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This is a routine screening examination, which should be offered to all women. For optimal visualisation, it is best performed after 19 weeks.
The purpose of the examination is to:
- assess fetal dates if there has been no earlier scan
- assess fetal biometry
- assess fetal anatomy and screen for structural abnormality
- establish placental location.
Section 88 codes: AN, AF, see Appendix 1.
Required clinical details
- First-trimester combined screening or NIPS result (see Appendix 5) if performed (Note: NIPS is not publicly funded.)
- EDD (and what the EDD is based on, eg, previous scan or LMP)
- Significant relevant obstetric history (maternal or family)
- Previous caesarean section.
Dating should be based on earlier scans, if available. The pregnancy should not be dated at the anatomy scan unless this is the first scan or there is no information available.
If there have been no earlier scans, second-trimester dating can be estimated by biometry, for example, BPD, head circumference (HC), femur length (FL), trans-cerebellar diameter, and is generally accurate to within 7 days (± 7 days).
The following should be assessed.
- Fetal number
- Fetal cardiac activity
- Ask about any previous caesarean section and document placental location in relation to the scar
- Location – anterior, posterior, fundal
- Transverse and longitudinal planes
- Distance from internal cervical os – measure in mm
- A full bladder can simulate a low-lying placenta – if in doubt, get the woman to empty her bladder
- Consider TV scan if not well visualised
- Less than 20 mm is considered low lying. Third-trimester follow-up is recommended.
- Placental cord insertion and its location
- For further information, see Placenta
- Amniotic fluid: subjective assessment
- Document maternal structures
- Uterine fibroids
- Fetal anatomy (as described below).
At a minimum, achieve the following.
Extended views, if achievable, are in italics and marked with *. Note: Failure to visualise these is not an isolated reason to recall for further imaging if the remaining anatomy is well visualised and normal.
- Nuchal fold measurement (measure in the plane of the cavum septum pellucidum (CSP), normal is ≤6 mm)
- Cerebral ventricles (measure lateral ventricle at atrium, normal ≤10 mm)
- Choroid plexus
- CSP and falx
- Cisterna magna (normal ≤10 mm)
- Sagittal corpus collosum*
- Sagittal vermis*.
- Orbits + lenses
- Nose/lips (coronal)
- Profile showing nasal bone and mandible
- Alveolar ridge*
- Uvula / soft palate*.
- Stomach (situs)
- Kidneys in two planes (transverse and longitudinal/coronal images (measure AP pelvis if renal dilatation is suspected)
- Diaphragm (right and left sides, and document organs in relation to the diaphragm)
- Anterior abdominal wall and cord insertion
- Situs, cardiac axis, position and size
- 4Ch heart and transverse view of the interventricular septum
- Outflow tracts: left/right ventricular outflow tract (LVOT and RVOT respectively)
- Three vessel and trachea (3VT) view / arrow view
- Assess fetal heart rate and rhythm (Note: Document M-mode if there is an abnormality of the heart rate or rhythm.)
- An axial sweep cine from stomach to outflow tracts is extremely helpful for offline review and when referring a suspected anomaly.
Include both colour and non-colour imaging on all heart views.
For more information, see the NZMFMN and ASUM guidelines on fetal heart assessment during the 18–20 week anatomy scan (PDF, 4.7 MB) (Necas and Bagnall 2014).
The following extended cardiac views should be considered in case of suspected anomaly, if the operator is experienced in cardiac assessment.
- Ductal arch and aortic arch*
- Superior/inferior vena cava* (SVC/IVC respectively)
- Pulmonary veins*
- Foramen ovale*
- Pulmonary arteries*
- Atrioventricular (AV) valves*
- Ductus venosus*
- Abdominal aorta and IVC* (for determination of situs).
If cardiac anomaly is suspected, prompt referral for fetal echocardiography is required.
For further information on the most common cardiac anomalies, see Cardiac anomalies.
Assess in three planes (sagittal, coronal and transverse, including skin line and sacrum). Two planes may be acceptable in suboptimal fetal position, but visualisation must be excellent.
- Cord insertion: both fetal and placental – see Appendix 6: Placental anomalies
- Three-vessel cord.
- Document all long bones
- Arms (upper arm and forearm)
- Hands observed open/parallel digits
- Fingers counted
- Legs (upper leg and lower leg)
The following sonographic findings previously referred to as soft markers are not significant and should not be reported if they are an isolated finding but should prompt careful review of the rest of the fetal anatomy.
- Choroid plexus cyst
- Echogenic cardiac focus
- Single umbilical artery
- Sandal gap toes
More than two soft markers should be discussed with the referrer for consideration of Fetal Medicine or local equivalent specialist review.
For more information, see the article Meta-analysis of second-trimester markers for trisomy 21 (Agathokleous et al 2013).
Markers that should be reported and may be indicative of an increased risk of chromosomal abnormality, requiring specialist review, include:
- increased nuchal fold thickness >6 mm (see Nuchal fold thickness below)
- absent or hypoplastic nasal bone
- ventriculomegaly >10 mm (see Fetal Ventriculomegaly [PDF, 388 KB], NZMFMN 2010)
- persisting clenched hand
- rocker bottom foot
- echogenic or thick-walled bowel (echogenicity greater than or equal to bone – see Fetal Echogenic Bowel [PDF, 287 KB], NZMFMN 2015c)
- pleural fluid or ascites
- pericardial fluid (>2 mm thickness) (see Fetal pericardial fluid below)
- perimembranous VSD (see Perimembranous VSD)
- aberrant right subclavian artery*.
