Am I being unreasonable to leave my in-laws’ at Christmas because MIL won’t stop with trying to guess if I'm pregnant

Tell her calmly and coldly how much her snide, hurtful comments are upsetting you. No lies about vasetomies, nothing about IVF, nothing about medical issues. Tell her that just one more hurtful comment and you will be leaving even if it just as the turkey is being served. If she says anything again, no need to get into a row, just pack up your things and go. Ignore the tears which will surely come.
The only way to stop this is to issue an ultimation and follow through. Ignore the subsequent pleading texts and calls for the time being at least - that is for DH to deal with. She will probably wail to the rest of her family. If they contact you, simply tell them that she has continued to make hurtful, snide comments despite being asked on multiple occasions not to. That ' it is just the way she is' is not an acceptable response.

Keep it factual and direct.

Mil you have been asked umpteen times to stop making inappropriate comments implying i am pregnant / harassing us regarding further children.
You might yhink its banter but i dont.
Ifind it intrusive, insensitive and rude.
I feel harassed and stressed by it and it means i cannot enjoy spending time in your company as i am always waiting for the next
dig from you regarding a 2nd GC
You may not be awate of how frequently you do it but its so bad I'm cutting short this visit, which is a shame as we were looking forward to this trip.
very honestly you can expect to see less of us if you dont stop making these inappropriate comments as its just too much.

Let her kick off cry whatever and ignore it all just make it clear calmly that all she has to do is stop making weird comments and normal service can resume. Her choice.

Every time she does it say "stop. I've asked you not to do that."and leave the room/ the house for a while

Just pack your things an leave - her 'friendly banter' is rude and stressful.
hopefully she'll be so offended she won't invite you back, may even go no contact if you're really lucky.
Don't leave it any later - you might cut yourself off without any opportunity to get to a supermarket on the way home.

Your DH isn't being firm enough with her about her comments.
If I were in your shoes, you have a few ways to deal with her.
Firstly you could simply roll your eyes, shake your head and say "Change the record MiL. It'll happen when it happens, alright?"
Or you could go utterly ballistic at her saying something like "Have you no feelings woman?? You have no idea the issues we're going through yet you keep repeating the same idiotic comments thinking they are funny but they are just so hurtful!"
Or you could run away crying every time she brings up the topic.
Or some sort of combination of the above might work.

All that said, she needs to be told that it will happen when it happens and until then, she shuts her gob!

Just let her know, We’re not trying for another child, and leave it at that. Keep it clear and direct. It’s not her business, and if you do get pregnant, you can decide when to share the news. Maybe she just needs to hear it once, but make sure it’s firm enough to leave no room for further discussion.

“We’ve decided we don’t want any more children, the constant pressure was too much for us and we were sick of people going on about it. So DH has had the snip”

Eh? Did you actually even read the OP?

You don't have to justify yourself with reasons why you're not pregnant yet, don't you dare!
She sounds foul, sorry
We’re currently doing IVF, but the in-laws don’t know. MIL would make it so much worse with her comments. She’s already said things to DH’s cousin like, “When are you finally going to get pregnant?” when they were having IVF, and when she heard about their miscarriage, her reaction was, “Oh, what a shame! I was really looking forward to baby cuddles this autumn.”

That's not banter, or jokes, it's MEMEMEME and what she wants, screw anyone else's feelings.
I wouldn't dream of doing that or pressuring /going on about grandkids to my DS and his partner, it's nothing to do with me!
Tell her to pack it in or you're going home. Then mean it if she starts up again. She clearly doesn't listen to reason so time to get as bolshy as she is.
It's tough, but you have to stand firm for your own mental health.
Hopefully your DH is on side.

I think your mother in law sounds awful and very nosey, and I would talk to her and make it clear that you find her comments intrusive, but of course you would be unreasonable to leave.

I think it's past that now. I think op needs to say something

You don't need to give an explanation. You don't owe this woman ANYTHING. You don't owe her ANY kind of information that relates to your private medical history. The fact you don't want to hear it should be enough.

"Id wait until she next does it and then say Mil, I know dh has repeatedly asked you not to make comments about pregnancy/ having more grandchildren and yet you are continuing to do it even though you know we don't like it. So let me be really clear. Any more comments or insinuations or jokes relating to pregnancy or having more children and we will pack up and leave. We've both been really clear with you on where we stand with this, so the rest is up to you. But that's all I'm prepared to say on the matter. "

Then you follow through if she does it again. The explanation or any mention of infertility etc is your business and she's not entitled to it. It's like that saying - no is a complete sentence. Same effect applies here and to be honest I'd avoid getting into it with her any more than that because it sounds like she would see it as another 'in'. And the pp who mentioned this is being directed at you is correct. So I think you need to be the one to challenge this very directly. And remember- it'll probably feel really uncomfortable but this is her doing. She should have stopped long before now but she made the choice not to. Anyone with a bit of sense knows these comments are never helpful and to be asked to stop and still continue is actually incredibly selfish and cruel.

Your DH needs to tell her again to stop it and to leave you alone. He needs to tell her that you will all be leaving if she doesn't stop.

I just burst into tears. That did the trick.

Not unreasonable at all. It sounds like absolute torture. Maybe if you actually leave she will finally learn her lesson about what an insufferable, selfish moron she is. If she doesn’t learn her lesson she won’t be spending much time with the grandchild she already has, let alone any future ones.

Excellent!

ask her

why do you keep talking like this when you know we can't have anymore children

dp told you it was sensitive we though you'd know what that meant

Look up Dr Becky Kennedy on setting boundaries - it's not about telling kids what not to do. Boundaries are about you saying what you will do - enforcing the boundary comes from you, and requires the other person to do nothing.

So you state your boundary 'i do not want to discuss, or have any one else refer to, whether i may be pregnant or trying to get pregnant. if this happens I will leave the room/house' (easier for you to leave than asking her to leave).

She does it again, you leave - calmly, not angry - you are simpley doing what you stated beforehand.

My mum was like this with my weight and how ugly I was etc etc
One day I sat there and thought I don't have to listen to this. Stood up, said "STFU about my weight, I'm leaving and I'll leave every time you keep going on about it" and walked out

She sounds like a right horror.

I'd pack your stuff, say, "We don't want to spend Christmas being interrogated about our plans for another baby when we've asked you numerous times to stop, so we're off", then stop off at the supermarket on the way home and do your Christmas food shop.

I wouldn't soft soap her at all. She's been told, she keeps doing it. Might be time for your husband to tell her 'any of this nonsense starts we'll be leaving and you won't see either of us or toddler again.' Then if she starts get up and go.

Oh I love this!

The trouble with 'friendly banter' is, if the recipient doesn't find it funny and is upset by it, then it's bullying.

I also agree with this. Make her feel really really uncomfortable.

If you are going to just blow things up by walking out then I think you should at least try to talk to her first. You could ask her for a word and explain to her that all the little comments speculating if you are pregnant are upsetting you and that if she does it again you will have no choice but to leave. Be firm but respectful.
Then if she does it again you look her straight in the eye and tell her that you are leaving and that she only has herself to blame.

I wouldn't be able to keep my trap shut. 'We've both asked you not to joke about pregnancy but there you go again. You must be doing it deliberately'.

I would honestly say to her that you have decided to have only one child and that no amount of comments will change your mind. Every time she makes a quip reiterate firmly that DC will be an only child and you’re getting tired of these conversations.

What she is doing is really inappropriate and unfair. If you cannot tell her about your infertility then just lie about DC being an only child.