How does gestational diabetes affect the baby negatively?

In many ways: The excess Insulin and glucose can make your baby grow larger than other babies.

Depends on control: The average blood sugar during pregnancy will affect the growth of the baby, potential for overgrowth of organs & general size& the risk of birth defects. A fetus makes their own Insulin & excess sugar is put into overgrowth of tissues, including the heart.Good control in pregnancy stops the problem. When mom's sugar stoops coming (cord clamped) babies Insulin production causes a low sugar problem.