I think you’re actually asking the wrong questions OP.
Just presume there is a reasonably decent likelihood of adverse things happening… your health being affected, baby having a genetic condition etc.
Because as soon as you accept a statistic, you’re still accepting the small likelihood that things won’t go the way you hope.
This really is a heart and not head-based decision.
Do you have the space and money for an additional child now?
What would happen in the next 20+ years if the child had additional needs?
What would happen in the next 20+ years if you, or your DH, or both of you, became seriously ill or died?
How do you feel about the whole nursery / starting school / birthday parties / supporting a child right through school scenario?
Before you found yourself pregnant, what would you have said your plans and ambitions for the next 20 years were? Would any of that be affected by having this DC?
What do you existing children require of you? Would your ability to give that change if you had this DC?
What if several of the above scenarios arose in combination?
Only you will know the answers OP.