minhag - Hiding pregnancy during the first 3 months

There is a tradition - or whatever you may call it - not to tell people about your pregnancy early on.

First off, is there a Torah basis to it? Or just common sense? (if things go wrong people will still say Mazal Tov...)

Secondly, when someone asks "are you pregnant?" or "is your wife pregnant?", is OK to lie and say NO??