can only tell yu my experiences, its a personal thing.
Had all teh tests etc for ds1 as recommended, he was clear so no amnio needed. Then PG went tits up at 4 ish weeks, developed eclampsia and had an emergency induction. will never know if it contributed to his Sn buut there you go.
With ds2 had bloods etc all clear, all ok.
With ds3- we were given a high risk of Downs atfter the blood tests. We said we categorically wouldn't have the amnio (miscarriage risk) but the MW rather rpessured us into it. eventually we capitulated and went to the unit for the tests, absolutely petrified. We saw a ocunsellor who was very about the rpessure we'd been under- and left the room to 'give us a minute'- which was all we needed to bolt!
DS3 doesn't have Dowsn Syndrome, he does however have ASD with complex speech disorders etc and certainly ahs defin ite special needs, may well be with us all our lives. And you know what? he's our teddy bear, an absolute joy who brightens every day. So very glad we amde the decision we did, our lives have improved so much since he came along.
this time we haven't had anything bar scans. TBH I do worry a little that the baby migt have a syndrome that emasn he won't live long, but we do know that with 2 ASD siblings, he ahs a stonkingly high chance of ASD anyway so thats far more a focus and one that cannot be tested for even if we wanted (dont think we would). having ajd 2 Sn children and lived through both the strains and the joys, it would seem wrong for us as a family to make the decison to put a baby at risk through testing now- and there's no real point to the blood tests if you won't continue ime, they just scare you.
But obv my family is unusual, and I ahve the benefits of having worked with and nursed kids with various SN before becoming a parent. It si however worth remembering the most common disabilties either cannot be tested for (ASD rates are above 1% now) or occur at or around birth (eg CP) so there's no such thing as a gurantee.
Persoanlly I would make the decision absed on what you would do if you were carrying a child with Edwards as opposed to Downs which ahs such a wide base of disability levels anyhow. Sometimes extremes are easier to work with.