Prenatal Testing

The antenatal period is unique when the clinician is taking care of two patients, the mother and the unborn child. Although the transgenerational impact of genomics is common to many genomic situations, it is perhaps most apparent in these scenarios.

There are different options for diagnosing CF during pregnancy or after birth depending on the couple’s wishes. If they are likely to use the results to make decisions about whether or not to continue the pregnancy, timing is important as termination of pregnancy becomes significantly more physically and emotionally traumatic after the first trimester.

Imaging

This is mentioned in a genomics course, as an abnormality identified here often leads to a genetic test being performed. A detailed fetal anomaly scan is often done between 18 and 20 weeks of gestation, with a wide range of anomalies potentially identified at this stage. An abnormality often leads to further investigation with amniocentesis.

Chorionic Villus Sampling

Usually performed between 11 and 13 weeks under ultrasound guidance either via a transabdominal or transcervical route.

After removing the material, the fetal material needs to be dissected from maternal tissue, and the villi can be used for DNA extraction or rapid cytogenetic analysis of dividing cells. The additional risk of miscarriage is approximately 1%.

Cytogenetic testing for chromosomal anomalies, such as Down Syndrome, Edwards Syndrome, or Patau syndrome can normally yield results in 2 or 3 days. DNA testing may take 2 to 3 weeks and has been largely used to date to look for specific conditions and usually only when the familial pathogenic variants are known. Such testing is always compared to the mother’s control sample to ensure that the result is the fetal genotype.

Prenatal WES (whole exome sequencing) is available in the national genomic testing directory for certain fetal anomalies, and WGS (whole genome sequencing) may also be adopted. WGS has the advantage that it captures both coding and non-coding point mutations as well as copy number variants (CNVs) and the ability to identify balanced chromosomal abnormalities in a single test. In contrast, WES typically needs to be combined with molecular cytogenetic methods to achieve the same comprehensive review. To read more about the evidence for, the opportunities and the limitations of prenatal WES and WGS look at this editorial on introduction of genomics into prenatal diagnostics

Amniocentesis

Performed between 14 and 20 weeks, and has a 0.5-1.0% risk of miscarriage. This involves drawing amniotic fluid via a transabdominal approach. Fetal cells can be isolated from the fluid and cultured for cytogenetic or molecular analysis, however culture of cells for cytogenetic analysis can take 2 weeks as there are fewer fetal cells than in the chorionic villi sample. Consequently methods that avoid the need for cell culture, such as quantitative PCR with fluorescent-labelled primers for specific trisomies or genome-wide microarrays are increasingly used. The amniotic fluid also can be tested for biochemical abnormalities, such as AFP, with elevated levels associated with neural tube defects.