Just tell them your pregnant, how far along and if they ask why you 'took so long' (as if there's some mandatory requirement that you 'confess' at the 12 weeks mark at the latest!!), just say you wanted to have it to yourselves for longer. And I've said to people that it's less time they have to spend watching me be pregnant and they fastrack to the birth
My mum was offended - but that was about her and what SHE wanted...not about me and what I was comfortable with...no, that's just not important in her world! People get offended when they think something's about them, and far too many people assume they have more involvement in a pregnancy than they actually do and then disappear when you actually would appreciate their involvement...after the baby's here. Meanwhile, there's only so much they need to know and if they've made plans for holidays for when you're due or whatever, too bad - you don't need them there for the birth, or you would have told them sooner