I am a mum of two DC myself, and we employ a nanny. In the years since we have employed a nanny, we have only ever had two nannies, and both of them have been pregnant in the time they have been with us.
Nanny number one told us she was pg after about 11 months or so with us. She was 12 weeks when she told us. She decided not to come back to work after we refused her request to bing her baby with her, for a variety of reasons. It is not appropriate for me to go into those reasons here.
I appreciated her letting us know early as i t meant that we could be more accommodating of her, knowing what it is like to be pregnant. As she went on maternity leave at about 31 weeks, we also had that time to make arrangements for her cover.
We hired another nanny on a fixed term maternity cover for our first nanny. When she decided not to return to work, we made our second nanny permanent. Now she is pregnant. She told us at about 19 weeks. She has her own valid and personal reasons for not telling us until then, but I wish she had said earlier. I would have adjusted my expectations of her, and done some kind of risk assessment to make sure everything was safe and easier or her. It would also have given us more time to get our heads around it and make plans for our DC's care.
I suppose what I am saying is that I think you should tell your boss, and quickly. I appreciate that you are trying to make plans for your return, and of course, you have the right to return to your job. But she will be trying to think through how to cover your maternity, and she will know that, whatever you say now, you have the right to take up to a year off. The uncertainty about how long you will take can be difficult for parents, as employers, as temp nannies my always be on the lookout for permanent jobs, which can lead to instability. Not that this is your issue, but I think that, as employers, parents would appreciate as much time as possible to make arrangements. Also, whatever you say now about your plans, things can and do change for you and your employers. So nothing is set in stone.
Also, I think it would be helpful if you indicated when you think you would like to start maternity leave, although you can at a later stage bring this forward.
Congrats on your pregnancy.