A 34-year-old G3P2 at 34 weeks gestation with pre-gestational diabetes has an ultrasound showing polyhydramnios (amniotic fluid index 32 cm). Which mechanism best explains polyhydramnios in diabetic pregnancies?
- A Poorly controlled diabetes impairs fetal swallowing due to decreased fetal central nervous system activity
- B Maternal hyperglycemia causes fetal osmotic diuresis via fetal hyperglycemia, increasing fetal urine output ✓
- C Diabetic vasculopathy reduces placental perfusion, leading to fetal hypoxia-induced polyuria
- D Maternal ketoacidosis alters amniotic fluid osmolality, drawing fluid from the fetal compartment
Correct answer: B. Maternal hyperglycemia causes fetal osmotic diuresis via fetal hyperglycemia, increasing fetal urine output
Explanation
In diabetic pregnancies, maternal hyperglycemia leads to fetal hyperglycemia, which causes fetal osmotic diuresis and increased fetal urine output, the primary mechanism of polyhydramnios. Distractor A describes a mechanism more associated with fetal neurological anomalies or anencephaly, not diabetes. Distractor C describes a mechanism of oligohydramnios in placental insufficiency. Distractor D is incorrect because diabetic ketoacidosis does not primarily cause polyhydramnios through osmotic shifts.
Reference: Williams Obstetrics, 26th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.