Nuchal Translucency / Early Anatomy / Pre-eclampsia Screening

Between 11 weeks and 14 weeks gestation, many of the ultrasound screening tests can be performed. QUFW recommends for this assessment to be done in the 13th week of pregnancy.

The routine combined first trimester screening (CFTS) test can be evaluated for the likelihood of chromosome abnormalities such as Trisomy 21, 18 and 13. It incorporates the ultrasound findings of the Nuchal Translucency (NT and the presence of the baby’s nasal bone), with blood tests of hormone markers, free bhCG and PAPP-A results to create a risk assessment.

At this stage of pregnancy, an early anatomy scan will also be performed. We can detect major structural anomalies which can impact on the management of your pregnancy. The first trimester early anatomy assessment is routinely performed during the CFTS or as its own examination if a NIPT has been performed previously.

As a NIPT has a higher sensitivity for detection of chromosomal abnormality, the Downs syndrome screening risk assessment from the Nuchal Translucency assessment is generally not recommended to be provided if a prior low probability NIPT result has been provided. 

Early onset preeclampsia is an additional screening test that QUFW performs at the Nuchal Translucency/First Trimester Anatomy scan. Hypertension or high blood pressure can develop into a serious medical condition for both the mother and the fetus that may require medication and closer monitoring.

Uterine artery Doppler trace for pre-eclampsia screening assessment

Combined First Trimester Screening (CFTS) Or Nuchal Translucency Scan (13 weeks – 13 weeks 6 days)

The nuchal translucency (NT) screening test is a non-invasive screening test to determine how likely your unborn baby will have a chromosomal abnormality, such as Down syndrome. The test combines the mother’s age related risk, a blood test determining the levels of hormones secreted by the placenta (free βhCG & PAPP-A), the length of the baby from head to bottom (crown to rump length or CRL) and the nuchal translucency measured on ultrasound. 

The CRL must be between 45 and 84 mm for the testing to be performed, that is, about 11 to 14 weeks pregnant. The nuchal translucency is a measurement of fluid behind the baby’s neck that is present in all babies during the first trimester of pregnancy. An increased NT measurement has an association with chromosome abnormalities but can also be associated with structural abnormalities e.g. heart. The fetus can have normal chromosomes even with an increased NT. The combined first trimester screening test (age + nuchal translucency + bloods) has a 90% accuracy for predicting your risk for Down Syndrome and some other chromosomal abnormalities.

The preferred timing for visualisation of the NT, NB and other anatomical structures is between 12 – 13 weeks 6 days. At QUFW, we prefer bookings to be between 13 weeks and 13 weeks 6 days. A week can make a difference in the visibility of anatomical structures.

First Trimester/Early anatomy ultrasound (13 weeks – 13 weeks 6 days)

At this stage of pregnancy, we can often detect major structural anomalies which can impact on the management of your pregnancy. The first trimester early anatomy assessment is routinely performed during the CFTS or as its own examination if a NIPT has been performed previously.

The following are some of the fetal structures assessed by our sonographers and doctors during a first trimester early anatomy ultrasound. These are assessed in keeping with international guidelines and with what we would expect to see structurally and embryologically at this gestation

• Cranial bones
• Fetal brain
• Nuchal translucency thickness
• Face/orbits/midline facial structures
• Nasal bone
• Fetal profile and mandible
• Chest including diaphragm
• Cardiac activity and early heart structures
• Abdomen including stomach, bladder, kidneys, abdominal cord insertionand 3 vessel cord 
• Spine and intact overlying skin
• Extremities – upper limbs/lower limbs including hands and feet

ISUOG Practice Guidelines: performance of firsttrimester fetal ultrasound scan

Chromosomal Abnormalities

Each of our cells has 46 chromosomes; 23 chromosomes come from the sperm and 23 from the egg. The most common chromosomal abnormality is Down syndrome (Trisomy 21) which occurs when a baby receives an extra or third chromosome 21. Some other less common chromosomal abnormalities are Trisomy 18 and 13, which also involve an extra chromosome 18 or 13 respectively. None of these conditions can be detected on ultrasound alone and it is important to know that the nuchal scan and NIPT are screening tests. The diagnostic test for chromosomal abnormalities is a chorionic villus sampling (CVS) or amniocentesis.

Preparation

It is recommended to empty your bladder 1 hour prior to your scan, then drink 2 glasses of water and then do not empty your bladder. You are welcome to bring your family members or a close friend to your scan. Children are welcome but their attention span may be limited for the duration of the ultrasound, so it is helpful if another adult is available to help look after them.

If you have decided to have the Combined First Trimester Screening or Nuchal Translucency assessment and have not had a NIPT, you are required to have a blood test (βhCG & PAPP-A) at least 3 working days prior to your scan, so that these results will be accessible for the risk assessment. Your Doctor will need to refer you for this test. 

Please bring with you:

• A referral from your Doctor
• Previous scans such as early pregnancy dating scans (if performed elsewhere)
• Information on any hereditary conditions

What to expect

Allow 1-1.5 hours for your appointment. The examination is performed by one of our highly trained obstetric Sonographers using state of the art ultrasound equipment. The scan includes an examination of the baby’s early anatomy, however not all structural abnormalities can be determined at this stage. We will also measure the length of the baby (crown rump length) to confirm the due date. Other anatomy that will be looked at include your ovaries, uterus and your cervix.

Transvaginal Scanning

Occasionally, a transvaginal scan is required to obtain the images needed to complete the examination. Transvaginal ultrasounds are performed with discretion at QUFW and are perfectly safe during pregnancy.

3D/4D Scanning

The majority of the scan is completed in normal 2D (greyscale) as this offers the best resolution for detecting abnormalities. We will also try to obtain some nice pictures in 3D/4D of your baby, but this is very dependent on the baby’s position, the amount of amniotic fluid and maternal issues like placental position, abdominal scarring and maternal habitus.

Results

A highly trained Sonographer or Doctor will discuss the results with you. In relation to the Combined First Trimester Screening ultrasound, the computer program that we use to calculate the risk, a risk of less than 1:300 implies that the baby has a low probability of having a chromosomal abnormality such as Down syndrome. If the risk calculated is greater than 1:300 (e.g. 1:50) it is implied that there is an increased probability that the baby has a chromosomal abnormality and further testing options (CVS/ amniocentesis) will be discussed with you. 

1 in 300 is a cut-off set by the scientists who have formulated the program at the level of about 5% of the pregnant population and approximately the risk of loss after amniocentesis. So it is a fair objective number to compare the result with. However the true assessor of risk is you, the patient, and the decision to proceed to further testing is one that can be discussed with your referring Doctor. You will receive a copy of the report and a copy will also be sent to your referring Doctor.