Level I - Early anomaly scan (NT-NB scan)

A Structured Approach to the 11–14-Week Ultrasound Scan

1. Initial Assessment

In early pregnancy, it is important to confirm viability, establish gestational age accurately, determine the number of fetuses and, in the presence of a multiple pregnancy, assess chorionicity and amnionicity.

Step	Details
Timing	Early anomaly scan should be performed when the Crown-Rump Length (CRL) is between 45 mm and 84 mm.
Viability	Defined by the presence of a fetal pole with cardiac activity (FHR).
Plurality	Determine the number of fetuses, establish chorionicity and amnionicity.

Safety:

• ALARA principle.

• B or M-mode ultrasound is considered safe during first trimester.

• However, if using Doppler, keep the Thermal Index (TI) ≤1.0 and exposure time minimal (usually no longer than 5–10 mins)

2. Fetal biometry

Accurate dating is performed via a CRL measurement.

Measurement	Criteria
Crown-Rump Length (CRL)	- Fetus must be in a True midsagittal plane. - Neutral position (not excessively flexed or extended). - The image should be magnified to fill most of the width of the ultrasound screen. - Calipers are placed from the outer tip of the head (crown) to the outer tip of the rump.
Biparietal Diameter (BPD) and Head Circumference (HC)	Measured in the largest symmetrical axial view of the head (including the midline falx and thalami).
Abdominal Circumference	- Transverse section of the fetal abdomen that is as circular as possible. - The plane must include the stomach bubble and the intra-abdominal portion of the umbilical vein where it joins the portal sinus. - The fetal kidneys must NOT be visible in this plane. - Calipers are placed on the outer borders of the body outline
Femur length	- Measured in the long-axis plane of the femur. - The calipers are placed at either end of the ossified diaphysis.

• The CRL is reduced in trisomy 18 and triploidy.

• Special attention is necessary if the CRL is smaller than anticipated from a previous ultrasound measurement.

3. Nuchal Translucency (NT)

The term NT describes the echolucent region seen at the back of the fetal neck during sonographic assessment.

Criterion	Requirement (Midsagittal Plane)
Image Plane	Perfect midsagittal view of the face and neck. The key markers for this plane are: the echogenic tip of the nose, the rectangular shape of the palate anteriorly, and the translucent diencephalon centrally.
Fetal Position	The fetus must be in a neutral position. Neck flexion leads to falsely decreased NT; hyperextension leads to falsely increased NT.
Magnification	The fetal head and thorax should occupy the entire screen.
Caliper Placement	Calipers are placed on the inner border of the lines defining the translucency. The crossbar of the caliper should merge with the white line of the border, not be placed in the nuchal fluid itself.
Amnion Check	Care must be taken to distinguish the fetal skin from the surrounding amnion.

• Increased NT is considered when it measures >95th percentile for a given CRL.

• The rate of aneuploidy is directly proportional to the value of nuchal translucency 9:

• <2 mm: risk of <1%.

• >3.5 mm: risk of >20%

4. Nasal Bone (NB)

A normal Nasal Bone appears as a brightly echogenic line situated under the skin line of the nose.  It helps to lower the calculated risk for Trisomy 21, even if the Nuchal Translucency (NT) is slightly increased.

• Absence or delayed ossification is strongly associated with an increased risk of chromosomal abnormalities, particularly Trisomy 21 (Down Syndrome).

• The inclusion of the Nasal Bone assessment significantly increases the sensitivity of first-trimester screening when combined with Nuchal Translucency (NT) and maternal serum biochemistry.

5. Ductus Venosus flow (DV)

Fetuses with aneuploidy are more prone to structural or functional cardiac defects between 11 and 14 weeks of gestation. Functional anomalies can involve abnormal flow in the ductus venosus and tricuspid regurgitation.

• An increase in ductus venosus pulsatility index for veins(PIV) is associated with an increased risk for common trisomies.

Ductus venosus is normally assessed in a right paramedial section.	DV identified by the area of aliasing - where it drains into the IVC.

6. Tricuspid Regurgitation (TR)

To assess flow through the tricuspid valve:

• Locate the four-chamber view in an axial section.

• Position the ultrasound transducer so the heart's apex is at 12 or 6 o'clock.

• Place a 2–4 mm gate across the anterior semilunar valve (tricuspid valve) to examine the waveform.

• Tricuspid regurgitation is characterized by a flow exceeding 60 cm/s for more than 50% of the cardiac cycle.

• Tricuspid flow is rarely abnormal in euploid fetuses and is linked to a high positive and negative likelihood ratio.

