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.

Improve your posture
Backache is one of the most common pregnancy niggles. There are many causes, but one of the main ones is bad posture caused primarily by the weight of your bump. This exercise should get you in good habits.
Stand with your feet hip-distance apart, making sure your knees are soft. Feel the weight of your feet on the floor and feel your trunk lifted and open.
Tilt your pelvis forward, your tailbone down towards the decking, and pull your baby up and in towards your spine. Make sure you have a natural curve in your back - it shouldn't be vaulted. Lengthen through the spine and aim for a nice straight back. Keep your shoulders back and down, and imagine you have a string pulling your head upright.

A good diet is central to overall good health. But which are the best foods to include in your meals, and which ones are best avoided?
Although it contains no nutrients, water is a vital component of our diets. It's essential for the growth and maintenance of our bodies, as it's involved in a number of biological processes. But most of us don't get nearly enough.
Water comprises 50 to 70 per cent of an adult's total body weight, and without regular top-ups, our body's survival time is limited to a matter of hours or days.
Water is lost from the body through urine and sweat, and must be replaced through our diets. Many people, though, don't consume enough and as a result may become dehydrated, causing symptoms such as headaches, tiredness and loss of concentration.