Is it based off of lmp or ovulation day

Okay so Is counting weeks of pregnancy based on first day of last menstrual period or the day you ovulated? If its based on first day of last period im 4 weeks pregnant and cant find out for another 2-4 weeks, however if its based on the ovulation day im only 2 weeks and four days, when can you find out if your pregnant like how many days past ovulation are you liable to get a positive result?