Macrocephaly is typical for achondroplasia although at birth head circumference is usually only marginally increased (mean 36.7 cm). Head size increases rapidly during the first year of life and about 90% of adult head circumference is achieved by 2 years of age, which is almost 1 year earlier than in the general population.
At 2 years of age, head circumference in about 70% of infants is above normal population ranges, i.e. above +2 SDS; in about 45% above +3 SDS. At adult ages, head circumference in roughly 45% remain above +2 SDS, in 25% above +3 SDS and is on average 59.8 ±1.9 cm for males and 58.2 ±2.0 cm for females.
Head circumference (cm) in achondroplasia shows less sex differences than found in the background/general population. From Andrea Merker 2018 growth in achondroplasia
Growth chart of two girls both shunted at two years of age with different medical background: The head grows slower in both girls after shunting resulting in a decrease in head circumference SDS. The increased tempi in head circumference growth could hypothetically influence tempo in height. From Andrea Merker 2018 development of body proportion