When to Order Imaging for Second and Third Trimester Screening for Fetal Anomaly: ACR Appropriateness Decoded
A patient arrives for their routine second-trimester visit. The pregnancy has been uncomplicated, but the time has come for the anatomic survey. You know an ultrasound is the standard of care, but which one? A standard anatomy scan, or a more detailed evaluation? What if the patient has risk factors, or if the initial scan reveals a subtle finding? Choosing the right initial and follow-up imaging is critical for accurate diagnosis, patient counseling, and perinatal planning. The American College of Radiology (ACR) Appropriateness Criteria provide an evidence-based framework to guide these decisions, ensuring the right study is ordered for the right clinical indication, balancing diagnostic yield with safety.
What Does ACR Second and Third Trimester Screening for Fetal Anomaly Cover?
This ACR guideline focuses on the use of imaging to detect and evaluate fetal structural anomalies during the second and third trimesters of pregnancy. The criteria address several distinct clinical situations, providing a roadmap for clinicians managing both low-risk and high-risk pregnancies.
The scope includes:
- Initial screening imaging for low-risk pregnancies, typically performed between 18 and 22 weeks of gestation.
- Initial screening imaging for high-risk pregnancies, where maternal or fetal factors increase the likelihood of an anomaly.
- Next-step imaging recommendations when an initial ultrasound identifies “soft markers,” which are nonspecific findings that can be associated with an increased risk of aneuploidy or other anomalies.
- Appropriate follow-up imaging when a major structural anomaly is suspected or confirmed on a prior ultrasound.
These guidelines do not cover first-trimester screening, such as nuchal translucency assessment, or non-imaging-based screening methods like maternal serum analyte testing or cell-free DNA analysis. The focus is strictly on the appropriate application of imaging modalities like ultrasound and magnetic resonance imaging (MRI) for fetal structural evaluation.
What Imaging Should I Order for Second and Third Trimester Screening for Fetal Anomaly? Recommendations by Clinical Scenario
The ACR panel provides clear, scenario-based recommendations to guide imaging selection. The choice of study depends heavily on the patient’s risk profile and the findings of any prior imaging.
For a low-risk pregnancy undergoing initial imaging, the standard of care is clear. The ACR rates a US pregnant uterus transabdominal anatomy scan as Usually appropriate. This comprehensive survey is designed to assess fetal number, viability, gestational age, and anatomy, as well as placental position and amniotic fluid volume. In this routine setting, more specialized studies like a detailed scan, fetal echocardiogram, or fetal MRI are considered Usually not appropriate as a first-line screening tool.
The approach changes for a high-risk pregnancy requiring initial imaging. In this context, a US pregnant uterus transabdominal detailed scan is rated Usually appropriate. This study involves a more in-depth evaluation of fetal anatomy by an experienced operator. A fetal echocardiogram May be appropriate, particularly if the maternal or fetal risk factors specifically point to an increased risk of congenital heart disease. Interestingly, a standard anatomy scan and a non-contrast fetal MRI are both rated May be appropriate (Disagreement), reflecting variability in practice and the need for case-by-case consideration.
When an initial ultrasound reveals an abnormal finding of soft markers, the next imaging study should be more focused. Both a US pregnant uterus transabdominal detailed scan and a US pregnant uterus transabdominal follow-up are considered Usually appropriate to further characterize the findings and look for associated anomalies. A fetal echocardiogram May be appropriate, as some soft markers are associated with cardiac defects.
Finally, if an ultrasound identifies major anomalies, a multi-modal approach is often warranted for the next imaging study. A US pregnant uterus transabdominal detailed scan and a US pregnant uterus transabdominal follow-up are Usually appropriate to confirm and fully delineate the extent of the anomaly. In this scenario, both a US echocardiography fetal and an MRI fetal without IV contrast are also rated Usually appropriate. Fetal echo is crucial for assessing associated cardiac defects, while MRI can provide superior characterization of complex brain, spine, or chest anomalies. MRI with IV contrast remains Usually not appropriate due to safety concerns.
