Newborn size at birth is an important indicator of fetal health, neonatal health, infant survival and childhood morbidity. The aim of our study was to assess and identify factors that significantly influence birth weight, length, and head circumference by performing a post hoc analysis of the data collected during the retrospective-prospective observational cohort study which was conducted at the Clinic for Gynecology and Obstetrics at the University Clinical Centre Kragujevac, Serbia. The influence of potential factors on birth weight, length, and head circumference was evaluated by multiple linear regression analysis. The study included 320 pregnant women and 332 newborns. Four factors had a significant negative effect on birth weight: smoking, twin pregnancy, use of methyldopa and corticosteroids during pregnancy. Smoking, twin pregnancy and corticosteroid use also had a significant negative effect on birth length. Negative effect on fetal head circumference at birth had smoking, use of corticosteroids and antibiotics during pregnancy. Maternal height and gestational age at birth showed a positive influence on fetal anthropometric measurements. Clinicians should pay attention to pregnant women with lower body height, twin pregnancy, who smoke and use corticosteroids, methyldopa and antibiotics.