Reproductive · PANCE / PANRE

Gestational Diabetes Mellitus (GDM)

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.

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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

Test	Timing	Cutoff	Interpretation
50 g 1-h GCT (screen)	24-28 weeks (universal)	≥130-140 mg/dL	Abnormal → proceed to OGTT
3-h 100 g OGTT	After abnormal GCT	Fasting ≥95, 1h ≥180, 2h ≥155, 3h ≥140	≥2 abnormal = GDM (Carpenter-Coustan)
2-h 75 g OGTT (1-step)	24-28 weeks	Fasting ≥92, 1h ≥180, 2h ≥153	≥1 abnormal = GDM (IADPSG)
Postpartum 75 g 2-h OGTT	4-12 weeks postpartum	Fasting ≥126 or 2h ≥200	Diagnose 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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