I had cause to research this not long ago when a young woman I know was pregnant.
The early ultrasound (called the dating scan) is a very, very accurate - to within 5 days. It's true that as the pregnancy progresses, the babies become more individual and dating the pregnancy becomes less accurate. However, at about 10-12 weeks (when the dating scan is normally performed), it gives a very accurate estimate. So if Stacey attended the scan at the usual point in her pregnancy, the date given is likely to be very close.
I've done some research to try and pinpoint the date we think Stacey possibly slept with Kush (when they spent the evening drinking wine at her flat) and the first time she slept with Martin. The dates are:
Kush: 21st May (here's the episode guide for that date:
http://www.bbc.co.uk/programmes/b05wgwsl)
Martin: 19th June (episode guide:
http://www.bbc.co.uk/programmes/b05zpsyz)
So that's a whole month apart. Plenty of time for Stacey to know the identity of the father from her menstrual cycle and from the dating scan.
I think that's unlikely since we saw her deliberately hide the second scan from Martin. She's keeping the photos from him for a reason, although that reason could turn out to have nothing to do with the dates.
I was still hoping Stacey would end up being mistaken and the baby would turn out to be Martin's after all, but the above dates make that unlikely -
unless she never did sleep with Kush. I hope that's the case and the hints we're being given are red herrings.