Oh memories! Personally, I was reluctant to tell my mom whenI had my first period before--all because I was so shy and I am not really comfortable of 1. the experience of having the unexpected visit and 2. the idea of sharing that to anyone because I think it is something way too personal.
But, having my mom to help me with all the things i needed to know made the situation a lot easier to handle. I guess that moms/parents should, at an early age, introduce girls the idea of having menstruation and what usually happens--and how to deal with it. In this way, they would more likely to open up because the topic is already out in the open and just make the conversation as casual as possible.