Obstetric and Gynecologic Imaging

What Imaging Is Best for Increased Nuchal Translucency in a Dichorionic Twin Pregnancy?

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What Imaging Is Best for Increased Nuchal Translucency in a Dichorionic Twin Pregnancy?

It’s late in the morning clinic, and you’re reviewing the first-trimester screening results for a 34-year-old patient with a dichorionic, diamniotic twin pregnancy at 12 weeks and 5 days of gestation. The nuchal translucency (NT) measurement for Twin A is within normal limits, but for Twin B, it is 3.8 mm, well above the 95th percentile. The immediate question is how to best evaluate this discordant finding to counsel the patient on potential risks and subsequent diagnostic options. This article provides a clinical workflow for this specific scenario, detailing why the American College of Radiology (ACR) recommends a targeted imaging approach. For this presentation, fetal echocardiography is rated as Usually Appropriate, directly addressing one of the most significant risks associated with an increased NT.

Who Fits This Clinical Scenario for Increased Twin Nuchal Translucency?

This guidance applies to a specific and increasingly common clinical situation: a patient with a confirmed dichorionic twin pregnancy between 11 weeks 0 days and 13 weeks 6 days of gestation, where at least one fetus demonstrates an increased nuchal translucency. An “increased” NT is typically defined as a measurement greater than 3.0 to 3.5 mm or above the 95th percentile for the specific crown-rump length (CRL). The chorionicity must be confirmed as dichorionic, meaning two separate placentas (which may be fused) and a thick inter-twin membrane (“lambda” or “twin peak” sign).

This workflow is distinct from several related scenarios and should not be applied to them:

  • Monochorionic Twins: If the pregnancy is monochorionic (a single shared placenta), the differential diagnosis and management pathway change significantly due to the risk of twin-to-twin transfusion syndrome (TTTS) and other complications related to shared circulation. This represents a different ACR variant.
  • Singleton Gestation: While the underlying concerns (aneuploidy, cardiac defects) are similar, the management of a singleton pregnancy with increased NT does not involve the complexities of a discordant twin, making it a separate clinical question.
  • Routine Screening: This guidance is for a pregnancy with an abnormal finding. It does not apply to the initial, routine NT screening performed for all pregnancies, which is covered under a different ACR scenario.

What Diagnoses Are You Working Up in This Scenario?

An increased nuchal translucency in one or both dichorionic twins is a nonspecific but powerful marker for several underlying conditions. The imaging and downstream workup are designed to differentiate among these possibilities.

The most pressing concern is chromosomal aneuploidy. Conditions like Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome) are strongly associated with increased NT. The risk of aneuploidy increases with the degree of NT thickening. In a dichorionic twin pregnancy, this finding can be discordant, affecting only one fetus.

Equally important, and the primary driver for the recommended imaging, is the risk of major congenital heart defects (CHD). Increased NT is one of the strongest early markers for structural cardiac anomalies, even in the absence of aneuploidy. The fluid accumulation may be related to early cardiac dysfunction or altered hemodynamics. This risk persists even if genetic testing returns normal results.

Less commonly, increased NT can be an early sign of other genetic syndromes not detectable on standard aneuploidy screening, such as Noonan syndrome or Smith-Lemli-Opitz syndrome. It can also be associated with a range of non-cardiac structural anomalies, including diaphragmatic hernia, skeletal dysplasias, or abdominal wall defects, which may become more apparent on a detailed anatomic survey later in gestation. In some cases, a thickened NT can be a transient finding with no identifiable underlying cause, ultimately leading to a healthy outcome, but this can only be concluded after a thorough evaluation.

Why Is Fetal Echocardiography the Recommended Study for This Presentation?

The ACR Appropriateness Criteria rate US echocardiography fetal as Usually Appropriate for evaluating increased nuchal translucency in dichorionic twins. This recommendation is driven by the high prevalence of congenital heart disease in this specific clinical context.

While a detailed transabdominal ultrasound of the pregnant uterus is also rated Usually Appropriate and is a necessary part of the overall evaluation, the fetal echocardiogram is a distinct, specialized examination. It provides a comprehensive assessment of cardiac anatomy, function, and blood flow patterns that is not achieved in a routine anatomic survey. Given that up to one-third of fetuses with increased NT and normal chromosomes may have a major anomaly, with cardiac defects being the most common, this targeted evaluation is critical. It can identify complex defects like tetralogy of Fallot, transposition of the great arteries, or hypoplastic left heart syndrome early in gestation, which is vital for parental counseling and delivery planning.

Other imaging modalities are rated lower for this initial workup:

  • US duplex Doppler pregnant uterus: This is rated May be appropriate. Doppler assessment is valuable for evaluating fetal growth, umbilical artery flow, and placental function, but it does not provide the detailed structural information needed to diagnose the cause of the increased NT. It is a complementary tool, not the primary diagnostic study for this indication.
  • US pregnant uterus transvaginal: Also rated May be appropriate, a transvaginal approach can sometimes provide higher-resolution images in the late first or early second trimester, especially with challenging maternal body habitus or a retroverted uterus. However, a comprehensive fetal echocardiogram typically requires a transabdominal approach later in gestation (around 18-22 weeks) for optimal views of the four-chamber heart and outflow tracts.

