First trimester ultrasound

Why is ultrasound performed in pregnancy?

Ultrasound screening (like cervical smear and breast cancer screening) helps to identify problems early. It allows time for care, management and effective treatment should an irregularity be detected.

The two main reasons for ultrasound during pregnancy are:

  • screening—a routine test to confirm pregnancy dates and the number of babies, to check your baby’s physical development and to identify possible problems or potential birth defects
  • diagnostic—to identify problems causing symptoms in pregnancy such as bleeding, abdominal pain or slow growth.

First trimester screening program

The following information will help you understand what first trimester screening involves and how it can assess your unborn baby’s health and development.

Why is ultrasound screening offered in the first trimester?

First trimester nuchal translucency (NT) ultrasound is a non-invasive screening method used to identify babies with an increased risk for chromosomal abnormalities such as Down syndrome.

This assessment is done between 11 and 14 weeks gestation by measuring the thickness of a skin fold at the back of the neck called the NT.

The baby’s length, NT and your age are combined to calculate your baby’s risk level of Down syndrome (Trisomy 21). A blood test measuring two hormones (PAPPA and HCG) can also be taken. If used together with the ultrasound measurements it can improve accuracy of the test. The detection rate for Down syndrome using NT and your age is 75-80%. By adding the blood test results, the detection rate is 85-90%.

It is important to remember that not all babies with a chromosomal abnormality will be detected by this method as the detection rate is not 100% guaranteed using either technique.

How is the scan result interpreted?

A report will be provided for you when results have been calculated and a copy sent to the doctor who referred you.

If your baby’s risk is high, further diagnostic tests will be fully discussed with you. Genetic counsellors are available to discuss high-risk results. You can choose to proceed to the diagnostic test at any stage.

Occasionally during the ultrasound scan, images are not clear enough for a variety of reasons. The scan may need to be performed using a vaginal ultrasound probe. This may cause some discomfort, but should not be painful.

Staff have undertaken special training and accreditation in ultrasonography for first trimester screening. The benefits of "seeing" your baby early and being reassured that all is well (in 95% of cases) needs to be balanced against the possibility (for five percent of patients) of finding potential problems with your baby. We understand this could be a stressful time for you and your family.

First trimester ultrasound screening is a new medical development. There are still many questions we may be unable to answer. Your doctor will help you understand the benefits, risks and options. If you would like to discuss your concerns with our Maternal Fetal Medicine team before deciding to have first trimester screening, you are welcome to contact us during business hours on 07 3163 1896.

Ultrasound scanning is a low-risk procedure, and more than one scan early in your pregnancy is usually unnecessary. We recommend another scan be performed at 18-20 weeks to check for other irregularities which may not be detectable before 14 weeks, such as spina bifida. Your doctor will refer you for this scan closer to the time.

Is a First Trimester Screening appointment appropriate for me?

Until recently, diagnostic tests for fetal abnormalities were only offered to women aged 35 years or more as the risk of having a baby with problems increases with mother’s age. However, first trimester ultrasound screening is appropriate for women of all ages who:

  • are aware of an increased risk of problems based on previous pregnancies and hereditary knowledge
  • have concerns about their unborn baby.

How does first trimester ultrasound screening compare with other antenatal diagnostic tests?

Ultrasound is a non-invasive screening method. It is different from diagnostic tests such as amniocentesis and chorionic villus sampling (CVS), where a needle is inserted into mother’s abdomen. CVS is performed from 11 to 14 weeks and amniocentesis from 16 weeks.

These tests are offered to mothers more likely to be carrying babies with chromosomal or other problems. They are offered to clarify results of ultrasound scans. Amniocentesis and CVS both carry a small risk of miscarriage.

How do I access the First Trimester Ultrasound Screening Program?

Patients must be referred by an obstetrician or a general practitioner for this service between 11 to 13 weeks of pregnancy. To make an appointment, please telephone 07 3163 1896 between 8.30 am and 4 pm, Monday to Friday. Please bring your referral form with you to the appointment.

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