A pregnancy is calculated starting on the first day of your last menstruation. If you are sure that your last period began on the 5th of September, then by the 30th of October you were exactly 8 weeks pregnant. Don’t expect pregnancy ages based on this calculation to be exactly the same as an ultrasound estimate. There is usually some difference. The ultrasound apparatus determines the age of a pregnancy according to the embryo/fetus’s dimensions compared to standard values. Sometimes women don’t remember the date of their last menstruation and in such cases only an ultrasound scan can relatively determine a pregnancy's age. You probably conceived around the 20th of September.