An ultrasound scan involves transmitting high frequency sound waves through the uterus. These bounce off the baby and the returning echoes are translated by a computer into an image on a screen that reveals the baby's position and movements. Hard tissues such as bone reflect the biggest echoes and are white in the image, and soft tissues appear grey and speckled. Fluids (such as the amniotic fluid that the baby lies in) do not reflect any echoes so appear black. It is the contrast between these different shades of white, grey and black that allows your sonographer to interpret the images.

Most parents look forward to their scan because it gives them the first glimpse of their baby. The sonographer will probably give you a printout of your baby as a keepsake, although some hospitals charge for this. However, it is important to remember that the main purpose of the scan is not to provide the first photo for your baby album, or to find out your baby's sex, either. It is to check that your baby is growing and developing normally.

Depending on which stage of pregnancy they are done at, ultrasound scans can: Check your baby has a heartbeat Say whether you are pregnant with one baby or more Detect an ectopic pregnancy, where the embryo implants outside of the womb, usually in the Fallopian tube. Find out the cause of any bleeding you may be having Accurately date your pregnancy by measuring your baby Assess the risk of Down's syndrome by measuring fluid at the back of your baby's neck at weeks (what's called the nuchal translucency scan).ectopic pregnancybleedingnuchal translucency scan

Find out why a blood screening test was abnormal Assist in performing diagnostic tests, such as CVS or amniocentesis safely by showing the position of the baby and placenta. Examine your baby to see if all the organs have developed normally Diagnose certain abnormalities, such as spina bifida Assess the amount of amniotic fluid you have and locate the placenta. Measure your baby's rate of growth over several scans.blood screening testCVSamniocentesis

Ultrasound has been used in pregnancy for nearly 30 years and medical research has found no side effects. No association has been shown between ultrasound exposure and the baby's birthweight, childhood leukaemias or other cancers, eyesight, hearing or dyslexia. Even so, most experts agree that the procedure shouldn't be done without clear medical reasons, and that all ultrasound exposure should be justified and limited to the minimum needed to make a diagnosis.

Dating scan at 8-12 weeks - the sonographer takes some basic measurements of your baby, for example from the top of his head to the end of his bottom (crown/rump measurement) in order to calculate how many weeks old he is, and therefore, your due date. Dating scans tend to be done before 13 weeks as they are less accurate after this. However, there can still be a discrepancy in your own date and that confirmed by the scan in which case you may have a range of time, for instance between 12th and 20th July, in which your baby is due. This can prove important later on if you go overdue and your obstetrician starts suggesting induction! Major abnormalities, such as limb deformities, may also be picked up at this stage, depending on the quality of the equipment and the experience of the sonographer.

18-22 weeks anomaly scan - this is the main scan at which the sonographer takes a much more detailed look at your baby. She takes various measurements to check that your baby's growth is 'on track' and looks for conditions such as cleft lip and palate, spina bifida, plus any abnormalities of the brain, heart and other organs. She'll also count all your baby's fingers and toes just to be on the safe side!

While there's no strong evidence to suggest that ultrasound harms babies, there is a small amount of research that suggests a few babies could be affected. Some people think that more long-term research needs to be done in order to make sure ultrasound is safe, but others feel it is so widely used that any serious problems would have shown up by now.

the quality of the scanning machine (some NHS machines are less than modern!) the training and skills of the person doing the scan the length of time for which you are scanned the way your baby is lying (if the scanner can't see certain organs such as the heart, you may be asked to go and walk round, then come back) whether you are very overweight.

"Ultrasound machines are getting better in that the resolution of images is improving but the accuracy of the results still depends to a large extent on the ability of the person using the machine," says independent sonographer Sally Hill who runs diagnostic ultrasound services for several NHS trusts in the south east of Engand.

Although scans are a useful aid, they can only supply a certain amount of information and they may miss small problems while identifying anomalies that rectify themselves by the time your baby is born. +

Depending on the type of scan you are having, the results may be diagnostic - it could show, for example, that your baby's brain hasn't formed properly (anencephaly) or that he has a gap in his spine (spina bifida). However, your scan is much more likely to act as a screening test which means that it may provide some evidence of developmental or medical problems that need further investigation.diagnosticscreening

Perhaps the hardest abnormalities to spot (and, therefore, most often missed) in unborn babies are heart defects. According to Echo, the charity supporting effective cardiac health in obstetrics, only 25 to 30 per cent of infant heart problems are diagnosed before birth in the UK. The heart is the most complex in the human body and at weeks of pregnancy (when the anomaly scan is done) is only the size of a peanut. But, with the right training and equipment, it is possible to check all of the heart's internal and external detail using computer- enhanced ultrasound and new technology may provide more help in this area in future.

Sally Hill says that if a sonographer picks up a serious heart abnormality or other major defect such as a facial tumour in the second or third trimester of pregnancy, the woman should be referred for an MRI scan, (magnetic resonance imaging which is safe for the baby). This gives the clearest image of the baby's internal organs and will show much more accurately the extent and seriousness of any problem.

If your scan suggests there is a problem with your baby, it's important to ask how serious the problem is likely to be, whether there are better scanning techniques at your hospital, (for instance MRI), that could be used to give a more accurate diagnosis.