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Ultrasound : Overview

Updated: Feb 5, 2021

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What is an ultrasound scan?

An ultrasound scan sends high-frequency sound waves through your uterus (womb). These sound waves bounce off your baby and the echoes are turned into an image on a screen that shows your baby's position and movements.

Hard tissues, such as bone, reflect the biggest echoes and are white in the image, and soft tissues are grey. Fluids, such as the amniotic fluid that your baby lies in, appear black because the echoes go through them. Your sonographer (the person performing the scan) will look at these shades to interpret the images.

Who performs the scan?

Scans are usually conducted by radiographers or midwives who are trained in ultrasound, and are known as sonographers. Most have a post-graduate certificate, diploma or master's degree in medical ultrasound.

Special scans may be needed in some pregnancies and these will be carried out by a doctor trained in ultrasound, known as a maternal fetal medicine specialist.

If you are seeing a private obstetrician or an obstetrician in a public hospital, she may also conduct a short scan at some of your antenatal appointments. These scans can be used to check the general health and growth of your baby and the how your placenta is working. Although the safety of ultrasounds is not in question, you should always be advised about what the purpose of a scan is and have the right to refuse, especially if there is no medical reason for its use.

What is an ultrasound scan used for?

Depending on which stage of pregnancy they are done at, scans can:

  • Accurately date your pregnancy by measuring your baby.

  • Determine whether you are pregnant with one baby or more.

  • Check that your baby has a heartbeat.

  • Examine your baby to see if all their organs are normal.

  • Detect an ectopic pregnancy, where the embryo implants outside the uterus, usually in the fallopian tube.

  • Find out the cause of any bleeding you may be having.

  • Assess your baby's risk of Down syndrome (nuchal translucency scan).

  • Determine why a blood screening test was abnormal.

  • Diagnose certain abnormalities, such as spina bifida.

  • Assess the amount of amniotic fluid you have and find out where the placenta lies.

  • Measure your baby's rate of growth over several scans.

How is an ultrasound performed?

If you're having a scan in early pregnancy, you will be required to drink a few glasses of water prior so that your uterus is pushed out of your pelvis by your bladder. This will give your sonographer a good view of your baby. Your sonographer will apply some gel on your tummy and move a hand-held device, or transducer, over your skin to pick up images of your baby. This is known as an abdominal scan.

If your baby is still too deep in your pelvis, or if you are overweight, the images will not be very clear, so a vaginal scan may be necessary. The vaginal transducer is long and narrow to fit comfortably inside your vagina. It is not necessary to go in very deeply, and it will not harm you or your baby in any way. Vaginal scans provide a much clearer picture of your baby, particularly during the very early stages of pregnancy.

Does an ultrasound hurt?

An abdominal scan is painless except for a mild discomfort from the transducer pressing on your tummy if you have a very full bladder. You won't usually be asked to fill your bladder for scans in later pregnancy, but check with your midwife or doctor first.