Your LMP started on October 11?
If so, you had sex wtih your boyfriend around day 8/9 of your cycle.
You had sex with someone else on day 15.
Normally you will ovulate between days 11 and 16 of your cycle. Sperm can survive up to about 5 days.
Timing wise it could be either. You don't get pregnant immediately.
The problem. Dates are based on your LMP. Why? Because almost every woman knows exactly when that was. 20 weeks prior to March 7 would have been October 18, not the 11th. Those dates should match. Are you positive as to when your LMP started?
It's not clear Boss. When did your last period START. The end date does not matter.
More than likely it is your boyfriend that is the father but even though "someone else" did not ejaculate there is a chance.
It could be either. I do think it is more likely to be the boyfriend though.
You will need a DNA test to confirm. There is NO way around it. The dates are just too close together.
You said your period started on the 11th. That's day 1 of your cycle. You normally will not ovulate until between days 11 and 16 of your cycle - the 22nd to the 27th of October. Until that point you aren't pregnant. Sperm can survive up to 5 days.
Remember, dates go off of your LMP.
Oct 11th - period starts
Oct 18th - one week. You aren't pregnant yet - haven't ovulated.
Oct 25th - two weeks - You'll ovulate and conception occurs
Nov 1st - three weeks - Implantation normally occurs.
Nov 8th - four weeks.