Registering self employed while claiming smp from an employer

[Yelloyello]

Hello, not really sure where to post this to get an answer so I’ve tried a few different boards.
Wondering if anybody knows the answer to this. Any help appreciated!
I’m expecting a baby next year and can claim SMP from my employer. I’d like to do some self employed work while I’m away on maternity, which I understand from the Gov.uk website I can do.
But my question is when do I have to have registered as self employed by? Is it by the 15th week before the baby is due? If I don’t register as self employed by then will it affect the SMP from my employer? Also will she be notified if I register? There’s nothing in my contract that says I can’t do self employed work while on maternity leave.

I checked with my employer who were happy with it once I emailed them the bit of the HMRC website that says it's allowed.

You're not allowed to work freelance for your employer though.

I'd say you may as well register now. I'd registered well before my maternity leave and there were no issues.

Thanks for that!

Do you know if you have to register by the 15th week before the baby is due?