What is Ultrasound?

Ultrasound is high frequency sound, much higher than humans can hear. It is passed into the body from an ultrasound probe (transducer) and the sound is reflected off the internal organs.

The ultrasound machine changes the returning echoes (sound waves) into visible images and displays them on a television screen.

Why Do I Need An Ultrasound Scan?

If you are pregnant, the most common reasons for your doctor to request an ultrasound scan are

• to establish the date when the baby is due
• to check the number of babies
• to establish that the pregnancy is continuing normally when there has been bleeding in the early part of pregnancy
• to show the position of the placenta when there has been bleeding in later pregnancy
• to check the physical development of the baby and, as far as possible, to check for birth defects (this is best done between 18 to 20 weeks)
• in later pregnancy, to see that the baby has grown appropriately for its stage of development.

What Happens During An Ultrasound Examination?

Most ultrasound examinations are performed on your lower abdomen. Gel is spread on your stomach area to allow the ultrasound waves to pass from transducer into your body. The waves reflect off internal organs (and the fetus if you are pregnant), creating a moving picture on a TV screen. This is known as real-time ultrasound, and you can view the picture on the monitor at the same time as the ultrasonologist.

Is Ultrasound safe?

Diagnostic ultrasound has been in clinical use since the late 1950s. To date, the results of numerous follow-up studies on patients and children who have been examined before birth have failed to demonstrate any confirmed harmful effects that could be attributed to the ultrasound examination.

Does My Bladder Need To Be “Bursting”?

If you are not pregnant, or in the early weeks of pregnancy, the uterus (womb) and the ovaries lie deep in the pelvis and are often covered by the bowel. With some urine in the bladder, the bowel is pushed aside and the pelvic organs may be more easily viewed.

If you have any bleeding complications in pregnancy, the lower part of the uterus needs to be viewed carefully to make sure that the afterbirth (placenta) is not in the way. Under these circumstances, a moderately filled bladder is need for a proper examination.

This is usually best achieved by drinking 500 mls of water or other fluid one hour before the ultrasound examination time.

For most of the other ultrasound examinations, a small amount of urine in your bladder is adequate. If you are not sure whether you can empty your bladder before the examination, please see the attending staff before your ultrasound scan.

What About Vaginal Scanning?

It is possible to scan through the vagina using a specially designed probe. A small transducer is placed gently into the vagina. This should only lead to minimal discomfort. If you are pregnant, or in the early stages of pregnancy, this method often provides better images and therefore more information.

You would usually be asked to empty your bladder before vaginal scanning.

How Limited is the Accuracy of a Diagnosis of Birth Defects by Ultrasound?

Although a number of birth defects can be identified by ultrasound, there are many that will not be detected. Diagnostic ultrasound cannot be used to guarantee a normal baby.

A scan at 18 to 20 weeks gestation is generally the best time to look for birth defects, although some will not become evident until late in the pregnancy. Anatomic areas such as the fetal heart, the face and the hands are difficult to assess and not all defects will be detected.

Other factors such as the pregnant woman's build, scars from previous operations, and the way the fetus is lying may limit the diagnostic ability of this test.

It is not possible to diagnose such conditions as Down syndrome by ultrasound, though sometimes there may be features seen during the examination which make the operator suspect that the baby could have this or another chromosomal problem. Further testing is required to diagnose these conditions.

As with adults, there can be normal variations in the appearance of the fetus or fetal structures. Sometimes these variations of “normal” can be puzzling. So, do not be alarmed if the operator requests a future scan or recommends that further tests be carried out.

Other Questions and Comments

We hope that you have found this information useful. If you would like any further information, please discuss your concerns with your doctor.