What ultrasound scans do I need?

By Jasmin Whinnett, Fetal Medicine

Now that you've fallen pregnant, you might be wondering what happens next? It can feel like there’s a mountain of information about pregnancy but also feel as if it’s going to take a science degree to work out what’s important, what’s relevant for you and your partner and which testing you should or could have, to check the health of your growing baby.

Ultrasounds are performed by very highly trained Sonographers and utilise sound waves to look inside the baby. Since ultrasound was first used in 1956, there has been no evidence to show any associated health risks - ultrasound scans are safe for you and your baby.

The black and white images most people are familiar with are 2D ultrasound images. 2D images provide diagnostic information about the baby and in the hands of experienced sonographers can pick up structural problems.

3D and 4D (real time imaging) ultrasound are commonly sepia coloured. In addition to providing us with a sneak peak of your growing baby, they may also provide better definition of some abnormalities such as cleft lip, abnormal positioning of hands and feet or Spina bifida lesions.

You can have an ultrasound at any time during your pregnancy but there are certain weeks which are used to provide specific, standardised growth and structural assessments for your baby.

Early dating scan

Commonly performed early in the first trimester, usually during week 6,7 or 8. These scans are used to confirm there is a fetal heartbeat, to check the expected date of delivery (EDD), to count the number of babies and can also help exclude an ectopic pregnancy (a pregnancy located outside of the uterus).

This scan is not a necessity but is helpful if you are unsure of your dates and it will help you identify the correct time to have the 12-13 week Nuchal Translucency (NT) Ultrasound.

12 - 13 week Nuchal Translucency Ultrasound/First Trimester Assessment

Ideally performed between 12.5 to 13.5 weeks, this is a screening test designed to identify whether your baby is at high or low risk for a chromosome abnormality such as Trisomy 21 (Down Syndrome), Trisomy 13 or Trisomy 18.

It also provides an early opportunity to check the baby for major defects and provided the ultrasound practice is accredited, can also screen for pregnancy complications such as baby having low birth weight, baby delivering prematurely and the chance you will develop pre-eclampsia.
Given the amount of information gathered from the 12-13 week NT ultrasound it is highly recommended.

The NT Ultrasound combines your age risk with the results of a blood test (done before the scan, around week 10 or 11) and ultrasound measurements of both the length of the baby from head to bottom and the fluid collection located at the back of the baby’s neck.

You should be given your risk result on completion of the ultrasound.

  • A low risk result indicates you have a low chance of having a baby with a chromosomal abnormality. This is a great result however it is recommended you have the Morphology ultrasound at 19-20 weeks for a more comprehensive structural assessment
  • A moderate risk may be handled in two ways. Firstly, further clarification of the fetal DNA via the Non Invasive Prenatal Test (NIPT* - see below); or secondly, skipping the NIPT and following up with the Morphology ultrasound at 19-20 weeks. You should be counselled at the time of the scan to help decide which path to take
  • A high risk result should accompany counselling regarding the types of diagnostic tests that are available to provide clarification for you and your baby. The diagnostic tests are also termed ‘invasive tests’ and are the only way to be 100 % certain of the baby’s chromosomal makeup. They include Chorionic Villi Sampling (CVS) or Amniocentesis with Microarray

*NIPT - Available in Australia since 2013, this involves a simple blood test taken from the you, the pregnant mother, after 10 weeks gestation. It is 99.5% accurate when screening for Down Syndrome but cannot assess the fetal structure so it should always be preceded by an ultrasound. It should NOT replace the NT ultrasound.

19 - 20 week Morphology Ultrasound

At this stage most of your baby’s organs have formed and can be assessed with ultrasound. Most women have already had an earlier ultrasound by now (usually the NT ultrasound).

This scan confirms fetal heartbeat, measures your baby’s size, checks the baby’s structural anatomical development, identifies the position of the placenta, checks the volume of amniotic fluid and may reveal the gender if the baby is obliging!

Non-structural abnormalities such as cerebral palsy, epilepsy, autism, deafness and blindness will never be seen on ultrasound. Sometimes markers can be seen that may have links with certain syndromes and in these situations you should be counselled regarding your available options and the likely outcome.

This ultrasound is highly recommended. If your baby is healthy and there are no concerns then this may be your last ultrasound for the remainder of your pregnancy!

Late pregnancy scan

These ultrasounds are usually only ordered by your Obstetrician or health care team when there is a clinical indication. Reasons for a late or third trimester ultrasound may include:

  • Growth and well being of the baby or to estimate fetal size
  • Amniotic fluid loss or leaking
  • Twins/multiple pregnancy
  • Diabetes/Gestational Diabetes
  • Previous history of small or large baby
  • Low lying placenta
  • Pain or decreased fetal movements
  • High blood pressure

The ultrasound technology we have today is so far advanced in terms of what was available even five to ten years ago and often allows incredible views into the secret life of your baby. However, even with this amazing technology our ability to clearly see the baby is still dependent on the position and age of the fetus. Other quality-limiting factors may include maternal BMI, previous abdominal surgery and reduced amniotic fluid. Therefore it is possible, but rare, to misdiagnose the gender or not identify structural abnormalities.

Note: The views and advice expressed on this blog post are those of the author and are not representative of the Pregnancy Babies & Children's Expo.

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