We had a scan on the 23rd of Oct and baby measured 43.2 mm. We were told 11w 4d. This dates to 17 Aug 2012 for average ovulation with preceding days being fertility days. I was across the country from 6 Aug to 23 Aug. How accurate is the test? Measurement taken several times. Afraid its not mine. Please help me put this out of my mind or confirm. Thank you.

Scans are a best guess, they're not 100% accurate.

If you fear that the baby isn't yours than get a DNA test after it's born. That's the only way to establish paternity.

They use average measurements based on average size of a large percentage of fetus at various times.

They are good, give or take about 2 weeks. There is no way to pick a exact day, not even a exact week