Hormones

• Hormones produced by the pituitary gland are responsible for the physical and sexual changes that occur during puberty. The University of Virginia Health System notes that the primary hormone is called follicle-stimulating hormone (FSH). It activates estrogen production in young girls.
  
  

Growth

• On average, girls will gain between 15 and 55 pounds and grow 2 to 10 inches between the ages of 10 and 14.
  
  

Early Changes

• Early in puberty, at around age 8, girls experience breast development. This is followed by the growth of pubic hair.
  
  

Later Changes

• Around age 12, underarm hair begins to appear in girls. Somewhere between 10 and 16.5, menstruation commences, note University of Virginia health experts.
  
  

Migraines

• Mayo Clinic doctors point out that between the ages of 4 and 7 migraines are more common in boys than girls. Once puberty starts, though, females are three times more likely than males to suffer from migraines. This coincides with the onset of menstruation, which often triggers migraines.