Obstetric and Gynecologic Imaging

What Is the Best Initial Imaging for High-Risk Fetal Growth Restriction?

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What Is the Best Initial Imaging for High-Risk Fetal Growth Restriction?

A 32-year-old G2P1 patient with chronic hypertension presents at 30 weeks gestation for a routine visit. Her fundal height measurement is lagging, measuring 27 cm, and has not increased appropriately since her last visit two weeks ago. Given her maternal risk factor and the concerning clinical finding, you suspect a potential growth disturbance and need to initiate an evaluation for fetal growth restriction (FGR). The central question is which imaging study provides the most direct and actionable information to assess fetal well-being and placental function. According to the American College of Radiology (ACR) Appropriateness Criteria, the initial evaluation in this high-risk scenario calls for specific ultrasound techniques, with US duplex Doppler of the fetal umbilical artery rated as Usually Appropriate.

Who Fits This Clinical Scenario?

This guidance applies to pregnant patients undergoing an initial evaluation for a growth disturbance where there is a high pre-test probability for fetal growth restriction (FGR). “High risk” is defined by the presence of maternal, fetal, or placental risk factors known to impair fetal growth. These include, but are not limited to:

  • Maternal Conditions: Chronic hypertension, pregestational diabetes, autoimmune diseases (e.g., lupus, antiphospholipid syndrome), severe renal or cardiac disease, or a history of a prior pregnancy affected by FGR.
  • Fetal Conditions: Known or suspected chromosomal abnormalities, congenital anomalies, or multiple gestations.
  • Placental/Uterine Factors: Known placental abnormalities or uterine anomalies.
  • Clinical Findings: A fundal height that is significantly smaller than expected for gestational age (typically >3 cm discrepancy) or static growth on serial measurements.

This workflow is distinct from the evaluation of a patient with low-risk factors, such as an otherwise healthy patient with a single soft marker or a borderline fundal height measurement. It also differs from the follow-up management of a patient with already diagnosed and established FGR, which involves a different cadence and set of imaging priorities focused on surveillance and delivery timing.

What Diagnoses Are You Working Up in This Scenario?

When ordering imaging for suspected FGR in a high-risk patient, you are primarily investigating the underlying cause of the poor growth. The differential diagnosis guides the interpretation of the imaging findings and subsequent management.

Placental Insufficiency
This is the most common cause of pathologic FGR. It refers to the placenta’s inability to deliver an adequate supply of oxygen and nutrients to the fetus, leading to impaired growth. This is often the primary concern in patients with maternal vascular conditions like chronic hypertension or preeclampsia. Imaging, particularly Doppler velocimetry, is crucial for directly assessing placental function.

Constitutional Smallness
Some fetuses are simply genetically destined to be small and are otherwise healthy, a condition known as small for gestational age (SGA) without FGR. These fetuses follow a consistent, albeit lower, growth trajectory and have normal placental function. Differentiating this benign state from pathologic FGR is a primary goal of the initial imaging workup to avoid unnecessary intervention.

Fetal Chromosomal or Genetic Syndromes
Underlying genetic conditions, such as trisomy 13, 18, or 21, can manifest as early and severe growth restriction. While ultrasound may reveal associated structural anomalies, the growth pattern itself is a key indicator that may prompt further diagnostic testing like amniocentesis.

Intrauterine Infection
Though less common, congenital infections (e.g., cytomegalovirus, toxoplasmosis) can directly impact fetal development and lead to growth restriction. Ultrasound findings might include associated signs like intracranial calcifications or ventriculomegaly, but FGR can be the initial presenting sign.

Why Is US Duplex Doppler of the Fetal Umbilical Artery the Recommended Study?

For the initial evaluation of a high-risk patient with suspected FGR, the ACR designates several ultrasound modalities as Usually Appropriate, but the umbilical artery (UA) Doppler provides the most direct physiologic information about the cause of the growth disturbance.

The core of the evaluation includes a standard transabdominal ultrasound for fetal biometry to calculate an estimated fetal weight (EFW) and determine if the fetus is small for gestational age (typically EFW <10th percentile). A biophysical profile (BPP) is also Usually Appropriate to provide a snapshot of acute fetal well-being. However, the US duplex Doppler of the fetal umbilical artery is the key study for differentiating pathologic FGR from constitutional smallness.

This study measures the resistance to blood flow in the feto-placental circulation. In a healthy pregnancy, the placenta is a low-resistance system, allowing for robust blood flow to the fetus, especially during diastole. In cases of placental insufficiency, progressive obliteration of small placental vessels causes resistance to rise. This is reflected in the UA Doppler waveform as decreased, then absent, and finally reversed end-diastolic velocity (AREDV). These findings are highly specific for placental dysfunction and are strongly associated with adverse perinatal outcomes.

Why are other studies rated lower for this initial step?

  • US duplex Doppler of the maternal uterine artery is rated as May be appropriate. While abnormal uterine artery waveforms can predict the risk of developing FGR or preeclampsia earlier in pregnancy (e.g., at 20-24 weeks), they have less diagnostic utility when a growth problem has already been identified in the third trimester. At this stage, fetal Doppler is a more direct measure of the current impact on the fetus.
  • US duplex Doppler of the fetal middle cerebral artery (MCA) is rated as Usually not appropriate for the initial evaluation. MCA Doppler is used to detect the “brain-sparing” phenomenon, a fetal adaptation to chronic hypoxia where blood flow is redistributed to the brain. While a critical finding, it represents a more advanced stage of fetal compromise and is typically reserved for the surveillance and management of established FGR, not the initial workup.

