The prenatal lifestyle, including maternal dietary behaviour, is an important determinant of offspring health. This secondary cohort analysis of the GeliS (“healthy living in pregnancy”) trial investigated associations between antenatal dietary factors and neonatal weight parameters. The cluster-randomised GeliS trial included 2286 pregnant women. Dietary information was collected with food frequency questionnaires before or in the 12th (T0) and after the 29th week of gestation (T1). Consumption of vegetables (41.28 g per portion at T0, p = 0.001; 36.67 g per portion at T1, p = 0.001), fruit (15.25 g per portion at T1, p = 0.010) and dietary quality, measured with a Healthy Eating Index (39.26 g per 10 points at T0, p = 0.004; 42.76 g per 10 points at T1, p = 0.002) were positively associated with birth weight. In contrast, sugar-sweetened beverages (10.90 g per portion at T0, p = 0.003; 8.19 g per portion at T1, p = 0.047), higher sugar consumption at T0 (8.27 g per 10 g, p = 0.032) and early pregnancy alcohol intake (15.32 g per g, p = 0.039) were inversely associated with birth weight. Most other dietary factors were not associated with neonatal weight. Some components reflecting a healthy maternal diet were associated with a modest increase in offspring birth weight, whereas some unhealthy components slightly reduced neonatal weight.