We declined as we had twins and results are not conclusive for them. We found out between 12-15 weeks that our twin boy had an insurmountable genetic problem via several scans.
It was only due to the nature of his problem that this was able to be picked up by a scan.
Knowing his issue was not about whether we would have a termination or not - it was about how best to manage it in terms of his pain, my health and his twins health. I imagine it would be the same in a singleton pregnancy.
I also have a close friend who declined testing - she had far too much amniotic fluid, was very poorly and after an early c section was found that her baby boy had T18, he passed away within 4 hours - she had never had time to prepare for it and the impact was huge.
I find it slightly naive to look at it as just an issue of 'well we wouldn't have an amnio or a termination'. Firstly, you don't know this until you have a diagnosis - would you carry a seriously ill child to birth only to pass away within minutes? Even if the answer is yes, many genetic disorders can carry a risk to the mother and being forearmed with knowledge is vital. Also if baby is undiagnosed and requires specialist treatment at birth, not knowing about their condition could cause real issues for baby and mum.
Most anomalies would show on a scan, but often not until 20 weeks, by which time a termination becomes very traumatic.
This is all highly, highly unlikely. However, I honestly think thy midwives do not tell you the full story as to why this testing is so very vital., it is most definitely not a simple question of 'would you have an amnio or termination?' It is far more complex than that.
I don't want to sound like the harbinger of doom but I do feel very strongly that we should be better informed with regards to the importance of these tests.