One of the dangers of gestational diabetes is giving birth to a big baby. The placenta produces hormones that spur growth of the baby and the baby gets more sugar than he needs. Large babies are a problem because they usually need to be delivered by Caesarean section. There is also increased risk of hypertension and preeclampsia.
The largest baby on record, according to the Guinness Book of World Records, was born in Canada in 1879. That baby weighed 23 lbs, 12 oz but died 11 hours later. Recently an Indonesian woman gave birth to a 19.2 lb infant.