Needtoknowasap, With your due date/GA data you ovulated around October 7 (give or take a day or two). If we take this day or two uncertainty into consideration, sex between Sept 30 and Oct 9 got you pregnant.
Using the precise date, Oct 3 to Oct 8 got you pregnant. As this is not possible according to your sexual history, it is safe to assume that the scan was likely off by 2 or 3 days and Sept 30 sex got you pregnant. By Oct 16 you the egg was already fertilized and implanted in your uterus.
There is just no scenario where sex on Oct 16 could have gotten you pregnant.
I hope this helps.