Nelson, 34, gave birth to her twins prematurely at 31 weeks, having previously revealed she had experienced rare complications during the pregnancy.
In an Instagram post in October, she said she had “never felt prouder” of her body following the birth of her girls, adding that becoming a mother made her “realise how incredible my body actually is”.
Nelson left Little Mix in December 2020, and has since released music as a solo artist, including the single, Boyz, featuring Nicki Minaj, in 2021.
SMA is a progressive muscle-wasting disease that can cause death within two years if untreated.
In 2021, a life-changing gene therapy drug called Zolgensma was approved by the NHS to treat babies with the disease.
According to SMA UK, the drug delivers a healthy copy of the affected gene to the body, but timing is critical because irreversible damage may have already occurred in the nervous system.
At present, screening for SMA is only carried out on those who have a sibling with the condition.
SMA UK wants the disease to be added to a blood spot test that already checks newborn babies for 10 rare but serious conditions.
According to the charity, an estimated 47 babies were born with the condition in the UK in 2024, although about one in 40 people carry the altered gene that can cause the disease.