No sense of humour?

We're only 25 weeks pregnant but my boyfriend text his mum and told her that we'd had the baby early and that it was a boy as a joke. His mum was in a panic thinking the baby was poorly, also were actually having a girl we just haven't told anyone yet. I don't think that's funny at all 🤷‍♀️he says I'm being oversensitive. Would you be annoyed?