Children's Hospital Colorado
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Feasibility of Prenatal Ultrasound for Diagnosis of Anorectal Malformations


Doctor doing an ultrasound on a patient.

Key takeaways

  • Prenatal detection of anorectal malformations is challenging, especially less severe forms.

  • Our researchers studied the feasibility of using routine prenatal ultrasounds to identify anal dimples (AD) to improve the detection of anorectal malformations.

  • In the longitudinal study of prenatal ultrasounds, the AD was observed in about 58% of cases.

Research background: prenatal diagnosis of anorectal malformations

It is difficult to detect and diagnose anorectal malformations prenatally, especially less severe forms. Identifying less severe forms could be possible if an abnormal location or absence of an opening is detected, such as rectovestibular fistula, rectourethral bulbar or prostatic fistula, rectobladder neck fistula and anorectal malformation without fistula in males and females.

The identification and evaluation of an anal dimple (AD) during routine prenatal ultrasounds may be a feasible way to detect the presence of an AD and make a diagnosis of an anorectal malformation. The appearance of the AD is described as a “predominantly rounded area of echogenicity surrounded by a rim of hypoechogenicity posterior to the fetal genitalia, resembling a ‘target sign.’”

Researchers from the International Center for Colorectal and Urogenital Care and Colorado Fetal Care Center at Children’s Hospital Colorado hypothesized that visualizing the AD in prenatal ultrasounds may help detect less severe types of anorectal malformations and impact pregnancy management and family counseling.

Research methods: prenatal ultrasounds

Researchers performed a longitudinal study of 372 pregnant women who underwent prenatal ultrasounds at the Colorado Fetal Care Center between January 2019 and October 2020. Fetuses with suspected anorectal malformations were excluded. A total of 900 ultrasounds were performed, evaluating 1,044 fetuses, with a gestational age range of 16 to 38 weeks. The mean number of ultrasounds per participant was two.

Data collection on fetuses included:

  • Gestational age
  • Singleton vs. multiple pregnancies
  • Gender
  • Visualization of AD
  • Non-visualization of AD
CFCC ARM Graphics 1

Research results: role of age and position of the fetus in visualizing AD

Researchers found the following:

  • In 612 fetuses (58.6%), the AD was visualized
  • In 432 fetuses (41.4%), the AD was not visualized
  • The two most common reasons for non-visualization:
    • extreme gestational age
    • fetal position
  • The AD was visualized most often in 28 to 33 weeks (+ 6 days) gestation, with 78.1% success and was a statistically significant difference.
  • The AD was most identifiable in singleton pregnancies, at 66.6% vs. 37.6% in multiple pregnancies.
  • Gender did not play a role in the ability to visualize an AD.
  • Of the 205 babies in the study at Children’s Colorado, only one had the most benign anorectal malformation type (recto-perineal fistula); all others had a normal anus.
CFCC ARM Graphics 2
Crosstable of Anal Dimple Visualization on Static Stratification (age)
  Stratification (age)
  16-21w+6d 22-27w+6d 28-33w+6d 34w or more Total
Anal Dimple no   174 86 79 93 432
Imaged on   % of total 16.7% 8.2% 7.6% 8.9% 41.4%
Static yes   4 132 281 195 612
    % of total 0.4% 12.6% 26.9% 18.7% 58.6%
Total     178 218 360 288 1044
    % of total 17.0% 20.9% 34.5% 27.6% 100.0%

Research discussion: benefits of prenatal recognition of anorectal malformations

In utero recognition of anorectal malformations benefits the mother and fetus. Once AD is identified and the anorectal malformation is diagnosed as the underlying cause, healthcare professionals are able to better counsel patients. This includes both prenatal and perinatal management, such as identifying the proper facility for birth (a hospital with a level III neonatal intensive care unit and surgical capabilities), and counseling patients on the need for surgical intervention (most newborns with anorectal malformations will require surgery about 24 hours after birth).

To the researchers’ knowledge, this is the first U.S. study addressing the feasibility of visualizing AD in prenatal ultrasounds for anorectal malformation. The U.S. does not routinely examine AD in prenatal ultrasound because it is considered optional in national guidelines. Incidence of AD identification outside of this study was 16 to 42%. The AD identification rate when assessed by ultrasound encounter was 58% and 81% when assessed by individual fetus in this study. These rates are also consistent with findings in international studies, including a prior study from South Korea that demonstrated high rates in visualization.

One caveat for the success of these studies and an indicator of success in AD visualization was the age of the fetus at the time of the ultrasound. The optimal gestational age for the visualization of AD was between 28 and 33 weeks. In a histological evaluation, researchers found that fetal anal sphincter development is most active during the later gestational periods. This observation supports follow-up ultrasounds in cases where the AD was not initially identified.

Research conclusion: AD visualization and anorectal malformation detection

Researchers demonstrated that AD visualization in a prenatal ultrasound to detect an anorectal malformation is feasible. The optimal timing for AD visualization is late in the second and third trimester.