The Bayley-4 Fine Motor domain assesses hand and finger coordination tasks such as reaching and grasping. Standard scores range from 40 to 160 (mean = 100; standard deviation = 15). Higher scores indicate better fine motor development.

The Bayley-4 Expressive Language Standard Score is derived by converting raw scores to age-normed standard scores based on normative data for children ages 1 to 42 months. Standard scores range from 40 to 160, with a mean of 100 and a standard deviation of 15. Higher scores indicate more advanced expressive language abilities.

The Bayley-4 Receptive Language domain assesses a child's ability to understand spoken language. Scores range from 40 to 160 (mean = 100; SD = 15). Higher scores reflect better language comprehension.

The WHO Motor Milestone (WHOMM) checklist records developmental attainment of 6 gross motor milestones, scored as achieved or not achieved. The score reflects the number of milestones achieved at each visit. Higher scores indicate greater motor milestone attainment for age.

This outcome measures the proportion of participants who have initiated any early intervention service (e.g., physical, occupational, or speech therapy), based on caregiver report or medical record review.

This outcome records the age (in months) at which participants first received early intervention services, based on caregiver report or documentation in the medical record.

This outcome measures the proportion of participants who began any DMD-targeted pharmacologic treatment (e.g., corticosteroids, exon-skipping therapies) during the study period, based on caregiver report or medical records.

This outcome records the age (in months) at which each participant initiated pharmacologic treatment for Duchenne Muscular Dystrophy, as reported by caregivers or documented in the medical record.

This outcome reports the percentage of participants diagnosed with a neurobehavioral condition (e.g., autism spectrum disorder, ADHD) during the study, based on caregiver report or medical record review.

This outcome records the age (in months) at which each participant received their first neurobehavioral diagnosis, as determined by caregiver report or medical record.

The DMD Caregiver Reported Health Index Short Form (DMD-HI-SF) is a caregiver-reported questionnaire assessing burden related to the child's health. Total score is derived by summing item responses and converting to a standardized T-score. Higher scores indicate greater symptom burden.

The NSAA is a 17-item clinician-rated motor function scale validated for ambulant children with DMD. Each item is scored from 0 (unable) to 2 (normal), for a total score range of 0 to 34. A single NSAA total score will be recorded at the time of the participant's first assessment at age 3 years.