I'm going to weigh in here with my story.
We had dodgy 12 week bloods with dd2 but we're told we were low risk so nothing was done. Scan at 19 weeks picked up no problems so anomaly was pushed back to 20+6 (latest it's advised). Sonographer picked up a small brain anomaly but didn't know what it was so referred us to kings, luckily they squeezed me in the next day as I was able to drop everything, next available appointment was a week later.
At our scan at kings they sucpected T13 or T18, both horrific an incompatible with life. Had an amnio on the day. They rushed our initial results back in 48 hours. The results came back as a positive screen for full T18. We arranged a post 20 week termination (to be clear terminations post 20 weeks and post 24 weeks are done in exactly the same way) by the time dd2 came I was 22+2. That was with lots of rushing to next day, squeezed in appointments and a 'clear' diagnosis. If she had had a mosaic form it would have taken longer for the result to come back.
We were close to that 24 week line. If thing had been delayed or her diagnosis hadn't been an 'easy' one then we would have easily passed the 24 week point. Our dd2 still would have had a lethal condition but I would have been forced to carry her to term, probably resulting in a still birth but quite possibly resulting in her dying in labour or very soon after. We made our decision based on how much suffering she would have gone through in her short life. I absolutely respect others choice to carry babies with trisomies to term however, as parents we should be allowed to make the heartbreaking choice that we think is right for our baby.
I have suffered huge trauma, ptsd and guilt at her loss and out tfmr. I cannot imagine how much worse that would have been if I'd had the choice taken away from me.