My last period was october 22-27 and i took a clear blue last night and it says 1-2 wks. i was to start my period on the 18. when did i conceive?

How to count weeks: If pregnant, and if one has regular periods, the start date of the pregnancy is the start of the last menstrual period (meaning Oct. 22 in this case). Conception usually occurs 2 weeks before the expected start of the next period, if having regular periods (conception would be around Nov. 4 in this case). If pregnant, an ultrasound scan done at 8-11 weeks of pregnancy can give a due date ESTIMATE.