Hi, what do you mean when you say " If ever as it state my ovulation falls on 26th of Dec" above?
OK, it sounds like you're saying the following (please check this list for accuracy):
First day of last menstrual period: December 10
Sex with ex:.
December 16 (unprotected)
December 19, 21, 23 (protected)
December 29, 30 (unprotected)
Sex with boyfriend:
January 4 (unprotected)
Ultrasounds:
January 29, giving 7weeks 1 day GA and
September 16 estimated due date
19th week, giving same EDD
If the early ultrasound (on January 29) gave a September 16 due date, it's hard to argue that anyone else but the person you had sex with in December could be the father, and it sounds like you had sex with only one guy in December (at least, after December 10). Your cycles recently lengthening don't make enough of a difference to move conception to January 4.
You mention your boyfriend and your ex. Is the "ex" from before 15 years ago?
With whom was the sex after the 16th with your ex and before the 4th with your boyfriend? You mention the dates of the 19-23 protected and the 29030 unprotected, but don't say if those were with your ex or your boyfriend.
Also, did you have any earlier scans than your 19th week?