Let’s be honest here, there really is no week one.
This is the first day of your last period and this is what you’ll be asked about 10 thousand times by medical professionals until your baby is born. Your LMP (Last Menstrual Period) is how they figure out how pregnant you are. So if your period started on X date, then you most likely conceived two weeks later, and they’ll do the math from there.
It’s like me saying I cleaned the house before I’ve even put down the bag of chips and got off the couch – the idea is there but nothing has happened yet.
You’re not pregnant. It’s where the benefit of hindsight comes into play. Unless you’re psychic in which case you know you’re going to conceive in two weeks, so congratulations and tell me where the hell my car keys are.