Aneuploidy Screening from 11 to 14 Weeks

1.  Open Spina Bifida
2.  Neural tube defects

• High-risk screening → Recommend NIPT or CVS/Amniocentesis.
• NT over 3.5mm, absent NB, reversed DV A-wave → Immediate referral needed.
• First-trimester suspicion of CHD → Arrange fetal echocardiography at 18-22 weeks.

PART 5: Advanced First-Trimester Screening – Practical Cases and Learning Opportunities

Having grasped the essential markers, let’s explore actual scenarios and their applications to ensure you can effectively implement this knowledge in your practice.

CASE 1: Elevated NT with Normal Other Markers – What’s the Next Step?

Situation:

At 12 weeks of gestation, NT measures 3.8mm (Above the 99th percentile

Nasal bone is visible

Ductus Venosus Doppler shows a normal A-wave

Tricuspid Valve Doppler indicates no regurgitation

No structural abnormalities detected
Your Plan of Action:

Step 1: Evaluate Risk Based on NT Measurement 

 An NT greater than 3.5mm indicates a high risk for aneuploidy and congenital heart disease (CHD), but with other markers being normal, the risk is mitigated. 

 ✅ Step 2: Discuss Follow-Up Options with the Patient:

 NIPT (the most reliable non-invasive test) Early fetal echocardiography at 14-16 weeks (If NT remains elevated, assess for CHD). 

 ✅ Step 3: Keep a Close Watch on the Pregnancy 

 Even if fetal karyotype and anatomy scan are normal, the risk of CHD persists at 2-5% → A detailed fetal echocardiography is warranted between 18-22 weeks.

✔ Key Takeaway: An elevated NT does not necessarily indicate aneuploidy, but thorough follow-up is crucial for detecting potential heart defects.

📌 CASE 2: Missing Nasal Bone – Is It Trisomy 21 or a Normal Variation?

📝 Situation:

NT is 1.8mm (Normal range)

Nasal Bone absent at 12 weeks

Ductus Venosus Doppler shows a normal A-wave

Tricuspid Valve Doppler indicates no regurgitation

🚀 Your Plan of Action:

✅ Step 1: Re-evaluate the Image Quality

Ensure the mid-sagittal view is accurate and the transducer is aligned parallel to the nose (to avoid misinterpretation).

✅ Step 2: Consider Ethnic Background

In certain African and Asian populations, nasal bone ossification may develop later, possibly resulting in false positives.

✅ Step 3: Provide Additional Testing Options

If risk factors are present (such as maternal age or previous T-21 pregnancy) → Suggest NIPT.

If risk factors are low, schedule a follow-up scan at 14-16 weeks to reassess the nasal bone.

✔ Key Takeaway:

The absence of a nasal bone is a significant marker for Trisomy 21 but can vary by ethnicity → Always confirm with additional markers before drawing conclusions.

CASE 3: Reversed A-Wave in DV – Does This Indicate High Risk?

📝 Situation:

At 11+5 weeks, NT is measured at 2.1mm (Within normal limits)

Nasal bone is present

Ductus Venosus Doppler shows a reversed A-wave

Tricuspid Valve Doppler indicates normal function

🚀 Your Plan of Action:

✅ Step 1: Verify the Doppler Image Quality

Use a small sample gate (0.5-1mm) to prevent contamination from the hepatic vein.

✅ Step 2: Analyze Overall Risk

A reversed A-wave alone increases the risk for aneuploidy and CHD by 2-3 times, but other markers are normal.

✅ Step 3: Suggest Further Testing

First option: NIPT

If parents desire definitive results: CVS (Chorionic Villus Sampling).

✔ Key Takeaway:

A reversed A-wave in DV is a notable risk factor for aneuploidy and CHD but should be evaluated alongside other indicators.

📌 CASE 4: Severe TR + Thick NT – Are We Looking at CHD or Trisomy 21?

📝 Situation:

At 12+2 weeks, NT measures a thick 4.8mm

Nasal bone is visible

Ductus Venosus shows a reversed A-wave

Severe Tricuspid Regurgitation >80 cm/s

Four-chamber heart view reveals right atrial enlargement

🚀 Your Plan of Action:

✅ Step 1: Identify High-Risk Indicators

Severe TR combined with a reversed DV A-wave and thick NT strongly suggests CHD (such as AVSD or HLHS).

✅ Step 2: Refer for Early Echocardiography (14-16 Weeks)

Fetal heart defects may be found in 30-40% of these instances!

✅ Step 3: Explore Genetic Testing Options

High likelihood of Trisomy 21 (Down Syndrome)

Offer NIPT or CVS.

✔ Key Takeaway:

Severe TR is more indicative of congenital heart disease than aneuploidy alone.

Final Cheat Sheet for Clinical Decision Making: