Why are some babies large who are born to women with diabetes?

In the past, babies born to women with diabetes tended to be large. This problem, called macrosomia, was the baby’s response to having extra amounts of glucose from the mother’s blood. 

Macrosomia is less common, but sometimes the baby is too large or the woman’s pelvis is too small for a safe vaginal delivery. Trying to deliver a too-large baby vaginally can result in shoulder damage or respiratory distress for the baby. In this case, a C-section is performed.