All recommended and alternative studies are ultrasound-based and carry no risk of ionizing radiation (0 mSv).

What’s Next After the Initial Ultrasound? Downstream Workflow

The results of the initial comprehensive ultrasound, including biometry and umbilical artery Doppler, will guide the subsequent clinical pathway.

If the study shows a small fetus (EFW <10th percentile) with abnormal umbilical artery Doppler findings (e.g., elevated S/D ratio, absent or reversed end-diastolic flow):

  • This confirms a diagnosis of FGR secondary to placental insufficiency.
  • The patient’s management plan will shift to intensive surveillance. This typically involves frequent (e.g., once or twice weekly) biophysical profiles and/or repeat Doppler studies.
  • This patient now fits the “Established fetal growth restriction. Follow-up evaluation” clinical scenario. Consultation with a maternal-fetal medicine specialist is warranted, and planning for the optimal timing of delivery becomes the primary focus.

If the study shows a small fetus but with normal umbilical artery Doppler findings:

  • This finding is more consistent with a constitutionally small fetus or a very early stage of FGR.
  • The next step is serial growth assessment. A follow-up ultrasound for fetal biometry is typically scheduled in 2-4 weeks to assess the growth interval. A fetus that continues to grow along its established curve is reassuring.
  • If other structural anomalies are present, or if growth is profoundly slow, further workup for genetic causes or congenital infection may be considered.

If the study is normal (appropriate fetal size and normal Dopplers):

  • This is reassuring. The lagging fundal height may have been due to measurement variability, fetal position, or other benign factors.
  • Routine prenatal care can typically be resumed, though a follow-up growth scan in several weeks may be considered to confirm continued normal growth.

Pitfalls to Avoid (and When to Get Help)

In evaluating a patient for high-risk FGR, several common pitfalls can complicate diagnosis and management:

  • Inaccurate Gestational Dating: The entire premise of FGR relies on accurate dating. Relying on an uncertain last menstrual period without a confirmed early ultrasound can lead to misclassification of a fetus as small.
  • Over-reliance on a Single Biometric Parameter: Using only one measurement, like abdominal circumference, can be misleading. A comprehensive EFW calculation using multiple parameters (biparietal diameter, head circumference, abdominal circumference, femur length) is standard.
  • Ignoring Doppler Velocimetry: Diagnosing FGR based solely on an EFW <10th percentile without performing umbilical artery Doppler misses the critical opportunity to assess placental function and risk-stratify the fetus.
  • Delaying Escalation: The finding of absent or reversed end-diastolic flow in the umbilical artery is an ominous sign indicating severe placental dysfunction and high risk of fetal demise. This requires immediate consultation with a maternal-fetal medicine specialist and likely inpatient admission for intensive monitoring.

Related ACR Topics and Tools

For a comprehensive overview of all clinical scenarios related to fetal growth disturbances, and for tools to assist in ordering the correct imaging studies, the following resources are available:

Frequently Asked Questions

What is the clinical difference between ‘high risk’ and ‘low risk’ for FGR?

A ‘high-risk’ patient has pre-existing maternal conditions (like chronic hypertension or prior FGR), known fetal anomalies, or significant clinical findings (like a major fundal height lag) that substantially increase the probability of pathologic growth restriction. A ‘low-risk’ patient is generally healthy with no major risk factors, where the concern might arise from a borderline measurement or an isolated soft marker on a prior scan.

Why is umbilical artery Doppler preferred over uterine artery Doppler for this initial evaluation?

Uterine artery Doppler assesses blood flow from the mother to the placenta and is primarily a screening tool used earlier in pregnancy (around 20-24 weeks) to predict future risk. Umbilical artery Doppler assesses blood flow from the placenta to the fetus, providing a direct, real-time measure of placental function and its current impact on the fetus. When FGR is already suspected in the late second or third trimester, the umbilical artery Doppler is the more relevant diagnostic test.

If the umbilical artery Doppler is normal, can I completely rule out FGR?

Not entirely. A normal umbilical artery Doppler in a small fetus (EFW <10th percentile) makes severe, early-onset placental insufficiency unlikely and is very reassuring. However, it could represent a constitutionally small but healthy fetus, or a milder or later-onset form of FGR. The key next step is a follow-up growth scan in 2-4 weeks to assess the growth velocity over time.

How often should imaging be repeated if FGR is confirmed?

Once FGR is diagnosed, especially with abnormal Doppler findings, surveillance frequency increases significantly. The exact interval depends on the severity of the findings and gestational age. It can range from twice-weekly biophysical profiles and weekly Dopplers for severe cases to bi-weekly growth scans for milder forms. This management falls under the ‘Established FGR, Follow-up’ scenario and should be guided by a maternal-fetal medicine specialist.

Does a biophysical profile (BPP) replace the need for Doppler studies in this scenario?

No, they serve different purposes. The BPP is a test of acute fetal well-being, assessing parameters like fetal movement, tone, breathing, and amniotic fluid volume. It tells you how the fetus is doing *right now*. Doppler studies assess the underlying pathophysiology of placental function, which is a chronic process. A fetus can have a normal BPP for a period even with worsening Doppler indices. Both are complementary and essential for managing high-risk FGR.

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