based on your lmp and assuming a regular 28 day cycle your most likely date of conception would be jan 17, making you 16 weeks pregnant, but giving you a due date of oct 10.

based on your due date (oct 4) your most likely date of conception would be jan 11, but this makes you 17 weeks pregnant and puts your lmp at dec 28.

This means the 5 days prior to and including your conception date are when you had sex to become pregnant.

this is not definitive and is based on the limited info supplied and assumes a regular 28 day cycle.

The only 2 ways to know 100% who the father is are:
1. only sleep with 1 person at a time and always use protection unless you are trying to get pregnant.
2. take a paternity test after the child is born.