The sex of a baby is determined primarily by the two sex chromosomes. In normal male cells, there’s an X and a Y sex chromosome. In normal female cells there are two X chromosomes.
Initially, the embryonic tissue that later becomes testes in boys or ovaries in girls is undifferentiated. At around six to eight weeks of pregnancy, the presence of a Y chromosome causes this tissue to develop into testes. If there’s no Y chromosome, ovaries develop.
As well as the hormones released by the testes and ovaries, which determine whether we grow into boys or girls, hormones also influence brain development and behaviour.
Before You Are Born
Most of the brain is similar in males and females, but some regions such as the hypothalamus and amygdala, which control functions related to sexuality, differ between the sexes. In these tissues there are receptors or receiving units that allow the cells to respond to androgens, or male sex hormones.
Early hormone environment permanently influences behaviour. Girls exposed to high levels of androgens in the womb show more interest in toys such as cars and less interest in dolls, are more likely to prefer boys as playmates and engage in male-typical rough-and-tumble play.
Puberty and Beyond
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 is constantly falling. However, it takes on average 12 years for the female human to develop to puberty and fertility.
It’s not clear why it takes this long, although there’s probably a critical weight – around seven and a half stone – 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 is 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 now just over halfway through their lives – and the exact trigger are unknown. It may be that thousands of years ago, 50 was the normal female lifespan.
Oestrogen production by the ovaries falls as the menopause occurs. This triggers the brain to release other hormones, LH and FSH, in an attempt to make the ovaries work harder. The number and quality of eggs released decreases, the lack of oestrogen means the vagina begins to dry and thin, and fertility falls.
Symptoms such as hot flushes, sweats, muscle and bone pains, irritability and poor concentration are all linked to these hormonal surges.
The extent of these symptoms varies immensely but the end result is the same for all – failure of the reproductive organs and the subsequent effects throughout the body.
This article was last medically reviewed by Dr Trisha Macnair in August 2005.
First published in November 1997.