Large-for-gestational-age infant and infants of diabetic mothers — formative SAQs

SAQ 1 (10)

A 39-week male infant weighs 4400 g (above the 97th centile). His mother has gestational diabetes diagnosed at 26 weeks, managed with metformin, with a high third-trimester HbA1c. The infant is plump and plethoric with disproportionate shoulder and truncal adiposity and a palpable liver. At two hours of age a pre-feed glucose is low despite a breastfeed, and he is jittery. [2] [5]

1. Define the terms large-for-gestational-age (LGA), macrosomia and infant of a diabetic mother (IDM), and explain how they relate. (3) [6]
2. Outline the mechanism by which this infant became large yet hypoglycaemic (the Pedersen hypothesis). (4) [4]
3. Describe your stepwise management of his hypoglycaemia. (3) [2] [3]

Model answer

1. LGA is a centile descriptor — birthweight above the 90th centile for gestational age and sex. Macrosomia is an absolute weight term — commonly above 4000 g, with above 4500 g considered severe. IDM is a maternal-history diagnosis — any infant whose mother had pregestational or gestational diabetes. They relate but are not synonymous: most LGA infants are constitutionally large (non-diabetic), while a poorly controlled IDM is typically macrosomic and LGA because maternal glucose drives fetal hyperinsulinaemia. [6]
2. Glucose crosses the placenta freely but maternal insulin does not. Maternal hyperglycaemia therefore drives fetal pancreatic hyperinsulinaemia; insulin is the dominant fetal growth hormone, laying down fat, glycogen and muscle as the disproportionate adiposity and visceromegaly seen at birth. After cord clamping the maternal glucose supply stops abruptly, but fetal insulin secretion persists, driving blood glucose down — the mechanism of IDM hypoglycaemia within the first hours. [4]
3. Feed early and frequently (breast milk first, at least 2–3 hourly), check pre-feed glucose on a schedule, and treat low glucose up the ladder: feed, then buccal 40% dextrose gel (200 mg/kg) with a feed and a recheck, then intravenous 10% dextrose (2 mL/kg bolus then an escalating infusion) for persistent hypoglycaemia, reserving glucagon or hydrocortisone for refractory cases. Treat the infant and the trend, not the number alone. [2] [3]