Every part of having a special needs child surely is thrown into sharp relief when you are very much at fault, surely? Would they even let me keep it post diagnosis? Would they start looking into my family generally?
Just re-read this. With kindness, it's sort of irrational: like you've decided the less-that-1%-risk is going to happen.
Do speak to an experienced Dr or Midwife for reassurance.
The issues regarding your family dynamic only you know... is there a chance you are underplaying these to the 'damaged baby' scenario to rationalise any difficult choice you are considering? It is not wrong to choose to end a pregnancy for reasons other than health (if your own personal beliefs don't forbid it) - but you need to be very clear with yourself about really why you are considering this. You need to let yourself be ok to exercise your right to choose based on actual risks and actual issues - not an unproven assumption which you fixate on.
If you choose to terminate because you say to yourself it's because there are huge risks of 'special needs' when deep down you know it is untrue and there are other reasons (which might well be very valid reasons in themselves), it could create psychological issues in future for you and indeed your DH.
The 'older dad', 'special needs' and 'like and only child' worries have been reassured on this thread but you keep coming back to them.
The fact you found looking after young kids very challenging and it made you unhappy (which to me sounds a vitally important consideration), your financial and career worries... these are almost said as an aside before going back to the 'I'm only thinking of the others' stuff.
But to me, they sound vitally important and actually possibly the crux of it.
This is your body - you are perfectly entitled and right to make choices based on YOUR hopes, plans, life and psychological experiences without 'I must think of the others' excuses.
It is very unlikely there is anything wrong with the embryo.
Putting that aside, who can you talk to about YOUR fears and the psychological pressure you feel from parenthood?