OP, people on Mumsnet often jump to 'your days are numbered' when it's actually a manager doing normal performance management. Contrary to popular believe, most employers would prefer that employees who aren't performing start performing, rather than wanting to go through the long process of hiring someone new. Obviously sometimes it's personal, and often the formal PIP process only starts once the employer has decided there is no way forwards and is just going through the motions.
I think it is more likely that what is happening is this (1) your manager has been holding back on either the volume of work you're being given, or the strategic value/complexity of that work as you go through the return period, but they now consider that grace period is up and if you want to continue on your former trajectory you need to up your game, and (2) your manager still has some concerns about your performance and wants you to fix them.
It is very common for managers to do (1) when women return from maternity leave and they think they're doing the woman a favour. Appreciate you haven't asked for it, and it may not even be obvious to you. I recommend you make it clear to your manager that you don't want to be treated differently because you have a child and that you are happy to take on strategic work and ask what you need to do for him to be confident you are able to take on more strategic work (don't say more work if you're already hitting targets). There was a suggestion you're a lawyer - if that's the case focus on what they need to have confidence you can manage bigger client relationships/run bigger matters. If you want to continue to progress (you don't have to want to!), try to objectively think about whether you are falling behind others at a similar level in terms of what you do, not just how much you do. Eg you may be billing the same hours as Bob, but is Bob responsible for finalizing the DD report, sending it out and managing client questions whereas for your matters the partner is far more involved and you are really doing the basics not the strategy. Effectively, do you think you've been out on the 'mummy track' and don't want to be.
For (2), I'd ask very specifically what areas your manager thinks you need to work on. Then go from there.
Don't email copying HR in. It's would be a ridiculous over reaction and will not make you look good. I wouldn't email at all at this stage. I would ask for another meeting.
If at the next meeting your manager won't provide clarity, and continues to make statements that indicate that their issue is you know having a child, not anything concrete, then consider moving onto email so you have a record. You may end up needing to go to HR, but that is the nuclear option and I would never do it unless I felt I had no other option.