nuchal scan on NHS and private - what's the difference?

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Can anyone explain?
Last time I was pregnant I paid for a private scan as wasn't offered on NHS, but this time round my PCT does offer it. Which is great, but have now heard that the NHS one isn't as accurate/good as the private one.
Anyone any info? I'm in Oxfordshire. TIA.

Where I live they do the nuchal scan and blood test on the NHS but the blood test results aren't available straight away so you dont get the combined risk results straight away. Also the machines are less sensitive and see less detail.
I went for a private scan as my DH would have been abroad on the date of the NHS one and I didn't want that due to previous scare in same situation last time. The blood results are ready straight away. On the scan they were able to see things like kidney function, spine that woulnt be seen so early on NHS one.