Prenatal tests and why we need them
Modern prenatal screening and diagnostic tests have shown a tremendous success in the early discovery of gross fetal abnormalities. Depending on the method, there are invasive and non-invasive tests. Non-invasive tests include BioChemical Screening (BCS) and DNA Screening.
Biochemical screening is early (11th – 13th +6 week of gestation) and late (15th – 20th week of gestation). It gives pregnant women a risk assessment of chromosomal abnormalities of the fetus. The most common chromosomal abnormalities are Down syndrome (trisomy (3 chromosomes instead of 2) 21), Edwards syndrome (trisomy 18), Patau syndrome (trisomy 13) and Turner syndrome (monosomy (1 chromosome instead of 2) X). The risk, determined by the biochemical screening is calculated based on a number of factors such as biochemical and ultrasound fetal parameters (nuchal translucency, etc.), maternal age and others.
DNA screening tests are the most advanced method for prenatal screening. They are non-invasive and use cell free fetal DNA found in the mother’s blood. They can be administered as early as the 9th gestational week at the request of the parents or after a positive biochemical screening result. In addition to its extreme sensitivity to chromosomal anomalies, DNA screening tests can tell you the baby’s sex much earlier than the ultrasound.
Invasive prenatal tests unlike screening tests diagnose chromosomal abnormalities rather than determine risk. These tests include amniocentesis and chorion biopsy. A needle or catheter is inserted into the uterus. It draws some amniotic fluid or takes a piece of the placenta which is then subjected to chromosomal analysis. Invasive prenatal test, however, do lead to an increased risk of preterm labor.