I told my manager at 7 week but I had complications and been in hospital, also had previous miscarriage. If hadn't been for complications I wouldn't told until 12 weeks
I told line manager at 4 weeks as I needed my duties altering and a risk assessment
I didn't tell my manager until I had my 3month scan. However if you're having symptoms that's affecting you're work then maybe tell her sooner. There's no right or wrong time it's just when you feel comfortable. I think they prefer to be told immediately so they can better "support" you but it's entirely up to you.