7. Detailed Anatomical survey (11-14 weeks scan)

A systematic examination of the fetal anatomy should include the following areas:

Anatomical Region	Detailed Structures to be Visualized
Overview	Fetus, Uterus, liquor volume and Placenta.
Head and Brain	- Calcification - Contour of the cranium - Interhemispheric falx separating two brain halves - Choroid plexuses almost filling the lateral ventricles (butterfly sign)
	Midsagittal View:  Intracranial Translucency (fourth ventricle) and Cisterna Magna.
Face and Neck	Face (Midsagittal View): Forehead, Nasal Bone, Maxilla, Upper Lip, and Mandible.
	Neck: Nuchal Translucency (NT) thickness
	Axial/Coronal View: Bilateral Orbits and the Retronasal Triangle.
Thorax	Shape of the thoracic wall. Diaphragmatic continuity - to ensure the stomach and liver are in the abdomen.
Heart	Four-Chamber View: Establish situs (position and axis, ≈30−60∘ to the left). Regular rhythm and size (≈1/3 of thoracic space).
	Doppler:  - Confirm two distinct ventricles. - Assess for Tricuspid Regurgitation or abnormal Ductus Venosus A-wave.
	Outflow Tracts: Visualize the Left Ventricular Outflow Tract and the Three-Vessel-and-Trachea View.
Abdomen	Stomach - normal position in left upper abdomen Bladder - visible and not dilated, longitudinal diameter <7 mm. Kidneys: Present bilaterally in normal position. Abdominal wall: Intact
Spine	Assess for the regular shape and continuity of the spine.
Extremities	Visualize the four limbs, each with three segments, and confirm free movement.

8. Further management

Combined Screening Approach is a dual-purpose assessment to assess the personalized risk for two major obstetric outcomes:

1. Aneuploidy (chromosomal abnormalities like Down Syndrome) and

2. Preterm Pre-eclampsia (PE).

The NT-NB scan is a component of the First-Trimester Combined Screening (FTCS) test, which includes:

1. Nuchal Translucency

2. PAPP-A

3. Free β-hCG

	Performed at	Components
Dual test	10-14 weeks	PAPP-A Free β-hCG
Quadruple test	15 - 20 weeks	AFP hCG Maternal E3-estrogen Inhibin A

9. Screening tests for Aneuploidy

1. Dual Test (First-Trimester Biochemistry)

The term "Dual Test" refers to the two blood markers measured in the first trimester.

• Timing: 10 to 14 weeks.

• Markers:

  • Free β-hCG (Human Chorionic Gonadotropin): Typically elevated in Trisomy 21.

  • PAPP-A (Pregnancy-Associated Plasma Protein-A): Typically reduced in Trisomy 21.

    
    

• The FTCS (NT + Dual Test) is the most effective screening method in the first trimester, with detection rates for Down Syndrome exceeding 90%.

2. NIPT Test (Non Invasive Prenatal test)

In recent years, screening by cfDNA has demonstrated excellent performance for common aneuploidies.

Feature	Description
Basis	Cell-Free DNA (cfDNA):  It refers to the fragmented DNA in the mother's blood, which is a mixture of her own DNA and fetal DNA shed by the placenta.
Timing	Can be performed as early as >10 weeks.
Primary Screened Conditions	Aneuploidies: Screens for the most common abnormalities, including: Trisomy 21 (Down Syndrome), Trisomy 18 (Edwards Syndrome), Trisomy 13 (Patau Syndrome), and Sex Chromosome Aneuploidies (e.g., Monosomy X).
Accuracy	Extremely high sensitivity and specificity, ≥99% for Trisomy 21.

10. Screening tests for Pre Eclampsia

1. Uterine artery doppler

• 11-14 Weeks Gestation: 

  • Normal mean PI: in the range of 1.4 to 1.9 .

  • A value above the 95th percentile (approximately PI >2.0 ) is considered high or abnormal.

    
    

• 20-24 Weeks Gestation: The mean PI should be significantly lower, often less than 1.0 - 1.2

2. Maternal biochemical markers

• PIGF:

  • PlGF is a pro-angiogenic marker that plays a key role in the development and health of the placenta's blood supply.

  • In Preeclampsia: Levels of PlGF are typically significantly lower.

  • Significance: This reduction is believed to be a consequence of impaired placental development and poor blood flow, a primary cause of preeclampsia.

    
    

• PAPP-A

  • PAPP-A is a protein produced by the placenta and is thought to be involved in placental growth and the regulation of insulin-like growth factors.

  • In Preeclampsia: Levels are reduced.