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Conception occurs when a man's sperm fertilises a woman's egg. To understand conception and pregnancy, it helps to know about the male and female sexual organs, and to understand the physical process behind a woman's monthly cycle and conception itself.

The male sexual organs
The penis is made of erectile tissue. This tissue acts like a sponge and, when it becomes filled with blood, the penis becomes hard and erect.

The testes are contained in a bag of skin that hangs outside the body, called the scrotum. It helps to keep the testes at a constant temperature, just below the temperature of the rest of the body. This is necessary for the sperm to be produced. When it's warm, the scrotum hangs down, away from the body to help keep the testes cool. When it's cold, the scrotum draws up closer to the body for warmth.

Two tubes, called the vas deferens, carry sperm from the testes (testicles) where sperm are made, to the prostate and other glands. These glands add secretions that are ejaculated along with the sperm.

The urethra is a tube that runs down the length of the penis from the bladder, through the prostate gland to an opening at the tip of the penis. Sperm travel down this tube to be ejaculated.

The female sexual organs
A woman's reproductive system is made up of both external and internal organs. These are found in what is usually referred to as the pelvic area, or the part of the body below the tummy button. The external organs are known as the vulva. This includes the opening of the vagina, the inner and outer lips (labia) and the clitoris.

The woman's internal organs are made up of:

Pelvis: this is the bony structure around the hip area, which the baby will pass through when he or she is born.

Womb or uterus: the womb is about the size and shape of a small, upside-down pear. It is made of muscle and grows in size as the baby grows.

Fallopian tubes: these lead from the ovaries to the womb. Eggs are released from the ovaries into the fallopian tubes each month. This is where fertilisation takes place.  

Ovaries: there are two ovaries, each about the size of an almond. They produce the eggs, or ova.

Cervix: this is the neck of the womb. It is normally almost closed, with just a small opening through which blood passes during the monthly period. During labour, the cervix dilates in order to let the baby move from the uterus into the vagina.

Vagina: the vagina is a tube about three inches (8cm) long. It leads from the cervix down to the vulva, where it opens between the legs. The vagina is very elastic so it can easily stretch around a man's penis, or around a baby during labour.

Hormones: Hormones are chemicals that circulate in the blood of both men and women. They carry messages to different parts of the body, regulating certain activities and causing certain changes to take place. The female hormones, which include oestrogen and progesterone, control many of the events of a woman's monthly cycle, such as the release of the egg from the ovary and the thickening of the womb lining. During pregnancy, your hormone levels change. As soon as you have conceived, the amount of oestrogen and progesterone in your blood increases. This causes the womb lining to build up, the blood supply to your womb and breasts to increase and the muscles of your womb to relax to make room for the growing baby. The increased hormone levels can affect how you feel. You may have mood swings, feel tearful or be easily irritated. For a while, you may feel that you can't control your emotions, but these symptoms should ease after the first three months of your pregnancy.