Wow, these 20 week scan pictures are amazing.
hg I fainted a few weeks ago which I think was just low blood pressure. My bp is usually pretty low and has got lower in pregnancy. It is scary, but I think the advice is just basically not to stand up too quickly.
basket I'm sorry that you're so worried. I would say the first result (the actual screener) is much more reliable and worth listening to. I'm not sure how yours was calculated but assuming it's done the same way mine was, it's based on blood test, nasal bone and nuchal fold- three very distinct and reliable measurements.
I don't know much about soft signs of downs etc but I do know a lot about percentiles/average ranges and how they are calculated. Statistically, there's not a huge amount of difference between being on the 50th percentile and being on the 33rd. I know it feels counter-intuitive but it's not really that the 50th is bang on average and that 33rd is much lower. Also, in terms of percentiles, the statistical average range is 16-84 (I.e 86 or over is seen as above the average range). So, you can see that 33rd percentile is not really proportionately much smaller. So, in terms of the numbers, your baby's femurs are not disproportionately small compared to their other measurements- I hope that helps the worry a little.
If you need to satisfy your google urge, look up statistical Normal Distribution and percentile ranks for more explanation of the above. However, it's maths, fairly boring and might put you to sleep!