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How do you know the date of conception so precisely?Gestational age is usually taken from the day of the last menstrual period because the exact date of conception is difficult to know,but if you are definite about the date of conception,you can probably start from that day.
Since it is not possible to be exact about the date of conception,the date is assumed to be the date of the LMP,ther may be a factual error ,but the assumption is still made.Correction by checking the growth on the ultrasound scan will probably give a better idea,the date will still be an approximation so a diffference of a couple of days should not matter.
I would trust your scans and assume the date to be 7 weeks from 15 oct ,this works out to around the last week of august,give or take an error of about a week.