1st baby

Can anyone help me , I just got my bfp today . And I'm just confused on how the dates work. Clearblue is saying I'm about 3-4 weeks pregnant. But I haven't had a period since April of this year 2weeks to 8 weeks is a huge jump and I just want to know about what time to tell my doctor. And also for my personal sanity . Because I just took a test last week and it was completely negative ..