What is ultrasound?

High pitched sound waves are produced by a probe. These waves pass through various tissues such as your baby, the placenta, the amniotic fluid, the uterus and the ovaries. Some of these waves are reflected back by these structures and these patterns are interpreted by a computer to produce images that we can recognise on a monitor. These images can be "real time" or frozen to obtain measurements. They are usually in two-dimensional "slices" but modern machines may be capable of producing 3-D images. Ultrasound can also be used to measure the flow of blood in the baby"s circulation (umbilical artery and occasionally other arteries and veins), and the blood flow to your uterus in the uterine arteries.

How is a scan performed?

The scan is performed by a doctor. It is painless and poses no risk to either you or your baby under the conditions used in normal obstetric ultrasound. You will be lying down during the examination but may be asked to turn to one side or the other to enable better views to be obtained. Gel is first applied to the skin so that the sound waves are able to pass from the probe to your body and back again. In certain circumstances, such as early pregnancy or low-lying placenta, it will be necessary to perform the scan using a special probe inserted into the vagina (the probe is covered with a condom that is discarded immediately after use). Again there are no risks associated with this procedure. It is not necessary to be nil-by-mouth or attend with a full bladder for obstetric scans. The results will be available to the requesting doctor immediately (on the internal computer system) or via fax or mail in the case of referring doctors outside the hospital.

What can the scan show?

The scan can be used to date the pregnancy precisely, identify the number of babies and placentae in the case of twins, identify certain structural problems in the baby and monitor the baby"s growth. It is also used as part of the triple test for the identification of babies who are at high risk of conditions such as Down"s syndrome.

When are the scans performed?

A minimum of 3 scans are offered during your pregnancy. The first at 11-14 weeks is called the nuchal scan and provides a measurement for the thickness of the skin overlying the back of the baby"s neck (this is found to be thickened in cases of Down"s syndrome). The second "detailed" scan (echo morphologique) is at around 22 weeks and aims to confirm that the structure of the baby (heart, lungs, spine etc ...), is normal. The third scan is at around 32 weeks and checks the baby"s growth and presentation (head-down or breech).
There are numerous reasons for other scans to be performed as well.

Are there any problems concerning the use of the scans?

Unfortunately ultrasound scans are not perfect. Routine scanning will pick up around 60-70% of all malformations. Therefore 30-40% of malformations will not be identified. This can be due to technical difficulties with the scan such as reduced amniotic fluid around the baby or the baby lying in a difficult position. Scans are also more difficult in larger women compared with thin women.
Often it is not possible to see all parts of the baby at one appointment. In this case you will be asked to return at a later date in order to complete the scan.

If you have any questions regarding the use of ultrasound in pregnancy then please feel free to discuss this with your obstetrician.