Yep, I reckon just tell them, tell them how far along and as much as you can don't offer any justifications - it's YOUR news, YOUR body, YOUR baby, they're not even along for the ride because they are spectating. There IS NO RULE on by when you have to tell anyone about your pregnancy - it's so arbitrary and people expect to be told, based on sticky-beakiness.
Consider it good practice for later on when people will ask you how long you'll breastfeed for...as if it's ANY of their business and going to affect them (worse when you say you'll let the child self-wean...THEN the judgments really begin!).
Just tell them you have news and then tell them the news. It's up to them how they digest it - not you, no matter how you package it