Jelly on the tum, a roving doppler and, hey presto, you see your baby on the screen. Dr Chris Brown looks at the role ultrasound scans play in antenatal care

Obstetric ultrasound was first introduced in the late 1950s. It provides a safe, non-invasive and accurate way of investigating pregnancy and the foetus, and has become a routine diagnostic tool in antenatal care. The technology has advanced considerably in the last 50 years.

There’s now a revolutionary, three-dimensional, scanner – only available in a few centres in the UK at the moment – which gives such clear pictures that medical staff can diagnose many foetal abnormalities more accurately, and at an earlier stage of the pregnancy.

Another valuable feature of this scanner is that the clear pictures encourage an early bonding between the parents and their, as yet, unborn baby. We know that couples develop a powerful emotional attachment to their baby, as pregnancy develops but, with a detailed image from this scanner, parents will be able to identify with the pregnancy in a completely different way, and this early bonding will help them through the labour-delivery-birth processes.

Though the routine scan that mothers have at the moment produces pictures that are blurred and difficult to make out, most parents find it a moving experience as they catch the first glimpse of their baby on screen.

• Establish how many weeks pregnant you are
• Say whether you have one or more babies
• Locate the placental position
• Measure the rate of growth of your baby (several scans are needed)
• Check that your baby has a heart beat
• Assess the amount of amniotic fluid
• Check for possible ectopic pregnancy
• Guide the needle position in amniocentesis
• Assist in other diagnostic procedures such as chorionic villus sampling (CVS) and umbilical cord blood sampling
• Diagnose certain abnormalities
• Assess vaginal bleeding in early pregnancy
• Confirm foetal presentation e.g breech in uncertain cases
• Diagnose and monitor gynaecological conditions such as fibroids and ovarian cysts during pregnancy

You will probably be offered two routine ultrasound scans during your pregnancy:

1. The ‘booking’ scan – usually around 10-13 weeks.
2. At about 20 weeks – specifically, for checking your baby and the gestational age, and to provide reassurance to you that your baby appears to have no obvious structural abnormalities.

Ultrasound used to be started at about 16 weeks gestation, but with skilled sonographers and excellent equipment that’s improving all the time, it’s now possible to detect problems and take decisions about pregnancies with earlier scans.

Between 5–11 weeks an early scan can detect ectopic pregnancy and foetal heartbeats, and usually, this procedure would use a vaginal probe for the scan.

At 10-14 weeks a scan can be used to assess the risk of a baby having Down’s syndrome (Nuchal Translucency Scan). Arms, legs, face and heartbeats can also be seen.

The 11-16 week scan (Booking or Dating Scan) is used to detect the number of babies and the expected date of delivery. Fully formed arms and legs can usually be seen.

Between 18-20 weeks (Anomaly Scan) Down’s syndrome, Spina Bifida, Placenta Praevia – where the afterbirth is developing low down on the wall of the womb – and the baby’s sex can be determined. Some hospitals do not allow scan operators to tell the parents the sex of the baby in case of error.

How is it done? Is it painful?

A doctor or radiographer (ultra-sonographer) carries out the ultrasound examination in hospital. You lie on a couch, while jelly is applied to your abdominal skin – to get a better contact between the ultrasound probe and your skin – then a sound-emitting probe is moved across your abdomen to pick up the reflected sound waves for the computer generated screen picture. There’s no pain at all.

Most obstetric departments are now using vaginal probes in early pregnancy because they give a clearer image and the results are more accurate, because the probe tip can be placed closer to the uterus. The probe, which only causes a minimal amount of 'pressure' discomfort, doesn't harm the baby.

Ultrasounds can pick up a variety of problems: The purpose of the anomaly scan at 18-20 weeks is to reassure the mother that her baby doesn't appear to have any gross structural abnormalities. This isn't however a guarantee, as not all abnormalities will be detected by a scan at this time. Conditions that have a high rate of detection at the 18-20 week scan include:

• Anencephaly (where the brain has not developed)
• Hydrocephaly (where there is excessive fluid in the cavities of the brain)
• Spina Bifida (where the spine does not develop fully)
• The limbs and internal organs can be checked, as well as the face for cleft palate.

Some of the most common problems of newborn babies, like heart defects and abnormalities, together with abnormalities of the major blood vessels, can’t always be detected.

Remember that certain abnormalities may be identified which correct themselves by the time your baby is born. For example, an early scan could indicate an abnormal position of the placenta. If this is the case, a review will be arranged for a later date, when it’s often discovered that the placenta has ‘corrected’ itself.

What if the scan shows a problem?

You will, of course, be frightened and upset if a scan suggests a problem. If this happens, several more scans may be taken at intervals and compared and it may be that other tests are also needed to confirm a detected abnormality.

About half of the major abnormalities will be seen on a scan and half will not. Some types of congenital abnormality are more likely to be detected than others:

• Anencephaly – absence of brain – has a 99% chance of being seen
• Spina Bifida – open spinal cord – has a 90% chance of being seen
• Hydrocephalus – excess fluid within the brain – has a 60 % of being seen
• Major defects of the abdominal wall have a 90% chance of being seen
• Major limb abnormalities have a 90% chance of being seen
• Major kidney problems have an 85% chance of being seen
• Down’s syndrome has a 40% chance of being seen
• Major congenital heart problems have a 25% chance of being seen
• Cerebral Palsy and autism are NEVER SEEN

If the scan predicts the sex of your baby it is usually 95% correct.

The value of identifying an abnormality at an early stage is that it offers parents options. Some couples – probably the majority for serious lesions – will elect to terminate the pregnancy. Couples who choose to continue the pregnancy have the opportunity to prepare themselves, through discussions with health care personnel and self-help groups, while medical staff can ensure the appropriate care during pregnancy and after delivery.

Are scans safe?

Medical researchers are continuing to monitor for possible long-term effects of ultrasound on the hearing of children and babies exposed to ultrasound in the womb, but ultrasound examinations have been in use in pregnancy for nearly 50 years, and to date, no side effects have been detected.

Reports of ill-effects such as low birthweight, speech and hearing problems and brain damage, have been investigated, but have not been confirmed in large controlled studies.

Random studies have suggested that at least one routine scan in pregnancy to confirm gestational age is valuable, but the final decision about whether or not to have the ultrasound scan is entirely yours. Certainly the benefits far outweigh any risks.