To think "they" can be singular or plural?

[RhymingRabbit3]

I'm expecting a baby and we dont know the sex. I feel bad calling the baby "it" so had been alternating between "he" and "she" but I had some raised eyebrows from people (mainly family members) saying "ooh I thought you didnt know, it's a girl then?".

So I started referring to the baby as "they". As in "they are really kicking me today" or "they are definitely head down". My husbands grandad looked horrified when i did this and said "you just said they - it's not twins is it!?"

AIBU to think that "they" can be used to refer to a single person whose sex is unknown? Or should I just go back to calling the baby "it" or "the baby"?

Thanks for your replies. Looks like it can be confusing and there are differing opinions. My husbands grandfather is (obviously) older and it seems maybe older people stick more rigidly to "they" only being plural. However I think we would have told him by now if we were having twins!

We could use a nickname but I find that a bit awkward around extended family. Between my husband and I we might call it "little bean" or something but I wouldnt refer to them as "little bean" with strangers, extended family, acquaintances etc.

I'm 37 weeks so wont be a problem for much longer anyway!