Like I said, you need a due date from an EARLY ultrasound, not one in the 22nd week or the 24th week, for it to be useful in dating conception.
If the lady had an ultrasound before her 9th week, it would be possible to make an informed guess from the first estimated due date she got, that was given to her at her first ultrasound.
But there is no way to precisely date the beginning of a pregnancy from an ultrasound from the 22nd or 24th week of pregnancy. By now, the margin for error in dating would be a couple of weeks plus or minus. All those later ultrasounds can tell you, given the differences in growth rates in babies, she could have conceived any time in late June through late July.
Did she get an early ultrasound, such as in her 7th or 8th week?
another test made by an experienced doctor and scan now puts the pregnancy at 24 weeks and 2 days on the 4th Dec 2013 and Edd is 24/3/2014.
Is it still possible that the sexual activity of the 8th july 2013 or thereabout is connected to the pregnancy?
You would need a little more information. If she had an early ultrasound, like in her 7th or 8th week, it would be more helpful than one in her 22nd week, since babies can grow at different rates and this is more pronounced as time goes by. If she did have an early ultrasound, they should have given her an estimated due date based on the measurements in that ultrasound. If they did, and if the ultrasound was early, the estimated due date can be used to count backwards and see when conception might have occurred. If you have access to that number, contact us again and we can run it through a calculator.
Those numbers sound about right. I'm 22.2 weeks myself via IVF. If he's the only one she slept with then yes it could certainly be his. There are due date calculators you can plug the info into to see for yourself.