This examination has become extremely important in the early diagnosis of congenital anomalies and chromosomal disorders. Combined with 1st trimester biochemistry, it is the most sensitive, non-invasive risk assessment for aneuploidy currently available in Australia. The test should be preceded by appropriate counselling in all cases. All women are at risk of having an aneuploid fetus. Many international professional organisations suggest the 12 Week NT Assessment be offered to all pregnant women regardless of age due to the benefits of early aneuploid detection and reduction in unnecessary prenatal invasive testing.
The scan, when combined with 1st trimester biochemical screening (bHCG, PAPP-A) has a detection rate for aneuploidy with a sensitivity of 85-90% with a false positive rate of 5%. A local Accreditation Board has been established in Australia since 2002 for the training, monitoring, auditing and supervision of this examination. At this time, the software for risk assessment is only available to operators registered with the Fetal Medicine Foundation (FMF) Nuchal Translucency – Ultrasound and Monitoring Program administered by RANZCOG.
The 12 week scan is also useful for early diagnosis of missed abortion, multiple pregnancies, major structural congenital anomalies, uterine fibroids and ovarian pathology that may influence the course of pregnancy. It also enables visualization of early fetal anatomy.
Details of accreditation and performance of this scan are provided and updated on the Nuchal Translucency – Ultrasound and Monitoring Program web site. http://www.nuchaltrans.edu.au/guidelines.shtml (see Appendix 6- Guidelines for Measuring the Nuchal Translucency, updated on 15 May 2007)
The regulations and guidelines for continued performance of this study are provided at the Program web site: http://www.nuchaltrans.edu.au/regulations.shtml
Auditing and Continued Credentialling: http://www.nuchaltrans.edu.au/continuing_audit.shtml
For details on the RANZCOG policy on screening for Down Syndrome please see: http://www.ranzcog.edu.au/publications/collegestatements.shtml
C-Obs 4: Antenatal Screening for Down Syndrome and other Fetal Aneuploidy.
- Factors which may affect chromosomal risk and outcome should also be stated including:
- Clinical concern and reason for examination;
The patient’s LNMP if known, and date of 1st positive pregnancy test;
- Relevant history – maternal age, maternal conditions, parity, pregnancy loss, assisted conception including donor oocyte, family history of congenital abnormalities;
- Relevant clinical examination; and
- Multiple pregnancy – and if know chorionicity.
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Indications for Nuchal Translucency ScanThere are many potential clinical indications for a 12 week Nuchal Translucency scan. (Refer to Appendix 7 – Indications for Ultrasound 12-16 Weeks Gestation.)
The main aims are to assess:
- Risk assessment for aneuploidy;
- Gestational age;
- Number of fetal poles;
- Presence of fetal cardiac activity;
- Gross fetal anatomy; and
- Pelvic masses and uterine malformations.
The Ultrasound ExaminationThe following should be imaged in addition to those described in 3.1:
- Fetal biometry including CRL, BPD;
- Fetal nuchal translucency as per Appendix 6 – True sagittal plane, etc
- Anatomical survey including head, skull, midline brain, stomach, bladder, cord insertion, spine, both upper and lower limbs, hands and feet; and facial profile;
- Placental site; and
- The nasal bone and tricuspid regurgitation may become elements of the risk assessment in the future.
Risk CalculationWhen including biochemistry in the risk assessment, the operator should also check that MoM’s have been calculated for correct gestational age, maternal weight, smoking, and ethnicity. If the pregnancy is multiple, it should also be checked that MoM’s have also been calculated with this included in the formula.
The ReportThe report should include:
- Indication for scan from the referral;
- Gestational Age from the LNMP;
- Gestational Age by Ultrasound measurements. This is best calculated from the flexed CRL position and the BPD. There is potential for error in the CRL measurement if used alone for establishing dates due to the flexion and extension movements of the fetus.
- CRL, BPD and NT in mm
- Any other measurements taken;
- Fetal heart activity – presence or absence;
- Fetal Anatomy:
- List anatomy seen (see scan);
- Placental position in uterus. No comment is necessary on whether it is low lying or not, as this cannot be reliably determined at this gestation. Hydropic change suggestive of trophoblastic disease may be seen at this gestation and should be reported if present;
- Nuchal Translucency measurement in mm;
- Risk assessment for aneuploidy (T21 and T18) as calculated by risk assessment software. The referring doctor should be informed of high risk results in a timely manner to expedite management choice for the pregnant woman; and
- Uterine, cervical or adenexal pathology.