Hi there, don't know if you're still interested in the reasons why different places give conflicting time frames for having the tests done but here is an excerpt from an article written by the guy that 'discovered/designed' the NT scan.
Essentially it says it's more accurate to have bloods at 10wks and he scan at 12-13 but it makes it easier for collating results and giving a quick answer to do it all on the one day. Hope that helps
Timing of ultrasound and blood testing within the first trimester
One option in first-trimester combined screening for trisomy 21 is to perform biochemical and ultrasonographic testing as well as to counsel women in one-stop clinics for assessment of risk (OSCAR) (Bindra et al., 2002; Spencer et al., 2000d). This has been made possible by the introduction of biochemical analyzers which provide automated, precise and reproducible measurements within 30 min of obtaining a blood sample. The ideal gestation for OSCAR is 12 weeks because the aim of the first-trimester scan is not just to screen for trisomy 21 but also to diagnose an increasing number of fetal malformations, and in this respect the ability to visualize fetal anatomy is best at 12 weeks. The detection rate of trisomy 21 with OSCAR at 12 weeks is about 90% at a false-positive rate of 5%.
An alternative strategy for first-trimester combined screening is for biochemical testing and ultrasound scanning to be carried out in two separate visits, with the first done at 9 to 10 weeks and the second at 12 weeks. It has been estimated that this approach would improve the detection rate from 90% to 93 to 94%.
A third option would be to perform the scan at 12 weeks and optimize the performance of biochemical testing by measuring PAPP-A at 9 weeks and free β-hCG at the time of the scan at 12 weeks or even later with an estimated detection rate of 95%.