Period every 26/27 days last period sept 30th. i'm wondering what day i conceived? can sex on the first fertile day possibly be the day i conceived?.

Possibly.: In your "regular" 26 day cycles, ovulation would tend to occur on day 12 or 13 ; if you'd sex with a fertile man around that time, fertilization could have taken place then or any time within the subsequent 48 to 72 hours. A pelvic sonogram performed between 6 and 12 weeks of pregnancy is a useful way to confirm your "gestational age" and estimate when fertilization may have been(+ or - 5 to 7d)