Lmp was 08/02 or so.. Dr says my conception date is 18th (i had unprot. Sex this day) but what if I ovulated early. Would gest age show this on us?

Life is not exact. All dates are estimates, so none of them are exact. Estimates can be a week early or a week later, which doesn't matter in almost all cases (because the woman is trying to have a baby with just one man). If there is more than one man, she should always separate them by two weeks, which allows the estimated dates to work pretty well.