I would tell them on day one, just to be as honest and transparent as you can from the start.
Any employer will understand that things could change after the 12-week scan.
This is a tough one. I've never been in that position myself, but a woman at my work got her job (very competitive industry, very sought after position) without revealing she was in fact pg at the interview.
She went off on a year's maternity after 6 months, meaning there is essentially a vacancy which has never properly been filled in a stable and long-term way, leaving pressures on the rest of the department.
Others - not me - feel she should have never taken the role, knowing she'd be leaving the position in a state of limbo. Because essentially we were training her up, then her mat cover, and when she's back she'll probably have to re-learn a lot.
If you value this job and see it as a long term career, I would be as open as you can, and try and impress the hell out of them for the next 6 months. I would also use all your KIT days during your mat leave to demonstrate that you're serious about continuous learning and keeping on top of things.
Congratulations on your pregnancy.