In this study, researchers will learn more about the effects and safety of BIIB115, also known as salanersen. Specifically, researchers will learn more about how salanersen works in babies who have been diagnosed with SMA through genetic testing but have not yet started showing signs or symptoms. Most people with SMA have changes in a gene called survival motor neuron 1, also known as SMN1. These changes lower the amount of SMN protein in their bodies. Without enough of this protein, motor neurons and muscles cannot work properly. A similar gene called SMN2 can help replace some of the lost SMN protein in the body. Salanersen works by helping the SMN2 gene to make more SMN protein. In this study, participants will have either 2 SMN2 copies or 3 SMN2 copies. The higher the copy number, the less severe the participant's SMA is. The main goal of this study is to see if starting salanersen before signs or symptoms appear can prevent signs or symptoms of SMA or make them less severe. Researchers will use different tests to learn if motor symptoms are changing, including the World Health Organization (WHO) motor milestones. The main questions researchers want to answer in this study are: * How many participants with 2 copies of the SMN2 gene can sit without support at 12 months? * How many participants with 3 copies of the SMN2 gene can walk alone at 18 months? Researchers will also learn more about: * The effects on participants' motor symptoms and how many new movement milestones participants achieve. * How many participants stay free of SMA symptoms * How much salanersen gets into the fluid surrounding the brain and spinal cord. * How much salanersen gets into the blood. * How many participants have adverse events or serious adverse events. Adverse events are health problems that may or may not be caused by the study drug. This study will be done as follows: * First, participants will be screened to check if they can join the study. The screening period will be up to 28 days. * This is an open label study. This is a study in which the participants, study doctor, and site staff know which study drug participants are receiving. In this study, all participants will receive salanersen through an intrathecal injection, or one that is given into the fluid surrounding the brain and spinal cord. * There will be 2 parts in this study. During Part 1, participants will receive 2 doses of salanersen, about 12 months apart from each other. Part 1 will last up to 25 months. * During Part 2, participants will continue to receive salanersen. They will receive up to 3 doses, 12 months apart from each other. Part 2 will last up to 36 months. * During Part 1, participants will have up to 11 clinic visits and up to 3 phone calls. During Part 2, participants will have up to 7 clinic visits and up to 12 phone calls.