One-page cheat sheet: Gestational diabetes

Gestational diabetes mellitus (GDM) is one of those “looks simple, tests deep” topics: it’s basically pregnancy-induced insulin resistance, but the exam loves the screening timeline, diagnostic cutoffs, and maternal vs fetal complications. Here’s a high-yield, one-page cheat sheet you can basically memorize and move on.

The 10-second definition (one-liner)

Gestational diabetes = glucose intolerance first recognized in pregnancy due to placental hormones causing insulin resistance (most notable in 2nd–3rd trimester).

Core pathophys (Step 1 favorite)

Pregnancy is a diabetogenic state because the placenta makes hormones that increase insulin resistance, including:

Human placental lactogen (hPL) (classic buzzword)
Progesterone, estrogen, cortisol, prolactin

Net effect:

Mom needs more insulin as pregnancy progresses
If pancreatic $\beta$ -cells can’t keep up → hyperglycemia (GDM)

Why it matters: Maternal glucose crosses the placenta → fetal hyperglycemia → fetal pancreas responds with hyperinsulinemia (key driver of complications).

Mnemonic / visual: “SUGAR BABY” (complications snapshot)

Think: Mom’s sugar → Baby’s insulin → Big + Breathless + Blue + Low sugar after birth

SUGAR BABY

S: Shoulder dystocia (from macrosomia)
U: Up-sized baby = macrosomia ( $> 4{,}000$ – $4{,}500$ g; varies by source)
G: Glucose low after delivery (neonatal hypoglycemia)
A: ARDS (delayed surfactant maturation)
R: Red cells up = polycythemia → hyperbilirubinemia
B: Birth trauma
A: Anomalies (mainly with pre-gestational diabetes, not typical isolated GDM)
B: BP high in mom (↑ risk preeclampsia)
Y: “Years later” → ↑ risk of type 2 diabetes (mom and child)

High-yield nuance:

Congenital malformations are most associated with pre-gestational diabetes (hyperglycemia during organogenesis, 1st trimester).
GDM usually presents after insulin resistance rises (2nd/3rd trimester), so structural anomalies are less emphasized—but the future metabolic risk is real.

Screening (Step 2 loves timing + test choice)

Who gets screened?

Universal screening is common in the US.
Screen earlier (first prenatal visit) if high risk:
- Prior GDM, obesity, strong FHx, PCOS, prior macrosomic infant, etc.

When?

24–28 weeks for routine screening.

How? Two main approaches

1) Two-step approach (common in the US)

1-hour 50 g glucose challenge test (non-fasting)
- If abnormal → proceed to diagnostic test
3-hour 100 g oral glucose tolerance test (OGTT) (fasting)

2) One-step approach

2-hour 75 g OGTT (fasting), used in some guidelines/settings

Diagnosis: high-yield cutoffs

Two-step diagnostic test: 3-hour 100 g OGTT

Diagnosis typically made if $\ge 2$ values are abnormal (Carpenter-Coustan thresholds are commonly tested):

Time	Plasma glucose threshold (mg/dL)
Fasting	≥ 95
1 hour	≥ 180
2 hour	≥ 155
3 hour	≥ 140

Tip: USMLE questions often won’t nitpick which exact criteria; they’ll test the concept: screen at 24–28 weeks and confirm with a diagnostic OGTT.

Management (what to do, in order)

First-line

Diet + exercise
Self-monitored blood glucose (fasting + postprandial)

Common glycemic targets (often used clinically/tested conceptually):

Fasting: < 95 mg/dL
1-hour postprandial: < 140 mg/dL
2-hour postprandial: < 120 mg/dL

If not controlled → medication

Insulin is classic first-line pharmacotherapy in pregnancy (most test-safe answer)
Some guidelines allow metformin or glyburide, but exams often prefer insulin due to longest safety track record and tight control.

Delivery considerations (conceptual, not calculator-heavy)

Concern for macrosomia → shoulder dystocia risk
Decisions depend on estimated fetal weight, glycemic control, and obstetric factors
(USMLE tends to ask complications rather than exact delivery thresholds.)

Maternal complications (testable list)

GDM increases risk of:

Preeclampsia
Polyhydramnios (from fetal osmotic diuresis → ↑ amniotic fluid)
Cesarean delivery
Future type 2 diabetes (major counseling point)

Fetal/neonatal complications (high yield)

Mechanism = fetal hyperinsulinemia.

Macrosomia → shoulder dystocia, birth trauma
Neonatal hypoglycemia after birth
- Placenta removed → maternal glucose supply stops
- Baby still has high insulin → glucose crashes
Respiratory distress syndrome (RDS)
- Insulin can antagonize cortisol effects → ↓ surfactant → tachypnea, grunting
Polycythemia → hyperbilirubinemia
Hypocalcemia (less common but classically associated)

Classic vignette patterns

Pattern 1: Screening question

26-week pregnant patient → 1-hour glucose challenge abnormal → next step is diagnostic OGTT

Pattern 2: “Big baby” plus neonatal hypoglycemia

Infant of diabetic mother with jitteriness/lethargy shortly after birth → check glucose (treat hypoglycemia)

Pattern 3: RDS in infant of diabetic mother

Preterm-ish newborn with respiratory distress; mom had diabetes → think delayed surfactant maturation

Postpartum follow-up (easy points)

GDM usually resolves after delivery, but risk persists
Do postpartum diabetes screening (commonly 4–12 weeks postpartum with OGTT in many guidelines)
Lifelong screening for diabetes every few years (test concept: increased T2DM risk)

“If you remember only 5 things”

Placental hormones (hPL) → insulin resistance in 2nd/3rd trimester.
Screen at 24–28 weeks.
Two-step: 50 g screen → 100 g 3-hour OGTT to diagnose.
Baby complications are driven by fetal hyperinsulinemia: macrosomia, hypoglycemia, RDS.
Insulin is the safest test answer if meds are needed; mom needs postpartum screening.