Puberty

At the beginning of puberty, a girl's face rounds out, her hips widen, and her breasts begin to develop. Breast development can occur as early as 8 but starts between 10 and 14 for most girls. Full breast development may take two to five years. Pubic hair begins to grow shortly afterwards, followed by the first menstrual period, or menarche. Like male puberty, female puberty is initiated by hypothalamic hormones. GRH secreted from the hypothalamus triggers LH and FSH release from the anterior pituitary. The LH and FSH, in turn, stimulate ova maturation. GHRH is also released from the hypothalamus and stimulates growth hormone secretion from the pituitary.

Breast development is called thelarche and can be measured in stages. The initial accumulation of tissue pads the underside of the areola around the nipple. Before puberty, the areola is usually about 0.5 in (1.2 cm) in diameter. By the end of puberty, it can be about 1.5 in (3.8 cm) in diameter. The breast enlarges developing a smooth curve. Then a secondary mound of tissue grows under the areola. Usually by age 18, a girl's breasts have reabsorbed the secondary mound giving a rounded contour to the now adult shape.

Breast budding is followed by menarche between 12 and 14 for most girls. However, normal menarche may occur between 10 and 16. Menstruation occurs as part of the menstrual cycle which lasts about 28 days. The initial hormonal cycles associated with the menstrual period usually begin months before menarche, so for a while a girl usually has hormonal cycles without menstruation. The menstrual cycle is divided into two halves, the follicular and the luteal phases. During the follicular phase, an immature egg follicle ripens and estrogen levels rise. On around day 14, LH and FSH trigger the egg to travel into the adjacent fallopian tube. During the luteal phase, high progesterone and estrogen levels prevent another egg from beginning another cycle. After about eight days, if the egg is not fertilized, then the uterine lining is shed as menstrual blood. Menstruation can last one to eight days but usually lasts three to five days. The amount of blood lost varies from slight to 2.7 oz (80 ml) with the average being 1 oz (30 ml) lost for the whole period.

A number of factors affect when menstruation begins. Normal menarche is associated with good nutrition and health. Girls who are malnourished or ill may have later menarche. In addition, girls who are particularly athletic or involved in strenuous physical activities such as ballet often start menstruating later. Once menarche occurs, cycles are usually irregular for up to two years. Because of this irregularity, girls may be less likely to conceive during this time. However, it is possible to conceive and therefore they should use contraception if they are sexually active and wish to prevent pregnancy.

The pubertal growth spurt, of height and weight, in girls usually occurs a year or two before boys, on average. Increases in height and weight are followed by the increases in hip size, breast size, and body fat percentage. The peak growth rate during this time is 3.2 in (8 cm) per year, on average. The average female is 4 ft 3 in (1.3 m) tall at the beginning of puberty and gains 13.5 in (34 cm) total during her pubertal growth spurt. At the end of puberty, the average female height is 5 ft 4.5 in (1.6 m) tall. Girls also increase body fat at the hips, stomach, and thighs.