Better nutrition, more prosperous lifestyles and exposure to medical with sexual connotations are all said to contribute to girls attaining puberty earlier than before.
Whereas 150 years ago, the average age of a girl starting to menstruate was mid to late teens, whereas now the average age is 12 and some children do get their first period as early as age 8.
So, really as a mother (or father as the case may be), it is important that you have the all important chat about the Period, as early as you may be comfortable. Just don’t leave it too late; for a girl, getting her period while not knowing what it is can be very traumatic.
It is not required that all the facts and details are explained all at once; it can be done in a phased manner, possibly by letting your daughter see some of the sanitary things (pads, tampons, etc) that are used and then letting her ask a few questions.
They can be answered in a matter of fact manner with more details to follow in later discussions where you can then introduce some anatomical details as well.
What is important is not to make a great big deal about the period chat, make it as matter of fact as possible, and understand that it is important to relay information in as non-embarrassing a way as possible.