All recommended and alternative ultrasound modalities are non-ionizing and carry a radiation level of 0 mSv, making them safe for both the patient and the fetuses. When ordering, it is crucial to specify “fetal echocardiogram” to ensure the performing specialist allocates the appropriate time and resources for this detailed study.

What’s Next After Fetal Echocardiography? Downstream Workflow

The results of the fetal echocardiogram, combined with genetic testing, guide the subsequent management of the pregnancy. The workflow branches based on the findings.

If the fetal echocardiogram is positive for a congenital heart defect in one or both twins, the next step is a consultation with a maternal-fetal medicine (MFM) specialist and a pediatric cardiologist. This allows for detailed counseling regarding the specific cardiac lesion, prognosis, potential for in-utero progression, and the need for specialized neonatal care and cardiac surgery after birth. Delivery should be planned at a tertiary care center with a neonatal intensive care unit (NICU) and pediatric cardiac services.

If the fetal echocardiogram is negative (normal cardiac anatomy), the focus remains on the risk of aneuploidy and other structural anomalies. The patient should have already been offered genetic counseling and options for diagnostic testing, such as chorionic villus sampling (CVS) or amniocentesis, which can be performed on each twin separately in a dichorionic pregnancy. A detailed anatomic survey around 18-20 weeks is still essential to screen for non-cardiac anomalies.

If the fetal echocardiogram is indeterminate or limited due to fetal position or early gestational age, a repeat study is often recommended in 2 to 4 weeks. An indeterminate finding should not be interpreted as normal, and the patient should be managed as high-risk until a definitive diagnosis can be made. Continued surveillance with MFM is warranted.

Pitfalls to Avoid (and When to Get Help)

Navigating an increased NT in a twin pregnancy requires careful attention to detail. Here are several common pitfalls to avoid:

  • Assuming Normal Genetics: Do not assume a normal cell-free DNA (cfDNA) screen rules out all problems. cfDNA has a lower positive predictive value in twin pregnancies and does not screen for the microdeletion/duplication syndromes or single-gene disorders associated with increased NT.
  • Delaying the Fetal Echocardiogram: Waiting until the mid-trimester anatomic survey to evaluate the heart can delay the diagnosis of a critical CHD, limiting time for counseling and planning. The echo should be scheduled as soon as technically feasible.
  • Misinterpreting Chorionicity: The entire management pathway depends on accurate determination of chorionicity. If there is any ambiguity, referral to an MFM specialist for confirmation is essential.
  • False Reassurance from a “Normal” Anatomic Survey: A standard anatomic survey is not a substitute for a formal fetal echocardiogram. If the indication is an increased NT, the dedicated cardiac study is required.

If you encounter discordant and severe NT thickening, complex cardiac findings, or signs of fetal hydrops, immediate escalation to a maternal-fetal medicine specialist is the appropriate next step for co-management.

Related ACR Topics and Tools

This article covers one specific scenario in depth. For a broader view of all clinical variants and imaging options for first-trimester aneuploidy screening, please consult our comprehensive parent guide. The following GigHz resources can also support your clinical decision-making:

Frequently Asked Questions

Is a fetal echocardiogram necessary if the cell-free DNA (cfDNA) screening is normal for both twins?

Yes. A normal cfDNA screen significantly reduces the risk of common aneuploidies like Trisomy 21, but it does not evaluate the risk for structural anomalies. Increased nuchal translucency is a strong independent risk factor for congenital heart defects, even in chromosomally normal (euploid) fetuses. The fetal echocardiogram is essential to directly assess cardiac anatomy.

What is the optimal gestational age to perform the fetal echocardiogram after an increased NT is found?

While some cardiac structures can be seen earlier, a comprehensive fetal echocardiogram is typically most successful between 18 and 22 weeks of gestation. However, some specialized centers may perform an earlier, preliminary cardiac assessment between 14 and 16 weeks, followed by a more detailed study later. The timing should be coordinated with a maternal-fetal medicine specialist.

If only one twin has an increased NT, does the other twin still need a fetal echocardiogram?

Yes. While the primary concern is the twin with the abnormal measurement, it is best practice to perform a detailed evaluation, including fetal echocardiography, on both twins. This ensures a complete assessment, as dichorionic twins are not genetically identical and can have independent anomalies. It also establishes a baseline for the unaffected twin.

Does this guidance change if the dichorionic placentas are fused?

No. The presence of fused placentas does not change the fact that the pregnancy is dichorionic (originating from two separate zygotes with two separate placentas). The key diagnostic feature is the ‘twin peak’ or ‘lambda’ sign at the membrane insertion site. As long as chorionicity is confirmed as dichorionic, this workflow applies, regardless of placental location or fusion.

What if the increased NT resolves on a follow-up ultrasound? Is the fetal echocardiogram still needed?

Yes. A transient or resolved increased nuchal translucency does not eliminate the underlying associated risks. The initial abnormal measurement indicates a potential developmental issue that occurred during a critical period of organogenesis. The risk for congenital heart defects and other anomalies remains elevated, and a thorough evaluation with a fetal echocardiogram and detailed anatomic survey is still strongly recommended.

Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 29, 2026