Sex on aug 21 is it correct on vaginal ultrasound taken on oct 10 is 9weeks and 4 days

Yes: An ultrasound date of 9weeks and 4 days is 'dating' from the last menstrual period forward and conception is about two weeks later (a little more for longer cycles and less for shorter cycles). Ultrasound in the first trimester also has a range of error of +/_ 5-7 days. In other words, an ultasound date of 9 weeks and 4 days on10/10 would make the last menstrual period the first week of august.

Maybe: Ultrasound is often more accurate in dating a pregnancy than date of sex or last period.