how much do girls grow after period starts
A girl's average age at the time of her first period is 12 years old. Periods can come earlier or later than that, however, and the average age of menarche has been dropping. Girls for whom menarche occurs after age 13 tend to have more irregular ovulation cycles until age 18 or 19 [source: MedicineNet ]. During puberty, a girl may experience a remarkable change in height: Girls often grow as much as 3.5 inches (8.9 centimeters) in height a year during puberty [source: Morgan, et al ].