Thanks for writing to us at DoctorSpring with your health concern.
Firstly, if she is sure that on 8th she had a proper period, then it is not possible that she conceived between Feb 3rd - 5th.
Even if conception occurs close to an approaching period, then the implantation bleeding is different from a proper period.
Implantation period is much lighter and lasts for a shorter duration.
It is far more likely that she conceived around the 24th because that is when she is likely to be ovulating, after a period on the 8th.
Again, regarding the pregnancy test, if she conceived on Feb 3rd - 5th, a test taken on March 5th would definitely show up positive, if she had regular cycles.
If she has an early pregnancy visit, then a scan would be done.
The scan is quite accurate, if done early on in pregnancy.
The margin of error 5 to 7 days, in early pregnancy scans, and the difference between the two acts of intercourse is almost 3 weeks.
So yes, with a scan done as early on in the pregnancy as possible, it is possible to make out who is the father.
Based on the facts provided by you, I do not think you are the father.
Please have a scan done and let me know the exact report, that would make my diagnosis more specific.
Please feel free to discuss further.