Macrosomia is defined as the condition during which the neonate's body weight at birth is higher than the 90th percentile for gestational age or higher than 4 kg (8.82 lb). Macrosomia occurs in every type of diabetic pregnancy unless there is angiopathy. Neonates are large and corpulent, with an excessive accumulation of fat in the abdomen and shoulders. Furthermore, visceral enlargement occurs. Macrosomia predisposes to trauma during delivery, especially shoulder dystocia, which is prevented by caesarian section.
The proposed mechanism for the abnormal foetal conditions (foeto-pathy) includes hypertrophy of the ß-cells of the pancreatic islets and hyperinsulinaemia of the foetus, due to the mother's hyperglycaemia. A result of this hyperinsulinaemia is an increased synthesis and storage of fat. The foetus is big for its gestational age (macrosomia). This condition is the result of poor glycaemic control of the mother, especially during the last two trimesters of pregnancy. Good glycaemic control minimizes the risk for this complication.
Furthermore, macrosomia causes hypoxaemia due to increased oxygen needs, as well as polycythemia, catecholamine overproduction, hypertension and cardiomegaly.
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