Are there symptoms of diabetes onset during pregnancy?

Yes: Symptoms are typically more thirst, more urination, more hunger, blurry vision, fatigue.

Large baby.: Typically the mother has no symptoms from gestational diabetes mellitus. Sometimes, the uterus grows too big too fast from a large baby and/or increased fluid as a result of uncontrolled diabetes mellitus. Other symptoms could be frequent urination and excessive thirst - both common in normal pregnancies.

Maybe: You may or may not have symptoms like increased thirst or frequent infections. Make sure you get tested (it's part of the testing they do anyway), because if the mother has diabetes the baby can have risks for macrosomia (large birth weight), increased amniotic fluid, fetal death, neonatal hypoglycemia, jaundice, shoulder dystocia, brachial plexus injury, cesarean section, or cardiomyopathy.