well first and foremost, B’sha Tovah! may you and your wife only have nachas from the little peanut(which is what it is now)!
Second, this is a conversation i’ve had with people. It depends on how close you are with your parents. if you want to wait till month 3 thats acceptable. but if your very close with your parents or the fact that your wife is probably closer with her mom she’s most likely to tell your in laws first. Also if you want to be cute you can say to them…”So I was wondering…Which of these terms would like to be called in ___months? Bubby/Zaidy etc.” that would be cute!