How do I bring up my first period to mom?

Honesty. Menstruation is a natural, normal part of a women' s life. As embarrasing and stressful as it might be, everyone knows it occurs. I see your above listed age as 40? I am not sure what your age or situation is. You might even want to speak with your doctor about it first, or if you are uncomfortable bring your mom with you to the doctor and let the doctor do the counselling.