The 12+ week scan should be considered an early anatomy scan and may include NT assessment as part of combined screening for Down syndrome and other conditions (see NSU, Antenatal Screening for Down syndrome and other conditions).
The scan is optimally performed at 12–13+6 weeks’ gestation (see Antenatal Screening for Down Syndrome and Other Conditions: Guidelines for health practitioners [PDF, 2.6 MB], Ministry of Health 2013).
- Dating of pregnancy
- Early anatomy assessment
- Detection of multiple pregnancy (chorionicity and amnionicity)
- Screening for chromosomal anomalies and other conditions.
Section 88 codes: NT and NF (see Appendix 1).
Required clinical details
- Any symptoms
- Previous relevant maternal or family history
- History of previous caesarean section.
- TA scan is usually adequate. Consider TV assessment if there are technical limitations, such as maternal habitus or retroverted uterus.
- Uterus – anteverted, retroverted
- Dating – CRL, BPD
- Developing placental location
- Subjective evaluation of amniotic fluid
- Fetal anatomy (see below)
- NT for risk assessment if the woman accepts screening (Note: NT ≥3.5 mm may be an independent marker of cardiac or skeletal abnormality and Fetal Medicine or other local equivalent specialist review should be offered even if chromosome screening is not requested.).
Early fetal anatomy
Assessment of fetal anatomy is the major component of the 12+ week scan. The following routine fetal anatomy should be assessed as a minimum at the time of the NT scan.
- Skull and brain
- Four limbs (document two arms, two legs, two hands and two feet)
- Cord insertion
- Three-vessel cord
- Four-chamber (4Ch) heart (if possible).
Other structures that may be examined, if possible, include:
- posterior fossa
- three-vessel view / cardiac outflow tracts
- facial triangle.
- NT increases with gestational age and CRL – see graph (Nicolaides et al 2001).
- An increased NT is associated with an increased risk of chromosomal abnormality, most commonly trisomies 21, 18 and 13.
- Cardiac and other structural and genetic anomalies may also be associated with an increased NT.
- NT measurement and combined screening assessment is not recommended in women with a previous non-invasive prenatal screening (NIPS)* result, and the NT should not be reported. The exception is if the NT is ≥3.5 mm, as this is an independent reason for Fetal Medicine or other local equivalent specialist referral.
- Risk assessment is performed as part of combined screening, with first-trimester bloods. The report must be sent to the appropriate laboratory, which is either:
- LabPLUS at Auckland District Health Board (DHB), for Taupō and north of Taupō
- Canterbury Health Laboratories at Canterbury DHB (for south of Taupō).
- If the CRL is greater than the accepted range at NT scan, NT / combined screening can be replaced by second-trimester maternal serum screening (MSS2) or NIPS.*
* Note: NIPS is not part of the publicly funded antenatal screening for Down syndrome and other conditions. See Appendix 5: Non-invasive prenatal screening for more information.
Nuchal translucency assessment criteria
- Optimally performed at 12–13+6 weeks, or CRL ≥56 mm, and must be ≤84 mm
- Midline sagittal view
- Fetus magnified to 75 percent of screen, including the fetal head and thorax
- Fetal head in neutral position (ie, not flexed or extended)
- Ensure not measuring amnion (visualise fetus bouncing in real time)
- Measure maximal NT (calipers on-to-on, not including skin) – ideally demonstrate amnion separate to NT
- If nuchal cord is present, measure the NT both above and below the cord, and average the measurements
- Obtain at least three satisfactory images.
For more detail, see the NSU’s Antenatal Screening for Down Syndrome and Other Conditions: Guidelines for nuchal translucency (NT) and crown rump length (CRL) measurements (Ministry of Health 2015).