What is the reason?

i guess its because the first trimester is when miscarriages are more common so...

theres not islamic reasoning for it, more cultural and also people afraid of the evil eye

It's cultural & just to avoid bein evil eyed. In my family my sisters told us & parents but apart from that everyone else knew when they sighted the bump.

I think it is a cultural thing. But its understandable. The first trimester is the most important in terms of development of the embryo, the development of major organs takes place at this stage. Its sometimes called the 'all or nothing' stage, where if the baby is affected by the mothers condition or agents that will cause birth defects it is usually affected t this stage.

I havent come across any islamic evidence with regards to it. I think there may be a hadith about being discreet in such matters to protect from envy...but I cant find it. I might be mistaken.

I think you do not need any Islamic evidence to support or negate being discreet about one's private things. I understand why some people would not want to announce to the world that they are pregnant or any other happiness. They just keep it to themselves and the closest people to them including true good friends. No need to update status on facebook "I am pregnant."