Obstetric and Gynecologic Imaging

What Imaging Is Best for Dichorionic Twin Growth Surveillance? ACR-Guided Workflow

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What Imaging Is Best for Dichorionic Twin Growth Surveillance? ACR-Guided Workflow

A 32-year-old G1P0 patient with a confirmed dichorionic-diamniotic twin pregnancy presents for her 28-week prenatal visit. Her fundal height measurement is slightly larger than expected, but she is otherwise asymptomatic. The immediate clinical question is how to best assess the growth and well-being of both fetuses, ensuring that any potential growth discordance or restriction is identified early. This article provides a detailed clinical workflow for this specific scenario: antepartum growth and surveillance in dichorionic or multichorionic multiple gestations. According to the American College of Radiology (ACR) Appropriateness Criteria, a transabdominal ultrasound of the pregnant uterus is the cornerstone of this evaluation and is rated Usually Appropriate.

Who Fits This Clinical Scenario for Dichorionic Twin Surveillance?

This guidance is specifically for clinicians managing a patient with a confirmed dichorionic twin pregnancy or a multichorionic higher-order multiple gestation (e.g., trichorionic triplets) during the second or third trimester for routine growth assessment and antepartum surveillance.

The key inclusion criteria are:

  • Established Dichorionicity: Chorionicity must have been definitively established, typically during a first-trimester ultrasound.
  • Second or Third Trimester: The pregnancy has progressed beyond the initial anatomy scan.
  • Routine Surveillance: The indication is for scheduled monitoring of fetal growth and well-being in an otherwise uncomplicated multiple gestation.

It is critical to distinguish this scenario from others that require a different diagnostic approach. This workflow does not apply if:

  • The pregnancy is monochorionic. Monochorionic twins have a unique set of risks, primarily Twin-to-Twin Transfusion Syndrome (TTTS), and follow a more intensive surveillance protocol detailed in a separate ACR variant.
  • A fetal abnormality is already known or strongly suspected. The presence of a known anomaly shifts the clinical question from routine surveillance to diagnostic characterization, which may involve different imaging modalities.
  • The assessment is in the first trimester. Initial dating, determination of viability, and establishing chorionicity are covered under a different clinical scenario.

What Diagnoses Are You Working Up During Growth Surveillance?

In this context, imaging is not used to work up a new maternal symptom but to proactively screen for conditions inherent to multiple gestations. The goal is to identify deviations from normal development that may require intervention.

Fetal Growth Restriction (FGR): This is the primary concern. FGR occurs when a fetus fails to achieve its genetic growth potential, typically defined as an estimated fetal weight (EFW) below the 10th percentile. In multiple gestations, this can affect one or both fetuses. The risk of FGR is substantially higher in twin pregnancies compared to singletons due to placental limitations and increased metabolic demands.

Discordant Fetal Growth: This refers to a significant size discrepancy between the fetuses, often defined as a difference in EFW of more than 20-25%. While some size variation is normal, significant discordance can be a marker of underlying pathology, such as unequal placental sharing, even if both twins technically remain above the FGR threshold. It is an independent risk factor for adverse perinatal outcomes.

Amniotic Fluid Abnormalities: Ultrasound allows for the assessment of amniotic fluid volume in each sac. Both oligohydramnios (too little fluid) and polyhydramnios (too much fluid) can be signs of underlying issues. Oligohydramnios, in particular, can be associated with placental insufficiency and FGR, while polyhydramnios is less common in dichorionic pregnancies but can be linked to fetal anomalies or gestational diabetes.

Why Is Transabdominal Ultrasound the Recommended Study for Dichorionic Twin Surveillance?

The ACR rates `US pregnant uterus transabdominal` as Usually Appropriate for this scenario because it is a safe, effective, and readily available tool that directly answers the key clinical questions without exposing the mother or fetuses to ionizing radiation (0 mSv).

The standard transabdominal ultrasound for growth surveillance provides several critical data points:

  • Fetal Biometry: Measurements of the head circumference (HC), biparietal diameter (BPD), abdominal circumference (AC), and femur length (FL) for each fetus. These are used to calculate an Estimated Fetal Weight (EFW) and plot growth trajectories over time.
  • Amniotic Fluid Assessment: Measurement of the maximum vertical pocket (MVP) or Amniotic Fluid Index (AFI) for each gestational sac.
  • Fetal Anatomy Check: A limited evaluation to ensure no new, obvious structural anomalies have developed.
  • Placental Location: Confirmation of placental position and its relationship to the cervix.

Why are other studies rated lower for routine surveillance?

Two key examples from the ACR criteria illustrate the specificity of the recommendation:
1. US assessment for TTTS: This study is rated Usually not appropriate. Twin-to-Twin Transfusion Syndrome is a disease of shared placental circulation (anastomoses) and is therefore a complication exclusive to monochorionic pregnancies. Performing this specialized assessment in a confirmed dichorionic pregnancy is clinically unwarranted.
2. MRI fetal without IV contrast: This is also rated Usually not appropriate for routine growth surveillance. While fetal MRI is an invaluable problem-solving tool for complex fetal anomalies suspected on ultrasound, it is not indicated for screening. It is more costly, less accessible, and provides no additional benefit over ultrasound for the standard biometry and fluid assessment required in this scenario.

