Girls hit puberty earlier

TERRE HAUTE, Ind. (WTHI) - Imagine your daughter having her first menstrual cycle when she's just seven years old.

Well that's not too far off for some girls these days.

Young girls are hitting puberty earlier than ever...and doctors aren't sure why.

15 percent of girls are reaching puberty as young as just seven years old.

Why is this happening early than it was just a few years ago?

Experts say there isn't a proven answer yet.. but there are many ideas out there.

"There are some theories that it may be from al the hormones that are being given to cows so maybe we're getting hormones in dairy products and even the meats," Marcee Everly, Indiana State University nursing professor and midwife said.

Long term effects of hitting puberty early could even include teen depression and breast cancer.

"You would think ok young girls are getting their first period earlier, then perhaps they're going to then have menopause earlier, but no. So we're actually having more years of fertility."

So what age is considered normal?

Thirteen is the average age for a girl's first period.

However, doctors say that adding or subtracting three from that is a good rule of thumb these days.

For boys, things haven't changed as much.

"Typically and historically, girls generally develop secondary sexual characters younger than boys usually do and at least that hasn't changed."

For now, researchers will continue to search for an answer as to why this is happening.

Experts say childhood obesity could also play role in early puberty.

Studies have shown if you're worried about the effects of hormones in milk on your child, soy milk is a good substitute.