I mean this kindly OP, but do you normally have a problem with not being assertive enough?
@AnotherEmma is right, this is absolutely a masterclass in how NOT to be assertive.
Once she said the invites were out what can I do
Tell her it's a shame she didn't ask you if she could use your house before she sent the invites, and tell her to let you know the alternative address so if any guests turn up looking lost on the day you can forward them to the correct one.
she's given a bunch of strangers my address
TELL her that that was categorically NOT OKAY.
The fact that the only response to message was I understand is a bs response.
It was a wiggle out of cancelling it response because you hinted, you didn't straight up say NO this party isn't going to happen in my home when I am 9 months pregnant and/or with a newborn.
I've heard nothing since so tells me that opinion doesn't matter.
None of them have been told a clear and unequivocable NO this 'aint happening.
You could go into labour while the party is in swing or hours before it FFS, and would your Darling Husband come with you or want to stay to be a good little host?? You could have had the baby and be coming home with babe in arms for the first time and there would a house full of bloody strangers and a shit ton of housework for your DH to do when he should be looking after you both. You could be having those precious first few weeks at home when you don't want to see anyboy and no matter how locked away in your room you want to be they would all want to traipse to see and cuddle the new pet. No, no, no no NO.
You are 9 months pregnant and what you need more than anything else, is to not have a party going on in your fucking house. No discussion. No heavy hinting.
Lose your shit at your husband if you have to.
Lose your shit at your MIL if you have to.
STOP worrying about offending everybody else and start BEING the person who is justifiably offended and pissed off that these fuckers had the audacity to arrange a party at your house, right when you are due, and nobody bloody well even thought to ask you if you minded! Yes you bloody well do mind, and SAY SO. LOUDLY, CLEARLY and if necessarily REPEATEDLY.
Cancel. The. Damn. Party.
Text your H at the same time as texting your MIL if you really can't say it over the phone or face to face. Be crystal fucking clear and leave no room for doubt. TELL don't hint to them that there will not be a party of any configuration in your house at this time. If you want some wording that you can copy and paste:
"As you know I learnt very recently that a party is being planned at my house for So and So's birthday on X date. As you also know I am heavily pregnant and on that date I will be either due any second, overdue, or will be recovering from childbirth and looking after a newborn baby. As I am sure you will all understand it is therefore COMPLETELY inappropriate for our house to be used to host a party at this time so I must insist on cancelling this party which didn't even have my blessing anyway and suggest that you make alternative arrangements. My decision is final and in order to avoid embarrassing situations like this in future please ask me before inviting people to my home because my dickhead husband didn't.
I hope you have a lovely party and that So and So has a lovely birthday. We will invite you to visit when baby is safely here after we have found our feet as new parents."
And then switch your phone off and if necessary yell/cry/rant and rave at your H when he comes to ask you what the hell that text message was all about. Dickhead.