ACR Imaging Recommendations Table
| Clinical Scenario | Top Procedure | ACR Rating | Adult RRL | Pediatric RRL |
|---|---|---|---|---|
| Second and third trimester screening for fetal anomaly. Low-risk pregnancy. Initial imaging. | US pregnant uterus transabdominal anatomy scan | Usually appropriate | O 0 mSv | O 0 mSv [ped] |
| Second and third trimester screening for fetal anomaly. High-risk pregnancy. Initial imaging. | US pregnant uterus transabdominal detailed scan | Usually appropriate | O 0 mSv | O 0 mSv [ped] |
| Second and third trimester screening for abnormal finding on ultrasound: soft markers. Next imaging study. | US pregnant uterus transabdominal detailed scan / follow-up | Usually appropriate | O 0 mSv | O 0 mSv [ped] |
| Second and third trimester screening for abnormal finding on ultrasound: major anomalies. Next imaging study. | US echocardiography fetal / US detailed scan / US follow-up / MRI fetal without IV contrast | Usually appropriate | O 0 mSv | O 0 mSv [ped] |
Adult vs. Pediatric Second and Third Trimester Screening for Fetal Anomaly Imaging: Radiation Dose Tradeoffs
In the context of fetal imaging, the “pediatric” patient is the fetus. The primary consideration is fetal safety, which is why imaging modalities that use ionizing radiation, such as CT scans or radiography, are not included in these guidelines for routine screening. The ACR recommendations exclusively feature ultrasound (US) and magnetic resonance imaging (MRI), both of which are non-ionizing and have excellent safety profiles in pregnancy.
The Relative Radiation Level (RRL) for all recommended studies is “O 0 mSv,” indicating no ionizing radiation is used. The “[ped]” notation in the pediatric RRL column explicitly refers to the dose received by the fetus. The principle of As Low As Reasonably Achievable (ALARA) is still relevant, particularly concerning ultrasound. While diagnostic ultrasound is considered safe, energy deposition in tissue (thermal and mechanical effects) should be minimized by using appropriate power levels and limiting scan time, especially with Doppler imaging. For MRI, the main safety consideration is the avoidance of gadolinium-based contrast agents, which can cross the placenta and have unknown long-term effects on the fetus. Therefore, MRI without IV contrast is the preferred study when indicated.
Imaging Protocol Details for Second and Third Trimester Screening for Fetal Anomaly
Once you’ve decided on the right study, the protocol matters. The diagnostic quality of an obstetric ultrasound or fetal MRI depends heavily on standardized techniques, proper machine settings, and comprehensive checklists. A standard anatomy scan, for example, requires specific views of the fetal brain, heart, abdomen, and limbs to be considered complete. A detailed scan or fetal MRI has even more rigorous protocol requirements. Our protocol guides cover technique, patient preparation, and reading principles for the studies recommended above to help ensure you acquire the necessary diagnostic information.
Tools to Help You Order the Right Study
Navigating imaging guidelines can be complex, but several tools can streamline the process of selecting the most appropriate study for your patient.
The Imaging Appropriateness Selector provides direct access to the full, searchable ACR guidelines. It’s an essential resource for finding evidence-based recommendations for hundreds of clinical scenarios beyond second and third trimester screening for fetal anomaly.
For detailed procedural steps, the Imaging Protocol Library offers a collection of standardized, scannable protocols for a wide range of ultrasound and MRI examinations. These guides help ensure that the ordered study is performed correctly and completely.
While the studies in this guideline do not use ionizing radiation, tracking cumulative dose is important for many patients. The Radiation Dose Calculator is a useful tool for estimating and communicating radiation exposure from other imaging studies a patient may have undergone.
What is the difference between a standard anatomy scan and a detailed scan?
A standard anatomy scan (CPT 76805) is a routine screening examination performed on all pregnancies, typically between 18-22 weeks, to assess fetal anatomy, growth, and the uterine environment. A detailed scan (CPT 76811) is a more targeted and comprehensive examination performed for specific indications, such as a high-risk pregnancy or a suspected anomaly on a prior scan. It often requires more advanced equipment and is performed by a provider with specific expertise in high-risk obstetric imaging.
When is fetal MRI the right choice?
