OBJECTIVE Children with cerebral palsy (CP) can show an increase in gross motor function until the age of 9 to 10 years under the standard of care. Additionally, the motor development can have large individual fluctuations. Therefore, in clinical setting, it is not trivial to estimate the effect of an additional therapeutic intervention at this age interval. The study aim was to develop a method which allows quantification of the gross motor function changes over 6 months of the individual child with CP. METHODS The present study was a single center retrospective analysis. Data were collected in children with CP who participated in a rehabilitation program between 2006 and 2016. The gross motor function of the children was measured with the Gross Motor Function Measurement (GMFM-66). Reference centiles for the GMFM-66 were created with data before starting the rehabilitation program. The variability of the evolution of the GMFM-66 was assessed with data at the start and the end of a 6-month observational phase of standard of care. RESULTS In total, the GMFM-66 data of 919 children before starting the rehabilitation program were available (age 6.49 ± 2.49 years, GMFCS-level I-V). For 515 study participants (6.76 ± 2.30 years, GMFCS-level I-V), data were also available at the start and the end of a 6-month observational phase. CONCLUSIONS The presented method helps to guide the clinician to track the individual patient's gross motor development and assess the additional effect of an additionally applied intervention while taking into account the expected progression of gross motor function under standard of care.