I use 4 syllables. Merriam-Webster online shows the 4 syllables and also a 3 syllable prononciation. (as if it were spelled "menstration"). They also have an audio function so you can listen to the word being spoken. I don't have speakers at my desk, so I can't listen. http://www.m-w.com/dictionary/menstruation Click on the little loudspeaker to hear it spoken.

I went googling under "menarche" and I ended up at http://www.celebrategirls.com/index.html which is the site of a company called First Moon. They created a ceremony to celebrate a woman's first period, and they are selling kits to help with the celebration.

g'luck

*daha*