From personal experience, I dont entirely trust dating scans. The reason being is when I was expecting the first time round, I was given 2 totally different dates. I had my first scan at 16 weeks due to me finding out quite late on I was in fact pregnant. From that scan I was told 26th December. At my 2nd scan, I was told 6th January. Actually it couldve been the other way round lo, It was a long time ago but you get the idea. Two scans, two dates. They decided to go by the first scan and gave me the due date of 26th December (boxing day!) I was induced after going 10 days over and having no signs or indications of ever going into labour! Baby was born 5th January. The first the midwife said was "she looks like an early baby". I explained about the dates and asked if maybe her due date shouldve been the 6th January and she said it was possible.
The period she said she had between the two, couldve been implantation bleeding.
Like red says, both men should ask for a DNA test!