Updated 22 September 2015

Pregnancy ultrasound

Ultrasound is high-frequency sound that is generated electronically by a machine and special crystals in a probe.


Alternative names:

Foetal scanning, obstetric scanning or pregnancy scan.

What is obstetric ultrasound in pregnancy?

Ultrasound is high-frequency sound that is generated electronically by a machine and special crystals in a probe. The sound travels through your body, and bounces off different tissues and back to a receiver on the same probe. These sound waves are interpreted into a digital image. Ultrasound is not the same as X-ray, and does not make use of ionising radiation.

What is it used for?

In pregnancy, ultrasound examination is used to examine your uterus and the contents thereof, including the foetus and placenta.

Many observations are made during pregnancy. The ultrasound is used to confirm that the pregnancy is in the right place, inside the uterus, as opposed to an ectopic pregnancy where the foetus has implanted in an abnormal area. Other findings of importance are:

  • to establish the presence of a living embryo/foetus,
  • determine if there are multiple pregnancies,
  • estimate the age of the pregnancy,
  • diagnose congenital abnormalities,
  • evaluate the position of the baby,
  • evaluate the placenta, including its position,
  • determine the amount of amniotic fluid around the baby, and
  • evaluate the cervix for opening or shortening thereof.

The first scan that you may have is at about six week's gestation. Apart from checking the above, this scan is particularly used to estimate the gestational age of the foetus. This will help predict the birth date of your baby.

The next scan, usually at about 12 weeks, is another measurement scan to check dates and monitor growth. The first checks for abnormalities are also done at this time.

The scan to follow is the 20-week scan; this is done as a detailed check for abnormalities. All organ systems are checked, and the sex of the child can be assessed.

How should I prepare?

Wear loose-fitting clothing, as the examiner needs to have access to your abdomen. You may be asked to wear a gown. If it is early in your pregnancy, you may be requested to have a full bladder for the examination. After about three months, a full bladder is usually no longer required.

How is the procedure performed?

As you lie on the examination table, the radiologist or sonographer will spread warm gel on your skin and press the transducer firmly against your body, moving it back and forth over the area of interest until the desired images are captured. They will regularly stop to measure the size of the foetus. The examinations are by and large painless, fast and easy. Occasionally a transvaginal ultrasound examination will be required. During this exam an ultrasound probe is inserted into the vagina and the images are acquired this way. For further information on this, read Pelvic Ultrasound.

Are there any risks?

There has not been one documented case of damage to either a child or mother as a result of the ultrasound examination. Occasionally it has been noted that the examination seems to wake the sleeping foetus, but nothing else has been reported. The benefits of these examinations are extensive.

Are there any limitations?

Ultrasound scanning cannot detect all foetal abnormalities. In cases where abnormality is suspected but not detected ultrasonographically, you may require further testing like amniocentesis or chorionic villus sampling.

What will happen with the results of the examination?

Before the scan is performed, you will be asked if you want to know any of the results. The doctor who referred you for the examination will discuss the results with you after the scan.

Is it confidential?

Yes, like all medical examinations, your name, medical condition, and any particulars of your life are not to be made public in any way. The person performing the scan may record on paper, or digitally on the machine hard-drive, certain pictures to help them or colleagues remember the aspect they were interested in.


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