When does your body know?

[Cricket]

Just wondering if anyone out there knows when your body "knows" that you've conceived? Do things start to change as soon as the egg is fertilized, or does it take until implantation? (or longer) I am about 7 DPO, and starting to wonder a little. Couldn't tell you anything specific, just a bit of a feeling. (If I am, this would be my third pg.) We are not actually TTC, but we're not using BC either. We would be thrilled with another baby, but I wouldn't be crushed really if I wasn't pg yet, either. Guess I'll have to wait and see. Thanks!