During the pregnancy, it is highly important to detect and diagnose genetic diseases in the fetus as early as possible. These diseases may be serious, which could lead parents to debate whether they wish to terminate the pregnancy or continue it, according to their personal choices and beliefs, and according to the severity of the disease. For this reason, prenatal genetic testing is especially important. However, these tests must be as safe as possible for both mother and fetus.
Until recently, prenatal genetic testing was only available via invasive procedures. Tissue samples from the fetus (and direct access to the uterus) were required to identify genetic defects. The two main methods for sampling were amniocentesis and CVS (Chorionic villus sampling).
Chorionic villous sampling uses samples from the placenta. Since both the placenta and the fetus have an identical tissue source, their genetic material is also identical. CVS is performed around 10-11 weeks into the pregnancy, but it carries a certain risk for a miscarriage.
Amniocentesis is performed at a later stage of pregnancy (usually around week 15-20). The amniotic fluid contains fetal cells – cells from the fetus’ urinary system. These cells can be examined to identify genetic diseases. There is a risk of miscarriage in amniocentesis, though it is considered lower than the risk in CVS tests.
Maternal blood tests – the future of prenatal genetic testing
A new method allows prenatal testing without the need for invasive procedures that may cause a miscarriage. Research has found that fetal blood cells find their way into the mother’s bloodstream. These cells contain fetal genetic material, not maternal genetic material.
Identifying the fetal blood cells and analyzing them can replace the need for full tissue samples drawn via CVS or amniocentesis.
The difficult part of these new tests is separating the fetal and maternal blood cells. This is done so the test does not accidently analyze the mother’s cells instead of the fetus’ cells.
When and how is the test performed?
The blood test is usually performed around week 10 of the pregnancy. The test is simple and includes drawing a small sample of blood from the mother. After the sample is collected, the fetal genetic material is separated in the lab. The fetal genetic material can then be used for genetic testing. These tests replace CVS and amniocentesis.
The American College of Obstetricians and Gynecologists (ACOG) currently recommends the blood test to women in the following cases: a maternal age of 35 and over; when irregular ultrasound test results may indicate a chromosomal defect; a family history of trisomies; a known genetic disorder in one of the parents that may increase the risk for chromosome abnormalities in the fetus (Robertsonian translocation).
These blood tests have high specificity and sensitivity levels. The test has a high accuracy level of detecting genetic defects in the fetus, and a very low false-positive rate.
What diseases can the test find?
Maternal blood testing can be used to perform the entire range of prenatal genetic testing available after an amniocentesis or CVS.
The test can help detect diseases caused by an extra chromosome such as Down’s syndrome (trisomy 21) and trisomies 18 and 13. It is sometimes used to diagnose other chromosomal diseases including Cri Du Chat Syndrome, Angelman syndrome, and Prader-Willi syndrome.
When there is a risk for a certain genetic disorder in the family, PCR testing can also be used to identify specific genes and changes in those genes in the fetus.
