Is this a MIL red flag?

[hugefatso]

Am late, late preg and DC1 is imminent. MIL called DH and asked him what we'd decided on for the name. DH said we hadn't told anyone and wanted it to be a surprise. (I don't really care either way, but for DH it's important no one knows til the birth.)

MIL hung up the phone. DH tried to call her back, she didn't answer. Three days passed. Now she's texting him saying that she cannot believe he would keep a secret from her and that they "have always had a policy of no secrets." After saying her piece, she is not talking to him/us again.

oh huge

as if about to pop wasn't stressful enough...

what a bloody drama queen your MIL is, instead of being supportive and kind and giving you too the space to prepare for this wonderful event, she is forcing you to concentrate on her petty little attention-seeking behaviour.
how small-minded.

I have to say though that this would bring out the Rumpelstiltskin in me if my children wouldn't tell me the names they were thinking about.

Just what you need at this point in the pregnancy....

Yes its a red flag. This is about her struggle to deal with change. Her relationship with her son has evolved, he is about to have an actual family unit of his own where she does not have all the knowledge and is not part of the management team, and she liked things as they were and wants them to stay that way. I can understand that's hard, but it's her issue and she's the one who is going to have to deal with it. It's not something you can help her with.

Let her sulk, let her realise that it isn't going to make you back down and let her have her way, and let her come and do some repairs when she's ready. This is a good opportunity to do some boundary work with her before there's a baby involved, and hurray it's being led by your dh, so it's both of you as a team.

A grown woman is not talking to you and your DH? Seriously? She's behaving like a spoiled child.

I'd just carry on as normal, don't even acknowledge that she's misbehaved. It's like training a naughty puppy, ignore the attention seeking bad behaviour.

I don't really think that keeping the name a secret is particularly different from keeping the sex a secret. And it does happen that as soon as you see the baby, you know the name you chose before birth is wrong so you have to choose another (happened to me).

Aside from that, yes reinforce boundaries very strongly from now on. Nip it in the bud.