The first time round I didn't know I had to have bloods so went and got the scan - it said 1:40000, they then sent me to get bloods cause my OB hadn't told me about it and with the combined results it came back 1:700 so it did make a difference (which at the time freaked me right out cause no one could me give me an explanation but everything was okay). This time I was told up front to have both and the combined results came back 1:20000 and the scan result on it's own was 1: 25000 - so not the huge variation like last time. Can't explain the why's and wherefors or what is "supposed" to happen but just thought I'd share my experience.