i think it might be your ex'.. that was a little more than 15 weeks ago.. i could be wrong.

the only way you'll know for sure is if you have a dna test

your last period and your due date don't match - there's like a 2 week discrepancy there.

do (did) you have regular periods or not?

based on your lmp then your likely conception would have been jan 3 and your due date would be sept 26 (meaning mostly likely your ex's)

based on your due date (is this ultrasound confirmed?) your likely conception would have been jan 15. (meaning most likely the other guy's)

the only way to know for sure is to have a paternity test done after the baby is born.

I've moved this to the appropriate forum.
Please do not type in all caps.

hi! my last period was december 19th and i am 16.5 weeks now. so did you have an u/s and you are just measuring smaller? because if you are then it would be your ex's i would guess, but the only way to be sure is with a paternity test afterwards.