You can announce it whenever you like. We waited til after the 12 week scan to announce it to most people (apart from close family who already knew).

My 12 week scan was quite a long one. The recorded the heartbeat, checked everything was developing as it should be and did the NT measurement which turned out to be quite high so it turned out to be a not so good experience for me. I knew something was wrong because they kept measuring and measuring in the one place before they finally told me how high it was. All turned out well in the end though, so don't go into a mad panic like I did if they do suggest there is a problem with the nuchal fold. It doesn't necessarily mean there is an issue at all.

I got one of the best 3D pics I ever got throughout the entire pregnancy at the 12 week scan. It was just amazing how much detail you can see if your scan is done on the best equipment.