Nananina, maybe you are right and I am playing devil's advocate a little.
But, I think you are also missing the point a little...
Maybe you are lucky and you get on well with your DIL and you are all happy to spend time together, regardless of whether or not your ds is there too. And maybe you all had relatively easy pregnancies which didn't leave you completely wiped out by 39 weeks. Which is fantastic for you but it doesn't necessarily follow that everybody will have a great relationship with their MIL or that they will all have an easy pregancy where they are still able to be super active until the very last minute.
The OP's said that her MIL has invited not just herself but several people to tea at a time of her own convenience and is being very pushy about going regardless - despite being told that one day is not convenient for the person who is expected to provide the tea and being given a separate day that is.
Surely you wouldn't expect to invite yourself to tea regardless of whether or not you are told that it is not convenient? And any circumstances the op should not have to give any reason as to 'why' it is inconvenient, the fact it is should be enough. If pregnant even more so - there can be a whole host of personal things that I wouldn't want to have discussed with my mil when I was pregnant that would have been reasons to not want to do something like this.
the op says she has been doing the rounds of visiting over christmas and just wants a bit of a rest and some me time before the baby comes. It doesn't sound like having the mil + entourage will be an easy few hours like it would be if she was a nice understanding MIL. The DIL/MIL relationship is a two way thing - and it doesn't sound here that the MIL is doing a whole lot of giving or understanding.
And sometimes, if a relationship is like that, then yes, you do need to pull out something stronger than you would need with your own family or friends in order to try to get the point across. It certainly wouldn't hurt to see what the reaction is if the OP mentions the MW - would help to give a better indication of whether or not the mil is coming because she is thinking she is helping the op or if she is doing it for her own needs.
OP - has your mil said WHY she wants to come and see you - do you know if she thinks that you will be at home and in need of company as red dragon thinks or does she want to show you off to the other relatives or ??? If you knew you might be able to head it off at the pass - if she thinks you want company then you can say that you don't, if she wants to show you off you can say you really don't feel like being in company or that you'll be good company etc.
OP, hold your ground - you know how you feel and what you feel up to. And how you want your relationship with your mil to go forward - does she often try to dictate things?