The anatomy scan, also known as the Level II ultrasound, is a routine, non-invasive prenatal screening tool performed between 18 and 22 weeks of pregnancy. This imaging exam provides a detailed look at the developing fetus, primarily to check for structural abnormalities and ensure proper growth. While its main purpose is not genetic testing, the scan can identify certain physical indicators that are statistically more common in fetuses with Trisomy 21 (Down Syndrome). The anatomy scan serves only to raise a suspicion or level of risk, and it is not a definitive diagnostic test for the condition. The information gathered dictates whether further, more specific testing may be necessary for an accurate chromosomal analysis.
Primary Purpose of the Anatomy Scan
The primary function of the anatomy scan is to conduct a thorough evaluation of the fetus’s physical development and general well-being. The sonographer takes numerous measurements to confirm that the baby is growing at an appropriate rate for the gestational age. This includes checking the size of the head, abdomen, and long bones, such as the femur.
The scan involves a meticulous inspection of all major organ systems to identify any structural defects. The heart is examined for congenital defects, the brain is checked for correct formation, and the kidneys and bladder are evaluated for proper function. Additionally, the sonographer assesses the position of the placenta, the amount of amniotic fluid, and the integrity of the umbilical cord. The goal is to establish a baseline of structural integrity and fetal health, which helps the healthcare team plan for the remainder of the pregnancy and delivery.
Soft Markers Associated with Down Syndrome
The anatomy scan relates to Trisomy 21 detection through the identification of “soft markers.” These are minor structural anomalies or variations that are seen more frequently in fetuses with chromosomal abnormalities, though they often appear in healthy fetuses as well. Their presence does not confirm a diagnosis but recalculates the statistical risk of the condition.
Soft markers include:
- A thickened nuchal fold (increased measurement of the skin at the back of the neck).
- Absence or a smaller-than-normal (hypoplastic) nasal bone.
- An echogenic intracardiac focus (a bright spot in the heart).
- Echogenic bowel (a bright appearance of the bowel).
- Shortening of the long bones, such as the femur or humerus.
- Mild kidney swelling (pyelectasis).
Why the Anatomy Scan is Not Diagnostic
The anatomy scan is considered a screening test, not a diagnostic one, because it relies on visualizing physical structures rather than analyzing the baby’s chromosomes directly. Ultrasound technology provides images of the body’s anatomy but cannot count the number of chromosomes present in the fetal cells. A finding of a soft marker only increases the statistical likelihood of Down Syndrome, which is why it is referred to as assigning a level of risk.
This limitation means the scan is susceptible to both false positives and false negatives. A false positive occurs when soft markers are seen, but the fetus is genetically healthy. Conversely, a false negative happens when no soft markers are identified on the scan, but the fetus actually has Trisomy 21. The overall detection rate for Down Syndrome using only the second-trimester anatomy scan is 50% to 70%.
Next Steps After Marker Detection
When soft markers are detected during the anatomy scan, the healthcare provider will recommend subsequent testing to clarify the risk. The first option is Non-Invasive Prenatal Screening (NIPS), a maternal blood test that analyzes cell-free fetal DNA circulating in the mother’s bloodstream. NIPS offers high sensitivity and specificity for Trisomy 21, with detection rates exceeding 99%.
While NIPS is highly accurate, it remains a screening tool, meaning its results are not definitive. To obtain a confirmed diagnosis, an invasive procedure is required, such as amniocentesis or Chorionic Villus Sampling (CVS). Amniocentesis involves taking a sample of amniotic fluid after 15 weeks of gestation, while CVS samples placental tissue earlier in the pregnancy. These invasive tests analyze the fetal chromosomes directly, providing a certain diagnosis by karyotyping, but they carry a small risk of miscarriage.