Girls' bodies undergo gradual changes during puberty, analogous to but distinct from those experienced by boys. Puberty is the process of physical changes by which a child's body matures into an adult body capable of sexual reproduction to enable fertilisation . It is initiated by hormonal signals from the brain to the gonads -either the ovaries or the testes . In response to the signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain, bones , muscle , blood , skin , hair , breasts, and sexual organs. Physical growth —height and weight—accelerates in the first half of puberty and is completed when the child has developed an adult body. Until the maturation of their reproductive capabilities, the pre-pubertal, physical differences between boys and girls are the genitalia , the penis and the vagina. Puberty is a process that usually takes place between the ages 10–16, but these ages differ from girl to girl. The major landmark of girls' puberty is menarche, the onset of menstruation, which occurs on average between ages 12–13.