Your pregnancy starts at conception. Conception can take place up to one day after ovulation. If you do not track ovulation via basal temp and cervical mucous it usually happens about 14 days after the start of last period. So, technically our due date is about 38 weeks from conception, but doctors go by last period, which is 40 weeks.
2 weeks before you conceived ish! It all depends on where in your cycle you conceived :)