Reporting guide and recommendations
Minimum reporting requirements
- Dating information by previous scan if available (see Second-trimester dating above)
- Fetal biometry
- Placenta: location and distance of the lower placental margin/marginal sinus from the internal cervical os
- Amniotic fluid: subjective assessment
- Fetal anatomy (with structures as per Ultrasound examination above)
- Maternal adnexa
- Documentation of uterine fibroids
- Cervical length, if appropriate (see Cervical length screening)
- Uterine artery Doppler, if appropriate.
See Anatomy scan reporting pro forma.
Where any fetal anomaly is suspected, the sonographer should inform the reporting radiologist, who must be available to review the images while the woman is in the scanning facility (Section 88 requirement).
For cases of confirmed or suspected fetal anomaly, the radiologist should contact the referring lead maternity carer (LMC) to discuss referral to Fetal Medicine or local equivalent specialist.
A copy of the report and images should be available for review at the appropriate DHB.
If, after two separate attempts, the anatomy scan remains incomplete, with required structures not visualised, then it should be reported as incomplete. If local services allow, a tertiary referral for completion may be available.
Fetal abnormalities that require specialist referral include:
- cardiac abnormality
- skeletal dysplasia (see Lethal Skeletal Dysplasia [PDF, 282 KB], NZMFMN 2015e)
- neural tube defect
- brain abnormalities
- cleft lip/palate (see Cleft Lip/Palate [PDF, 270 KB], NZMFMN 2011a)
- abdominal wall defects (see Gastroschisis [PDF, 277 KB], NZMFMN 2015d)
- congenital diaphragmatic hernia (see Congenital Diaphragmatic Hernia [PDF, 293 KB], NZMFMN 2015a)
- hydrops (see Nonimmune Hydrops Fetalis [PDF, 389 KB], NZMFMN 2012)
- abnormal limb position / akinesia
- genitourinary abnormalities.
Other findings that may indicate underlying abnormality and for which specialist referral is recommended include:
- persistent / absent small stomach (see Persistent small stomach below)
- renal dilatation – see Appendix 7: Fetal renal tract dilation charts
- umbilical vein varix >9 mm (see Umbilical Vein Varix [PDF, 283 KB], NZMFMN 2017).
Nuchal fold thickness
- When increased (>6 mm), this is a hard marker for chromosomal abnormality.
- It should only be assessed between 16 and 21+6 weeks gestational age.
- Take care to avoid ‘drop-out’ artefact caused by shadowing from the posterior calvarium (angle the probe so that the posterior fossa and nuchal fold are slightly anterior rather than in the transverse plane on the image).
- A Fetal Medicine (or local equivalent) specialist review should be recommended for nuchal fold thickness of >6 mm.
- Careful examination for further anomalies should be performed.
Fetal pericardial fluid
- Greater than 2 mm fluid around a significant proportion of the heart (generally not more than one isolated pocket) may reflect an increased risk of chromosomal abnormality.
- If isolated, <3 mm and with a normal detailed fetal echo, there is usually a normal outcome.
Measurement of the pericardial fluid should be made in diastole (with the AV valves open).
Mimics: the outer myocardium may be hypoechoic and may mimic pericardial fluid.
Extended ultrasound examination should only be performed by individuals with sufficient clinical expertise. Referral to Fetal Medicine should be recommended if appropriate.
Further evaluation may include:
- detailed fetal cardiac scan
- assessment for other features of aneuploidy
- assessment for other signs of hydrops (pleural effusions, ascites, subcutaneous oedema)
- assessment of the placenta for placentomegaly
- assessment for polyhydramnios
- middle cerebral artery (MCA) peak systolic velocity (PSV) to exclude fetal anaemia (such as maternal rhesus disease or parvovirus infection)
- features of fetal growth restriction (FGR)
- toxoplasmosis, other (syphilis, varicella-zoster, parvovirus B19), rubella, cytomegalovirus – CMV – and herpes infections (TORCH)
- high-output shunt lesions, for example, placental chorioangioma, vein of Galen aneurysm, tumours.
Persistent small stomach
- May indicate an increased risk of chromosomal abnormality.
- It can also be seen in non-chromosomal structural fetal abnormalities, such as, oesophageal atresia and trachea-oesophageal fistula.
- Extended ultrasound examination should be performed to look for other markers of chromosomal anomaly, to exclude a structural abnormality of the fetal face and brain and to assess amniotic fluid to exclude secondary polyhydramnios.
- A follow-up scan within a week should be considered. If this is a persistent finding, Fetal Medicine referral (or local equivalent) is required.
- May be a marker for aneuploidy if bilateral, in particular trisomy 18.
- May also be found in non-chromosomal structural fetal abnormalities, such as sacral agenesis, spina bifida, arthrogryposis and caudal regression syndrome.
- Fetal Medicine referral (or local equivalent) is required, even if isolated.
- Open cervix ± bulging membranes
- Fetal hydrops
- Major structural fetal anomaly
- Unexpected fetal demise
- Significant fetal anomaly
- Suspicion of chromosomal anomaly
- Fetal biometry <5th centile
- Significant oligohydramnios
- Short cervix <25 mm.