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.
Procedure.
This scan is conducted between 18 to 22 weeks' gestation, but most often performed at 19 weeks, 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.
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 features.
Components.
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. Placental evaluation is vital for planning method of delivery. Additionally, placental and umbilical cord abnormalities are also associated with certain fetal genetic abnormalities.
Sex determination.
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.
Limitations.
Although ultrasound is a useful screening tool for fetal anomalies, the sensitivity of this scan is highly variable and potentially detects only 40% of all fetal anomalies. The ability of this scan to detect structural fetal abnormalities depends on a variety of factors. These factors include 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.