Page 162 - 86 human physiology part-2

Development: Birth Through Death

characteristics. Menarche, the first menstrual cycle, occurs at about 12.5 years of age as a result of the
release of FSH.

Breast development

The first physical sign of puberty in girls is usually a firm, tender lump under the center of the
areola(e) of one or both breasts, occurring on average at about 10.5 years. This is referred to as
thelarche. By the widely used Tanner staging of puberty, this is stage 2 of breast development (stage 1
is a flat, prepubertal breast). Within 6-12 months, the swelling has clearly begun in both sides,
softened, and can be felt and seen extending beyond the edges of the areolae. This is stage 3 of breast
development. By another 12 months (stage 4), the breasts are approaching mature size and shape, with
areolae and papillae forming a secondary mound. In most young women, this mound disappears into
the contour of the mature breast (stage 5), although there is so much variation in sizes and shapes of
adult breasts that distinguishing advanced stages is of little clinical value.

Pubic hair in girls

Pubic hair is often the second unequivocal change of puberty. It is referred to as pubarche and the
pubic hairs are usually visible first along the labia. The first few hairs are described as Tanner stage 2.
Stage 3 is usually reached within another 6–12 months, when the hairs are too numerous to count and
appear on the mons as well. By stage 4, the pubic hairs densely fill the "pubic triangle." Stage 5 refers
to spread of pubic hair to the thighs and sometimes as abdominal hair upward towards the umbilicus. In
about 15% of girls, the earliest pubic hair appears before breast development begins.

Vagina, uterus, ovaries

The mucosal surface of the vagina also changes in response to increasing levels of estrogen,
becoming thicker and a duller pink in color (in contrast to the brighter red of the prepubertal vaginal
mucosa). Whitish secretions (physiologic leukorrhea) are a normal effect of estrogen as well. In the
next 2 years following thelarche, the uterus and ovaries increase in size. The ovaries usually contain
small cysts visible by ultrasound.

Menstruation and fertility

The first menstrual bleeding is referred to as menarche. The average age of menarche in American
girls is about 12.7 years, usually about 2 years after thelarche. Menses (menstrual periods) are not
always regular and monthly in the first 2 years after menarche. Ovulation is necessary for fertility, and
may or may not accompany the earliest menses. By 2 years after menarche, most girls are ovulating at
least several times a year. Over 90% of girls who experience menarche before age 13 years are
experiencing very regular, predictable menses accompanied by ovulation within 2 years, and a higher
proportion of those with later menarche may not establish regular ovulation for 4 years or more.
However, initiation of ovulation after menarche is not inevitable, and a high proportion of girls with
continued irregularity several years from menarche will continue to have prolonged irregularity and
anovulation, and are at higher risk for reduced fertility.

Pelvic shape, fat distribution, and body composition

During this period, also in response to rising levels of estrogen, the lower half of the pelvis widens
(providing a larger birth canal). Fat tissue increases to a greater percentage of the body composition

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