This is a decision made by the pregnant person, informed by her/their doctor. I can also see her/their family having some input in informing the pregnant person's decision, to the extent that they are not being coerced by their family.

As far as outside agents, such as ourselves, are involved, pregnancy is a medical issue. This is the only responsible way for society to handle these questions. Society is able to provide support for parents, esp those who are caretakers of children with severe disabilities, and we can improve education and access to contraceptives. But it is irresponsible and unethical to become involved in the medical decisions of others under any circumstances.

If we really care about unborn children we should care doubly about born children and the families who take on the responsibility of ensuring those disabled individuals are able to experience love and joy. Trying to collapse ethics and morals into pure individual choice is a scam.