Prenatal ultrasound is a non-invasive test that uses sound waves to create an image of the fetus. This procedure’s biggest advantage is the ability to detect abnormalities in the developing fetus without putting the pregnancy at risk, unlike invasive prenatal tests such as amniocentesis that come with a risk for complications.
The main disadvantage of the ultrasound test is its inability to fully diagnose the level of function of the different organs. In some cases, although ultrasound results appear normal, genetic disorders may still occur and go undiagnosed by this test. For instance, the scan may show the fetus’ liver to be fully developed but fail to diagnose the liver’s inability to produce certain enzymes due to a genetic disorder.
Nuchal translucency scan for genetic disorders
A nuchal translucency scan is performed between weeks 11-14 of pregnancy. The test can be performed abdominally or vaginally. A sonographer (or other healthcare professional) scans the area behind the fetus’ neck to measure the amount of fluid filled in that space.
The thicker the nuchal fold, the higher the risk for Down syndrome.
It is important to note that abnormal nuchal scan results do not diagnose Down syndrome for certain. The scan is considered a screening test and is not a diagnostic test. Further tests required for a diagnosis include CVS (chorionic villus sampling), or amniocentesis.
First trimester anatomy assessment
The first trimester fetal anatomy scan is usually performed between weeks 14-16 of pregnancy. The scan examines different important organs of the developing fetus, the organs are scanned morphologically. Findings can be divided into four groups:
- Normal results
- In less than 1% – detection of a severe birth defect that may require terminating the pregnancy
- In certain cases, birth defects that are not severe and do not require a pregnancy termination but do require observation and further testing
- Findings that are not considered defects but may be related to other medical conditions and diseases. These generally require further testing and observation.
None of these groups include findings that are enough for a definite diagnosis of a genetics disease. Genetic testing is required for a diagnosis of genetic disorders.
Level II ultrasound – anatomy scan
When for various reasons a first trimester ultrasound was not carried out, an anatomy scan (also known as level II ultrasound) is recommended between weeks 19-22 of pregnancy. For women who have undergone a first trimester scan, the anomaly scan is recommended in weeks 22-25. This scan detects defects that develop in more advanced stages of pregnancy.
Abnormal results are not enough to diagnose a genetic disease or chromosome abnormality. Following abnormal anatomy scan results, a geneticist usually joins the diagnosis process.
What genetic diseases can an ultrasound test detect?
Ultrasound scans can detect various defects that may indicate genetic diseases and chromosome abnormalities, such as Down syndrome.
In some cases, defects found in an ultrasound scan may indicate several diseases such as microcephaly, polydactyly, limb defects, heart defects, polyhydramnios, and more.
In many instances, the defects found in the scan are not related to genetic disorders, but are caused by infections, obstructions of oxygen flow, or other reasons.