What a picture!

Ultrasound scans explained

• When are scans done?
• Are they safe and accurate?
• What problems can they detect?
• Placental problems and doppler ultrasound
• Nuchal translucency testing
• 3D/4D scans

You'll probably be offered two NHS ultrasound scans in pregnancy. Scans performed before eight weeks are usually done vaginally, using a special probe, to allow a clearer picture of the tiny baby while later scans are all done using a scanner on the bump.

• 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!

Girl or boy? At the 18-22 week scan you may be able to find out the gender of your baby, if you are interested. If this is important to you check beforehand as some hospitals have a rule that they don't tell parents what the sex of their baby is.

As it is quite common nowadays for parents to expect to find out the gender of their baby make sure you tell the operator if you don't want to know.

Remember, that there's no guarantee that the sonographer will either be able to see the sex organs or see enough to guess the sex of your baby - so it's not a completely reliable way to find out if you are having a boy or girl!

Extra scans If you develop a complication in your pregnancy or have a multiple pregnancy, you may be offered further scans.Common reasons for further scans include multiple pregnancies, fibroids and a low-lying placents (placenta praevia), see placental problems, below.

Are they safe and accurate?

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.

In terms of accuracy in detecting problems with your baby, scanning depends on:

• 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.

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 18-22 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.

The sonographer at your anomaly scan will also check the position, size and function of the placenta that supplies your baby with blood (and oxygen) plus nutrients from your own body. If your placenta is low-lying, ie across or near to the neck of the womb, you will be offered another scan later on in pregnancy to see if it has moved - the most likely outcome - in which case it won't present any further problems. If the placenta continues to block the womb, you'll be booked in for a caesarean section around your due date.

Doppler scanning uses black and white or colour images to trace the blood flow between the placenta and your baby through the umbilical cord. Using colour allows the sonographer to identify more easily the various different blood vessels. If needed, doppler scanning can usually be done during the anomaly scan, as long as the hospital's equipment is up to speed, and may be used at regular intervals throughout the pregnancy to keep a check on any known placental problems.

If your baby doesn't grow at the expected rate and is thought to be 'small for dates', you may also be offered doppler scanning as having a small baby could indicate placental problems. Doppler can also be used to predict your risk of pre-eclampsia, the high blood pressure disease of pregnancy, so that early preventive treatment can be given.

The nuchal fold translucency test is the latest and most accurate screening test for Down's Syndrome. It uses ultrasound to measure the amount of fluid between two layers of skin behind your baby's neck as those with Down's have a great collection of fluid than those who don't. This measurement is then fed into a computer, along with your age (the risk of Down's increases naturally with age), your baby's heart rate and any blood test results. The result will give you a risk factor, for example, one in 400, of having a baby with Down's and it's then up to you to decide whether to go on and have a diagnostic test such as amniocentesis.

Research is currently underway into another possible indication of Down's that can be detected by ultrasound. Sally Hill is investigating whether the absence of a nasal bone at the 12-week scan could be a signpost that Down's is present. 'This, together with the nuchal fold measurement, may give a more accurate indication of the risk of Down's and reduce the percentage of women who need an invasive diagnostic test,' says Sally.

Only 7 per cent of women are offered the nuchal scan on the NHS but it is available privately across the country with fees ranging from £90-£150. The Fetal Medicine Foundation, which first developed the nuchal scan, runs a training and accreditation scheme to maintain standards and you can find your nearest accredited centre by logging onto the foundation's website, (see below).

Various manufacturers of ultrasound equipment, universities and private clinics are developing 3D technology with ultrasound and it is now widely available. The term 3D means that a life-like picture of your baby can be built from images of the three 'planes' of vision that make up 3D, stored on the computer after the ultrasound, while 4D refers to a moving 3D image. The ultrasound takes the same as a conventional scan because the images are recorded simultaneously. Experts using 3D/4D scanning say this 'surface reconstruction' is faster and gives a clearer image of the baby's external anatomy including the face, hands and feet as well as the baby's activity in the womb. However, a well trained sonographer should pick up any problems using conventional 2D technology.

Where to next?

• Visit the Fetal Medicine Foundation
• Join babyworld's antenatal classes
• Get the lowdown on Health and wellbeing in pregnancy
• Find out more about diagnostic tests with Sally Hill Diagnostic Services
• Get more information about ultrasound from the Echo Charity.