For all the physical changes girls go through during puberty, a classification system called "Tanner Stages" has been developed to categorize how far along a girl has developed. The Tanner Stages don't address all the pubertal changes; rather, they focus on breast development and pubic hair growth.
The first stage is before the onset of puberty; there are no outward physical signs of puberty. During Stage 2, breast development is often the first sign of puberty. At Tanner Stage 2, girls will have developed breast buds, which are generally small mounds just larger than the nipple. This stage also includes the initial appearance of pubic hair, which is sparse and mostly on the labia. Some girls will see some pubic hair before their breasts begin to develop, although this is rare.
In the third stage, breasts continue to grow, but there is no separation between the nipple and breast mound. Pubic hair grows darker and coarser, and begins to cover more area. At Stage 4, the nipple begins to develop as a secondary mound on the breast as the breast itself also continues to grow. Pubic hair now covers the typical expanse as for a woman, but is sparser than on a fully developed woman. By Stage 5, girls are essentially out of puberty and are fully developed adults. Their breasts have reached their full size. The pubic hair is now fully grown in and looks like the traditional inverted triangle.
It is entirely natural for a girl's breast development and pubic hair growth to be in different Tanner stages at the same time. The Tanner scale does not address when menstruation should likely first start. Typically, a girl will begin to menstruate around two years after her breasts start to grow (Stage 2). However, her periods will not regulate and become predictable for another year or two. Finally, a girl will usually hit her major growth spurt prior to getting her first period.