They contacted my partner after I didn’t respond for 12 hours

Bonkers, and really intrusive. If your OH hasn't messaged back yet, I'd ask him not to (so she knows that approach isn't going to wash) and message something along the lines of:

"[Partner] passed on your message - no cause for concern, all good here :) Sorry I didn't message back straight away re meeting up. This precious time is flying by so fast and I'm really keen to cherish it and not spend loads of time on my phone answering messages.'

I think i'd feel exactly same as you , and as a much older mum i'd say you don't have to say people with babies are busy, people with lives are too busy for this nonsense, people with parents to look after are busy, people with toddlers and school kids on holidays are busy, people with jobs the list is endless, even if you are not busy you have a right to not respond till it suits you, you could well have decided to wait till plans are clearer for next week before replying . No reply after 3 /4 days might warrant a follow up text to you only, in case you forgot, but to your partner no. Short of a family member or very very close friend who knows you always get back in say a few hours, no one should have messaged your partner after 12 hours. I read it as fake, fake concern and trying to make drama out of nothing, its at best overbearing and bossy- people show you who they are and you don't need this in your life.

sounds like its time to take a bit of a step back.. if a talk with her doesn't clear it all up...

" in addition their partner says bad things about literally everyone."

This would make me wary of confiding in her.. in case it got passed on to her partner.

Maybe she has Post Partum Anxiety. I know a couple of people who would get anxious/irrational about stuff like this.

I'd probably just message her and say that you're fine, you're just busy with the baby and you're rarely on your phone, and then follow it with something like "Are you okay? Messaging X after I didn't reply for a day was a very OTT reaction, so I just want to check in and see how you're doing because you're right, motherhood is a massive adjustment".

If she's not always been like this then maybe she's not okay

I agree. Bit dramatic of that poster to jump to suicide!

I would be fuming too OP. Massive overstep on her part! I’d be telling her to back off.

Did your partner reply? What did your partner say to you? Concerned or laughed off as silly?

This issue aside… you like her?

I’d just write back that everyone’s fine, but very busy. While you understand that it probably came from a place caring, you have never been the type of person to be on your phone all day, and find her texting your partner intrusive and over-stepping.
*I suspect the intention was actually condescension rather than genuine care, and she wants attention.

I'd be backing off. I've had experience of overbearing friends and this woman sounds like she'd be one. I'd get out before it escalates.

Sorry missed this

People suffering do sometimes give zero indication that they are struggling. How’s your friend meant to know the state of your mental health?
Of course she could of text you again or called I do agree with that, my point was you were still rude to that poster

I'd be backing right off with that friendship. 12 hours and she's messaging your partner without even a follow up message to you first? Smothering and controlling.

That seems pushy to me.

I'd be LIVID, and would end the 'friendship'. However, I'd do it in a fade out way, rather than calling her out on her smothering, intrusive, deceitful ways, as I actually wouldn't put it past her to call SS telling them you aren't coping.

I cannot believe people actually think this acquaintance was being genuinely concerned by doing what she did. She's overbearing and out of order.

Not if you read all of OP's posts. She's told us:
"I’ve known them a long time but only really started being friends in the later stages of pregnancy. We have hung out maybe 7 times total over the space of 2 years." and
"... She has his number from a long time ago through mutual friends they have exchanged a couple texts before this point but nothing significant."

The fact this person has the partner’s number indicates a family friend or at least a fairly close relationship

I agree the reaction was OTT but do you think perhaps your friend is going through a rough patch and has post natal anxiety and maybe due to saying your baby has colic and you’ve been finding it hard to get out some days your friend has panicked (projecting her own worries) that you haven’t responded and that is why she messaged your partner.

My guess is she worried you were having a horrible day which is why she messaged your partner, again over thinking and anxiety on her end.

A lot of people are saying to stay clear but I’d give her the benefit of the doubt or speak to her in person about what happened if you want to continue the friendship.

Being a new mum can do crazy things to people

Tbf, she had a newborn herself

She’s unhinged. I’d block her immediately.

No, she wrote it at the end of her first post "I really don’t think I want to be friends with this person anymore as I find this extremely overbearing, in addition their partner says bad things about literally everyone. Thoughts on this and how you would feel?"

I'd be very annoyed at that too. I'd message back saying, I'm sorry I was busy with the baby and catching up on sleep! Please do t text my husband again. Just wait for a response which may be the next day!

That’s a bit of a rude reply to that poster. Women do suffer with PND, thoughts of suicide and mental health issues during these first weeks. You aren’t but how is she supposed to know that. You ask ‘after 12 hours??’ But when was the last time you spoke to her, 12 hours since she text which you didn’t reply to but if she’s not heard from you in a few days she may be checking in that you’re ok. I wouldn’t be as rude to discourage other people from checking in with friends by questioning their ‘paranoid tendencies’ because one day one woman might need that friend to check in on her!!