Due Date question after stopping BC pills

kati737

I stopped taking my birth control pills 18 days after I had my regular period, and I had some bleeding for the next couple days.  I took a pregnancy test that was negative 4 days after I missed my period.  Then, I took one the following week and it was positive.  Any idea if I should count my LMP for the due date purpose from the date I had my last period OR from the date I had the bleeding after stopping birth control?  It's a gap of 18 days, so I'd appreciate any help!