Objectives: Gestational diabetes mellitus (GDM) affects up to 25% of all pregnancies worldwide. If untreated, GDM leads to increased complication rates both, in the mother and the fetus. Early diagnosis and adequate management of GDM are essential to avoid macrosomia. Nonetheless, neonates born to GDM mothers often have high birth weight. The aim of the study was to evaluate selected factors which can affect neonatal birth weight. Material and methods: The study included 152 women with GDM and 58 healthy pregnant controls. Anthropometric data of both parents, maternal biochemical parameters, and neonatal birth weight were collected. Results: The independent factors influencing neonatal birth weight were pregnancy duration, maternal smoking, as well as birth weight and current weight of the father. The risk of delivering a large for gestational age (LGA) infant increases with the diagnosis of GDM, higher maternal pre-pregnancy weight, and higher fasting glycaemia. No correlation between maternal fasting glycaemia, HbA1c, 1,5-AG, lipids and neonatal birth weight was found. Conclusions: Risk factors for LGA include gestational diabetes, high maternal pre-pregnancy weight, and current body weight of the father. Neither HbA1c nor 1,5-AG were reliable predictors of neonatal birth weight and occurrence of LGA in the studied population.