What’s Next After the Ultrasound? Downstream Workflow for Dichorionic Twins

The results of the growth ultrasound guide the subsequent management and frequency of monitoring. The workflow typically branches based on the findings for growth, discordance, and amniotic fluid.

  • If the study shows appropriate, concordant growth: The patient continues with routine surveillance. The standard of care, supported by organizations like ACOG and SMFM, is typically serial growth ultrasounds every 4 weeks starting in the early third trimester (around 28-32 weeks).
  • If the study shows FGR or significant growth discordance: Management intensifies. Surveillance frequency increases to every 1-2 weeks. The next step often involves adding Doppler velocimetry studies, which are rated May be appropriate. Specifically, `US duplex Doppler fetal umbilical artery` is essential. Abnormal umbilical artery Doppler findings (such as absent or reversed end-diastolic flow) indicate significant placental insufficiency and are a critical factor in deciding the timing of antenatal corticosteroid administration and delivery.
  • If the study is indeterminate or shows other concerns: If fetal movement seems decreased or fluid levels are borderline, a `US pregnant uterus biophysical profile` (May be appropriate) may be ordered. This provides a more acute assessment of fetal well-being by scoring fetal tone, movement, breathing, and amniotic fluid volume, often combined with a nonstress test. Similarly, a `US cervix transvaginal` (May be appropriate) may be performed to assess for cervical shortening if there is a concern for preterm labor.

Pitfalls to Avoid (and When to Get Help)

Managing dichorionic multiple gestations requires meticulous attention to detail. Here are several common pitfalls to avoid:

  • Inconsistent Fetal Labeling: Failing to consistently label the fetuses (e.g., Twin A and Twin B, based on proximity to the cervix or laterality) at every scan can make it impossible to track individual growth trajectories accurately.
  • Overlooking Chorionicity: The entire surveillance strategy hinges on the correct initial determination of chorionicity. If there is any ambiguity from the first-trimester scan, it is a critical error to assume dichorionicity and not follow the more intensive monochorionic protocol.
  • Infrequent Surveillance: Extending the interval between growth scans beyond 4 weeks in the third trimester can lead to a missed diagnosis of late-onset FGR or evolving growth discordance.
  • Ignoring Maternal Factors: While this workflow focuses on fetal surveillance, remember to integrate maternal health status (e.g., development of preeclampsia, gestational diabetes) into the overall management plan, as these conditions can directly impact fetal growth.

If umbilical artery Dopplers become persistently abnormal or if severe FGR (EFW <3rd percentile) is diagnosed, it is time to escalate care. This typically involves consultation with a Maternal-Fetal Medicine (MFM) specialist and planning for delivery at a center with an appropriate level neonatal intensive care unit (NICU).

Related ACR Topics and Tools

For a comprehensive overview of imaging across all scenarios in this topic, from the first trimester to delivery, please see our parent guide. Additional tools from GigHz can help you navigate adjacent clinical questions and understand imaging protocols.

Frequently Asked Questions

How often should growth ultrasounds be performed for dichorionic twins?

For uncomplicated dichorionic twin pregnancies, professional societies generally recommend serial growth ultrasounds every 4 weeks, beginning in the third trimester (around 28-32 weeks) until delivery. If complications like fetal growth restriction or significant discordance are found, the frequency should be increased.

What is the definition of significant growth discordance in twins?

Significant growth discordance is most commonly defined as a 20% to 25% or greater difference in the estimated fetal weights (EFW) between the larger and smaller twin. The calculation is: [(EFW of larger twin – EFW of smaller twin) / EFW of larger twin] x 100%.

Why are umbilical artery Dopplers not part of the routine initial surveillance?

Umbilical artery (UA) Doppler assessment is rated as ‘May be appropriate’ rather than ‘Usually appropriate’ because it is not a primary screening tool for all dichorionic twins. Its main utility is in the further evaluation and management of pregnancies already complicated by suspected fetal growth restriction (FGR) to assess the degree of placental insufficiency and guide decisions about delivery timing.

If I suspect a problem, should I order a fetal MRI?

No, not for routine growth concerns. Fetal MRI is rated ‘Usually not appropriate’ for growth surveillance. Its role is as a problem-solving tool for complex cases, typically to better characterize a suspected structural or central nervous system anomaly that was inconclusively evaluated by a detailed ultrasound. For growth assessment, ultrasound remains the gold standard.

Does this guidance apply to triplets or quadruplets?

Yes, this guidance applies to ‘multichorionic higher order multiples,’ which includes trichorionic-triamniotic triplets, for example. The principles of monitoring each fetus’s growth, assessing for discordance, and checking amniotic fluid levels with serial transabdominal ultrasound remain the same, though the complexity and risks increase with each additional fetus.

Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 30, 2026