Who needs to know?
That last comment on the 'stepkids are a blessing' got me thinking.
It was suggested that if a parent prepares their child good enough there is no reason to tell the other adults in the child's life of physical issues that might come up while away from home.
I don't understand this way of thinking and completely disagree. In my opinion it's the parent's job to make sure the child is safe.
If my dd had her period or became sexually active (or I suspected either might be coming soon) I would tell her father. Not because I particularly want him to know but because it will help keep the lines of communication open and protect my daughter.
I would also tell my parents because DD spends extended amounts of time with them and I wouldn't want them to be caught off guard...
As a side note, young girls who become women are INCREDIBLY moody and I would want my DD cut a little slack rather than be treated as if she's simply being a little grouchy brat because she's not able to navigate the hormones yet.