Introduction. Skeletal dysplasias are rare genetic disorders that affect bone and cartilage development, with a prevalence of 3.2 cases per 10,000 newborns in South America. Neonatal auxologic evaluation enables the early detection of these conditions, serving as a cost-effective and accessible tool for early intervention. This study aims to determine the prevalence of low birth weight, short body length, and body disproportion at birth in patients with a molecular diagnosis of achondroplasia (ACH), hypochondroplasia (HCH), SHOX gene alterations (SHOX), and familial hypophosphatemic rickets (FHR). Population and methods. Retrospective descriptive study based on medical records of patients evaluated between 2002 and 2023 in a high-complexity pediatric hospital. Patients with skeletal dysplasia and complete anthropometric data at birth were included in the study. Weight, body length, and head circumference were analyzed, with the calculation of Z-scores according to the INTERGROWTH-21st standards and the head circumference/body length index using Argentine references. Results. Of the 581 patients, 453 were included (ACH 62%, HCH 12%, SHOX 8%, FHR 18%); 31% of the neonates with ACH and 12% with HCH had body length < ̵2 SD. True macrocephaly (>2 SD) was observed in 47% (ACH) and 32% (HCH), and relative macrocephaly in 57% and 28%, respectively. Conclusion. Low body length at birth was more frequent in achondroplasia and hypochondroplasia. Relative macrocephaly, also prevalent in these groups, highlights the value of the head circumference/body length index as a neonatal screening tool.