Maternal Diabetes Mellitus is the Most Likely Cause
A full-term infant presenting with a plethoric (ruddy, red) face and appropriate skin weight is most consistent with maternal diabetes mellitus (DM), which causes polycythemia in the neonate due to chronic intrauterine hypoxia and compensatory erythropoiesis.
Clinical Reasoning for Maternal Diabetes
Why Diabetes is the Primary Answer
Polycythemia mechanism: Maternal hyperglycemia leads to fetal hyperglycemia and hyperinsulinemia, which increases fetal oxygen consumption and causes relative chronic hypoxia, stimulating erythropoietin production and resulting in polycythemia that manifests as plethora (ruddy/red appearance) 1
Full-term delivery: Infants of diabetic mothers are typically delivered at or near term (unlike placental insufficiency which causes preterm delivery), and the infant described is full-term 1
Appropriate weight: While the question mentions "skin wt 1.8" (likely referring to appropriate size), infants of diabetic mothers can be large-for-gestational-age or appropriate-for-gestational-age, both consistent with this presentation 1
Why Other Maternal Conditions Are Less Likely
Placental Insufficiency - Unlikely
Contradictory presentation: Placental insufficiency typically causes intrauterine growth restriction (IUGR), low birth weight, and a pale, wasted appearance—not plethora 2, 3
Preterm delivery: Placental insufficiency usually necessitates early delivery due to fetal compromise, not full-term gestation 1, 3
Systemic Lupus Erythematosus (SLE) - Possible but Less Direct
Indirect mechanism: While maternal SLE is associated with placental dysfunction, preeclampsia, and IUGR, it does not directly cause neonatal polycythemia/plethora 2, 4, 3
Expected findings differ: SLE pregnancies more commonly result in growth-restricted, premature infants with low placental weight—not plethoric, full-term infants 4, 3
Neonatal lupus manifestations: When SLE affects the neonate directly (neonatal lupus), it presents with cutaneous lesions, congenital heart block, or transient cytopenias—not polycythemia 2, 1
Maternal Infection - Unlikely
No characteristic plethora: Maternal infections do not typically cause neonatal polycythemia or plethoric appearance 1
Different presentation: Infected neonates present with sepsis signs (temperature instability, poor feeding, lethargy), not ruddy complexion 1
Immediate Neonatal Management Priorities
For Polycythemia from Maternal Diabetes
Confirm polycythemia: Obtain venous hematocrit immediately; polycythemia is defined as venous hematocrit >65% in term infants 1
Monitor glucose: Screen for hypoglycemia within first hour of life and continue frequent monitoring, as hyperinsulinemia persists after birth while maternal glucose supply is interrupted 1
Assess for other complications: Evaluate for hypocalcemia, hyperbilirubinemia, respiratory distress, and cardiac abnormalities (hypertrophic cardiomyopathy) that commonly accompany infant of diabetic mother syndrome 1
Partial exchange transfusion: Consider if symptomatic polycythemia (lethargy, poor feeding, respiratory distress) or if venous hematocrit >70%, though this is controversial and should be individualized based on symptoms 1
Critical Pitfall to Avoid
Do not assume plethora is benign or normal variation—it indicates significant polycythemia requiring immediate evaluation and monitoring for associated metabolic derangements, particularly hypoglycemia, which can cause permanent neurologic damage if untreated 1.