Anomaly scan

The anomaly scan, also sometimes called the anatomy scan, 20 week ultrasound, or level 2 ultrasound, evaluates anatomic structures of the fetus, placenta, and maternal pelvic organs. This scan is an important component of routine prenatal care.

Anomaly scan
SynonymsAnatomy scan, Second-trimester ultrasound, 20 week ultrasound, Level 2 ultrasound
PurposeTo evaluate fetal anatomy and size

Procedure

This scan is conducted between 18 to 22 weeks gestation as a component of routine prenatal care. Prior to 18 weeks gestation, the fetal organs may be of insufficient size and development to allow for ultrasound evaluation. Scans performed beyond 22 weeks gestation may limit the ability to seek pregnancy termination, depending on local legislation.[1]

Two-dimensional (2D) is used to evaluate fetal structures, placenta, and amniotic fluid volume. Maternal pelvic organs are also evaluated. Views are obtained using an abdominal ultrasound probe, but a vaginal ultrasound probe may also be used to evaluate for placenta previa and cervical length. Three-dimensional (3D) ultrasound is not recommended for routine use during anomaly scan, but 3D ultrasound may be utilized to further evaluate suspected abnormalities in specific fetal structures.[2]

Components
Anatomy scan image of a human placenta and umbilical cord (colour Doppler rendering) showing central placement of the cord in the placenta and three vessels in the cord, which is the normal physiology.

A standard anatomy scan typically includes:

Medical uses

Second-trimester ultrasound screening for aneuploidies, such as Edwards syndrome and Patau syndrome, are based on looking for soft markers and some predefined structural abnormalities. Soft markers are variations from normal anatomy, which are more common in aneuploid fetuses compared to euploid ones. These markers are often not clinically significant and do not cause adverse pregnancy outcomes.[4] Placenta evaluation is vital to planning method of delivery. Additionally, placental and umblical cord abnormalities are also associated with certain fetal genetic abnormalities.[2]

Conditions Identified During Anomaly Scan
Fetal Conditions[5] Placental Conditions[2] Maternal Conditions[2]

Sex determination
Main article: Prenatal sex discernment

Fetal sex is often noted during the anomaly scan. However, performing an ultrasound for the sole purpose of determining fetal sex without a medical indication is not recommended.[1]

Limitations

Although ultrasound is a useful screening tool for fetal anomalies, the sensitivity of this scan is highly variable, potentially detecting only 40% of all fetal anomalies. The ability of this scan to detect structural fetal abnormalities varies depending on the severity of the anomaly, the background risk of the study population, gestational age at time of scan, the expertise of the ultrasound technician and interpreting obstetrician, and maternal obesity.[1]

See also

Nuchal scan

References
  1. Committee on Practice Bulletins—Obstetrics the American Institute of Ultrasound in Medicine (December 2016). "Practice Bulletin No. 175: Ultrasound in Pregnancy". Obstetrics & Gynecology. 128 (6): e241–e256. doi:10.1097/AOG.0000000000001815. ISSN 0029-7844. PMID 27875472.
  2. Creasy and Resnik's maternal-fetal medicine : principles and practice. Creasy, Robert K.,, Resnik, Robert,, Greene, Michael F.,, Iams, Jay D.,, Lockwood, Charles J. (Seventh ed.). Philadelphia, PA. 17 September 2013. ISBN 978-0-323-18665-0. OCLC 859526325.CS1 maint: others (link)
  3. Cunningham, F; Leveno, KJ; Bloom, SL; Spong, CY; Dashe, JS; Hoffman, BL; Casey BM, BM; Sheffield, JS (2013). "Fetal Imaging". Williams Obstetrics, Twenty-Fourth Edition. McGraw-Hill.
  4. Zare Mehrjardi, Mohammad; Keshavarz, Elham (2017-04-16). "Prefrontal Space Ratio—A Novel Ultrasound Marker in the Second Trimester Screening for Trisomy 21: Systematic Review and Meta-Analysis". Journal of Diagnostic Medical Sonography. 33 (4): 269–277. doi:10.1177/8756479317702619.
  5. NHS Choices. "Mid-pregnancy anomaly scan - Pregnancy and baby - NHS Choices". www.nhs.uk. Retrieved 2017-12-04.
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