Body Story

• The origins of change

• Physical changes

• Menstruation

• The origins of change
As with boys, puberty for girls starts years before there are outward signs. And it starts in precisely the same way as in boys: gonadotrophins are released into the blood, carrying chemical signals around the body. But whereas in boys gonadotrophins cause cell division in the testicles, in girls they stimulate the Graafian follicles located inside the ovaries, so that the eggs inside them begin to mature. As this happens, the eggs produce a new hormone, oestrogen, which flows out of the follicles and enters the blood stream.

One of oestrogen's early effects is to make permanent changes in the brain: as previously dormant areas of the brain are brought to life, powerful emotions are triggered.

• Physical changes
Oestrogen also has dramatic physical effects, and the physical changes of puberty are obvious at an earlier stage in girls than in boys. Boys have to wait as testosterone levels build up gradually in their bodies to the point at which it starts producing visible changes, such as hair production, and excess testosterone is converted into oestrogen, leading to a burst of growth. This is why girls often appear to mature more rapidly than boys, and why they often find older boys, who appear as sexually mature as themselves, more attractive.

Once oestrogen reaches a critical level, it triggers a burst of growth and weight gain. Fortunately, the way in which that weight is distributed changes at puberty. Oestrogen causes fat globules in the chest to be sucked out of the blood by surrounding fat cells. These soak up the fat and expand, causing girls' breasts to grow. Between the ages of 11 and 16, a similar process takes place around the hips and buttocks, giving girls the beginnings of an 'hourglass' shape.

Girls are luckier than boys in the early stages of puberty, in that oestrogen keeps the levels of sebum production down, meaning that girls are less likely to develop spots. However, girls also produce small amounts of testosterone, the male hormone, in order to give them a sex drive. When this is not countered by oestrogen — and oestrogen levels fall just before a period — girls suffer from spots too.

Another side-effect of testosterone production in girls is that they become hairier. Pubic and armpit hair grow, and hair on the legs becomes thicker and darker.

• Menstruation
But for girls, the telling sign of sexual maturity is menstruation. This usually happens between the ages of 11 and 16, but it can happen earlier or later.

The biological process that leads up to the first period is complex. Gonadotrophins cause one follicle within the ovaries to grow so large that the egg within in it is almost mature. When it reaches this stage, this follicle produces around three-quarters of all the oestrogen in the body. In order to keep growing, the follicle needs more gonadotrophins than can be supplied, and so it begins to starve and die, as does the egg within it. As a result, levels of oestrogen within the body plummet. When this happens, the blood vessels that feed the lining of the womb are cut off, and the womb lining dies and gradually bleeds away. This is the first period, or menarche.

Menarche is unique: it is the only period that is not triggered by the creation of a fertile egg. After the first period, the brain locks into a monthly cycle, in which gonadotrophins are released, stimulating follicles, inside which eggs grow. The eggs produce oestrogen, which stimulates the womb lining to re-grow. When one follicle reaches a critical size, it sends a signal to the brain, triggering a massive hormone surge. Gonadotrophins flood into the bloodstream, stimulating an egg to mature. The egg bursts through the follicle wall and is carried towards the womb. It will then either be fertilised, or it will be swept away the next time that oestrogen levels plummet and the womb lining dies as part of the monthly cycle.

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