This double-blind, Phase 2, multiple-dose study will be conducted to evaluate the PK/PD, efficacy, safety, and tolerability of apitegromab in subjects <2 years old with 5q autosomal recessive SMA who have delayed motor milestones for their age attributed to SMA at the discretion of the Investigator or a Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders (CHOP-INTEND) score <55.

Inclusion Criteria:

1. Is <2 years old at the time of the informed consent
2. Had a gestational age of ≥35 weeks and gestational body weight ≥2.0 kg at birth
3. Has confirmed diagnosis of 5q autosomal recessive SMA
4. Has confirmed presence of SMN2 gene copy(ies)
5. Must have been treated with an approved SMN1-targeted therapy (ie, onasemnogene abeparvovec-xioi) or are continuing to be treated with an approved SMN2-targeted therapy (ie, nusinersen or risdiplam)
6. Body weight for age is no less than 1st percentile based on the WHO Child Growth Standards at the Screening Visit
7. Has delayed motor milestones for age attributed to SMA at the discretion of the Investigator or a CHOP-INTEND score <55

Exclusion Criteria:

1. Nutritional status that is not anticipated to be stable throughout the study or medical necessity for a gastric feeding tube, where most feeds are administered by this route
2. Major orthopedic issues such as severe scoliosis or severe contractures or interventional procedure, including spine or hip surgery, which is considered to have the potential to substantially limit the ability of the subject to be evaluated on any motor function outcome measures, within 6 months before Screening or anticipated during the study
3. Any other physical limitations (eg, the subject requires cast for contractures) that would prevent the subject from undergoing motor function outcome measures throughout the study.

The Sponsor, the subject's caregiver, Investigators, and site personnel, with the exception of the designated unblinded personnel (eg, site Pharmacist), will be blinded to the treatment assignment.