HbA1c 5.7-6.4% before pregnancy

Pathophysiology

Placental hormones (human placental lactogen, growth hormone variant, cortisol, progesterone) progressively induce insulin resistance starting at ~20 weeks, requiring 2-3x baseline insulin secretion to maintain normoglycemia. Women with insufficient pancreatic β-cell reserve develop hyperglycemia. Maternal hyperglycemia → fetal hyperglycemia → fetal hyperinsulinemia → macrosomia, organomegaly, neonatal hypoglycemia.

Clinical presentation

Symptoms

  • Usually asymptomatic — identified by screening
  • Polyuria, polydipsia (uncommon)
  • Macrosomia or polyhydramnios may be the presenting clue on ultrasound

Signs / physical exam

  • Often normal exam
  • Obesity, acanthosis nigricans
  • Fundal height larger than dates

Differential diagnosis

  • Undiagnosed pregestational type 2 diabetes — A1c ≥6.5% or fasting glucose ≥126 at first prenatal visit; warrants pregestational management
  • Type 1 diabetes onset in pregnancy — Rare; ketoacidosis, severe hyperglycemia, often weight loss
  • MODY — Family history pattern; genetic testing
  • Stress hyperglycemia — Transient, related to acute illness

Diagnostic workup

Labs

  • Early screening at first prenatal visit for high-risk women (BMI ≥30, prior GDM, A1c ≥5.7%, etc.) — A1c, fasting glucose, or 1-h GCT
  • Universal screening at 24-28 weeks gestation
  • Two-step approach (most common in US): 50 g 1-hour glucose challenge test (non-fasting) → if ≥130-140 mg/dL (institutional cutoff), proceed to 3-hour 100 g OGTT
  • 3-hour OGTT diagnostic: ≥2 abnormal values (fasting ≥95, 1-h ≥180, 2-h ≥155, 3-h ≥140 mg/dL — Carpenter-Coustan criteria)
  • One-step IADPSG/ADA alternative: 75 g 2-hour OGTT, abnormal if fasting ≥92, 1-h ≥180, or 2-h ≥153

Imaging

  • Ultrasound for fetal growth at 28-32 weeks and again at 36 weeks; assess for macrosomia, polyhydramnios
  • Antepartum testing (NST, BPP) starting 32-36 weeks in patients on medications or with poor glycemic control

Diagnostic algorithm

TestTimingCutoffInterpretation
50 g 1-h GCT (screen)24-28 weeks (universal)≥130-140 mg/dLAbnormal → proceed to OGTT
3-h 100 g OGTTAfter abnormal GCTFasting ≥95, 1h ≥180, 2h ≥155, 3h ≥140≥2 abnormal = GDM (Carpenter-Coustan)
2-h 75 g OGTT (1-step)24-28 weeksFasting ≥92, 1h ≥180, 2h ≥153≥1 abnormal = GDM (IADPSG)
Postpartum 75 g 2-h OGTT4-12 weeks postpartumFasting ≥126 or 2h ≥200Diagnose persistent T2DM
GDM screening and diagnostic thresholds.

Treatment

First-line

  • Medical nutrition therapy — referral to registered dietitian; carbohydrate-controlled diet (~40% carbs, 20% protein, 40% fat), 3 meals + 2-3 snacks
  • Exercise — 30 min moderate activity most days
  • Self-monitoring of blood glucose 4 times daily: fasting + 1- or 2-hour postprandial
  • Glycemic targets: fasting <95 mg/dL, 1-h postprandial <140, 2-h postprandial <120

Pharmacologic therapy (if MNT fails)

  • Insulin — first-line pharmacotherapy per ACOG; does not cross placenta
  • • Basal: NPH or detemir at bedtime ± morning
  • • Bolus: regular insulin, aspart, or lispro before meals
  • • Start ~0.7-1.0 U/kg/day total (gestational age dependent), split per pattern
  • Metformin — alternative; crosses placenta; reasonable for women who decline insulin
  • Glyburide — historically used but has fallen out of favor due to higher rates of macrosomia and neonatal hypoglycemia

Intrapartum

  • Monitor glucose every 1-2 hours; target 70-110 mg/dL
  • Insulin drip if hyperglycemic
  • Avoid prolonged fasting; D5 in fluids if needed
  • Continuous fetal monitoring
  • Consider scheduled C-section if estimated fetal weight ≥4500 g (per ACOG)

Postpartum

  • Discontinue insulin/oral agents after delivery
  • Encourage breastfeeding
  • 75 g 2-h OGTT at 4-12 weeks postpartum to screen for persistent type 2 diabetes
  • Lifelong diabetes screening every 1-3 years (50% lifetime T2DM risk)
  • Counsel about future pregnancy planning

Complications

  • Maternal: preeclampsia (2-4x risk), polyhydramnios, cesarean delivery, type 2 diabetes after pregnancy (~50% within 10 years), recurrent GDM
  • Fetal: macrosomia (>4000 g), shoulder dystocia, birth trauma (brachial plexus injury, fracture), stillbirth (poor control)
  • Neonatal: hypoglycemia, hyperbilirubinemia, polycythemia, respiratory distress syndrome (delayed surfactant), hypocalcemia, cardiomyopathy
  • Long-term offspring: childhood obesity, metabolic syndrome, type 2 diabetes

PANCE pearls

  • Universal screening at 24-28 weeks gestation is recommended by ACOG; high-risk women should be screened at the first prenatal visit.
  • Insulin is first-line pharmacologic therapy for GDM per ACOG; metformin is an alternative for women who decline insulin.
  • Glyburide is associated with worse outcomes (macrosomia, neonatal hypoglycemia) than insulin or metformin; ACOG no longer recommends it as first-line.
  • 75 g 2-hour OGTT at 4-12 weeks postpartum identifies persistent type 2 diabetes; lifetime follow-up screening is essential.
  • Macrosomia warrants discussion of scheduled cesarean at EFW ≥4500 g (or ≥4000 g in diabetic pregnancies per some sources) to reduce shoulder dystocia.
  • Diabetic ketoacidosis can occur at lower glucose levels in pregnancy (euglycemic DKA) — maintain high suspicion in symptomatic patients.

References

  • ACOG PB 190 — ACOG Practice Bulletin No. 190: Gestational Diabetes Mellitus
  • ADA 2024 — ADA Standards of Care: Management of Diabetes in Pregnancy (Diabetes Care 2024)
  • USPSTF 2021 — Screening for Gestational Diabetes: USPSTF Recommendation Statement (JAMA 2021)
  • HAPO Study — Hyperglycemia and Adverse Pregnancy Outcomes (HAPO Study Cooperative Research Group, NEJM 2008)

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