Fetal MRI is not a screening tool. It is a problem-solving examination used to further evaluate a known or suspected anomaly found on ultrasound. It is rated Usually appropriate for evaluating major anomalies. It is particularly valuable for complex central nervous system anomalies (like ventriculomegaly or suspected cortical malformations), chest masses (like congenital diaphragmatic hernia), and certain complex genitourinary or gastrointestinal issues where ultrasound provides incomplete information.
Why is MRI with IV contrast usually not appropriate?
Gadolinium-based contrast agents can cross the placenta and enter the fetal circulation, after which they are excreted into the amniotic fluid and subsequently swallowed by the fetus. The long-term effects of gadolinium exposure on the developing fetus are not well understood. Due to these theoretical risks and a lack of clear benefit in most clinical scenarios, the ACR rates MRI with IV contrast as Usually not appropriate for fetal anomaly screening.
What are “soft markers” and how do they impact the imaging plan?
Soft markers are nonspecific ultrasound findings, such as an echogenic intracardiac focus, pyelectasis, or a thickened nuchal fold in the second trimester. While often transient and benign, they can be associated with a small increased risk of aneuploidy (e.g., Down syndrome) or other structural anomalies. When soft markers are detected, the ACR recommends a more detailed ultrasound examination (either a detailed scan or a follow-up scan) to look for any other associated findings and to ensure a comprehensive anatomic survey has been performed.
Is diagnostic ultrasound safe for the fetus?
Yes. Decades of clinical use and extensive research have shown no evidence of harm to the fetus from diagnostic ultrasound. It does not use ionizing radiation. Professional organizations like the American Institute of Ultrasound in Medicine (AIUM) and the ACR have concluded that the benefits of prudent use of diagnostic ultrasound in pregnancy far outweigh any theoretical risks. The principle of ALARA (As Low As Reasonably Achievable) is still applied, meaning scan times and power levels should be optimized for diagnostic purposes.
Frequently Asked Questions
What is the difference between a standard anatomy scan and a detailed scan?
A standard anatomy scan (CPT 76805) is a routine screening examination performed on all pregnancies, typically between 18-22 weeks, to assess fetal anatomy, growth, and the uterine environment. A detailed scan (CPT 76811) is a more targeted and comprehensive examination performed for specific indications, such as a high-risk pregnancy or a suspected anomaly on a prior scan. It often requires more advanced equipment and is performed by a provider with specific expertise in high-risk obstetric imaging.
When is fetal MRI the right choice?
Fetal MRI is not a screening tool. It is a problem-solving examination used to further evaluate a known or suspected anomaly found on ultrasound. It is rated Usually appropriate for evaluating major anomalies. It is particularly valuable for complex central nervous system anomalies (like ventriculomegaly or suspected cortical malformations), chest masses (like congenital diaphragmatic hernia), and certain complex genitourinary or gastrointestinal issues where ultrasound provides incomplete information.
Why is MRI with IV contrast usually not appropriate?
Gadolinium-based contrast agents can cross the placenta and enter the fetal circulation, after which they are excreted into the amniotic fluid and subsequently swallowed by the fetus. The long-term effects of gadolinium exposure on the developing fetus are not well understood. Due to these theoretical risks and a lack of clear benefit in most clinical scenarios, the ACR rates MRI with IV contrast as Usually not appropriate for fetal anomaly screening.
What are “soft markers” and how do they impact the imaging plan?
Soft markers are nonspecific ultrasound findings, such as an echogenic intracardiac focus, pyelectasis, or a thickened nuchal fold in the second trimester. While often transient and benign, they can be associated with a small increased risk of aneuploidy (e.g., Down syndrome) or other structural anomalies. When soft markers are detected, the ACR recommends a more detailed ultrasound examination (either a detailed scan or a follow-up scan) to look for any other associated findings and to ensure a comprehensive anatomic survey has been performed.
Is diagnostic ultrasound safe for the fetus?
Yes. Decades of clinical use and extensive research have shown no evidence of harm to the fetus from diagnostic ultrasound. It does not use ionizing radiation. Professional organizations like the American Institute of Ultrasound in Medicine (AIUM) and the ACR have concluded that the benefits of prudent use of diagnostic ultrasound in pregnancy far outweigh any theoretical risks. The principle of ALARA (As Low As Reasonably Achievable) is still applied, meaning scan times and power levels should be optimized for diagnostic purposes.
Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 26, 2026
