Maternal Hyperglycemia and Placental Glucose Transfer
Maternal glucose crosses the placenta unchecked, unlike insulin.
- GLUT1: Transfer mechanism (facilitated diffusion)
- ↑↑: Maternal glucose (poor glycemic control)
- None: Insulin crossing (insulin does not cross placenta)
- OGTT: Screening test (oral glucose tolerance test)
Why glucose but not insulin crosses
Placeholder: glucose diffuses via GLUT transporters while maternal insulin cannot cross.
Fetal Pancreatic Beta-Cell Response to Glucose Load
The fetus compensates with its own excess insulin production.
- Hyperplasia: Fetal beta-cells (compensatory growth)
- Anabolic: Insulin effect (growth-promoting hormone)
- 1952: Pedersen hypothesis (glucose-insulin theory)
- Hypoglycemia: Postnatal risk (after cord clamping)
The Pedersen hypothesis
Placeholder: maternal hyperglycemia drives fetal hyperinsulinemia and overgrowth.
Excess Fat Deposition and Fetal Overgrowth
Insulin drives disproportionate fat deposition on the trunk and shoulders.
- >4000g: Macrosomia threshold (birth weight definition)
- Truncal: Fat distribution (shoulders & abdomen)
- Ultrasound: Diagnosis (estimated fetal weight)
- ↑15–45%: Incidence in GDM (vs general population)
Asymmetric fetal overgrowth
Placeholder: diabetic macrosomia concentrates fat on shoulders and trunk, not head.
Shoulder Dystocia — Obstruction of Fetal Shoulders at Delivery
Enlarged shoulders can impact behind the maternal pubic bone.
- Impaction: Dystocia definition (anterior shoulder vs symphysis)
- ↑↑: Risk at >4500g (sharp risk increase)
- McRoberts: Key maneuver (hip flexion technique)
- Brachial plexus: Complication (possible nerve injury)
Recognizing and managing dystocia
Placeholder: shoulder dystocia is an obstetric emergency needing prompt maneuvers.
Weighing Cesarean Delivery Against Dystocia Risk
Clinicians balance surgical risk against the danger of a difficult vaginal birth.
- >4500g: EFW threshold (diabetes) (consider planned cesarean)
- >5000g: EFW threshold (no diabetes) (consider planned cesarean)
- Shared decision: ACOG guidance (individualized counseling)
- ±10–15%: Ultrasound error (EFW estimation limits)
Balancing surgical and dystocia risk
Placeholder: EFW guides but does not mandate the mode of delivery decision.
Placeholder: decisions combine EFW, maternal pelvis, and glycemic history.