In nature, as for any living organism, the probable sole aim of the human body is to reach maturity and reproduce before it dies, in order for the species to continue. Girls are born with all the eggs they'll ever have - in fact, the number of viable eggs falls steadily throughout life until the menopause. It takes, on average, 12 years for the female human to develop to puberty and become fertile.
It's not clear why it takes this long, although there's probably a critical weight - around 47kg (7.5st) - at which menstruation is triggered, dependent on the other hormone systems functioning properly, including growth hormone.
The changes puberty brings are vast, prompted by rapidly increasing levels of many hormones including oestrogen, progesterone and androgens.
During puberty, girls go through obvious outward changes, such as breast development, shape and hair pattern changes. The complex hormonal rhythm of the ovulatory cycle, involving brain, glands, ovaries and other organs, also begins.
Every month the body's hormones coordinate the production of an egg with a thickened uterine lining and receptive cervical mucus. If the egg's fertilised, the different series of hormonal changes that support pregnancy will follow, suppressing ovulation.
In our culture, girls of this age aren't considered emotionally mature enough for motherhood, but their bodies are capable of having children and in most cases continue to be so until the onset of the menopause between 45 and 55.
After the birth of a child, falling levels of hormone signal the brain to start the ovulatory cycle again, although if the mother is breastfeeding this will be suppressed and periods may not start for several months.
Why does it all stop?
Why women go through the menopause when they do - at what is just over halfway through their lifespan - and the exact trigger are unknown. It may be that thousands of years ago, 50 was the normal female lifespan.
No comments:
Post a Comment