Chromosomal Abnormalities and Prenatal Testing

Chromosomal abnormalities affect approximately 1 in 200 live births, though many affected fetuses do not survive to birth. Risk factors include maternal age over 35, a history of previous birth defects, and a family history of chromosomal disorders. Over the years, several medical techniques have been developed to screen and diagnose genetic abnormalities in the fetus.

Ultrasound for Early Detection

• After the third month of pregnancy, ultrasound can be used to detect major structural defects in the fetus.
• While it does not provide a definitive genetic diagnosis, it helps in early identification of visible abnormalities.

Amniocentesis: A Major Breakthrough

• The removal of amniotic fluid for medical purposes dates back to the late 1870s.
• In 1956, John H. Edwards highlighted its use for detecting hereditary disorders.
• Amniocentesis is usually performed between the 15th and 20th week of pregnancy.
• It allows the preparation of a karyotype, a chromosome map used to detect conditions such as:
• Down syndrome (Trisomy 21)
• Spina bifida
• Cystic fibrosis
• Tay-Sachs disease

Chorionic Villus Sampling (CVS): An Alternative Diagnostic Test

• Developed in 1983 by Italian biologist Giuseppe Simoni, CVS provides results earlier than amniocentesis (between the 10th and 12th weeks).
• It involves extracting chorionic villi (finger-like projections from the fetal membrane) to study fetal genetic makeup.
• CVS can detect over 200 genetic abnormalities.

Cell-Free Fetal DNA Testing: A Non-Invasive Option

• Introduced in 2011, this test requires only a blood sample from the mother, making it non-invasive.
• It detects DNA fragments from the fetus and screens for conditions like Down syndrome as early as the 10th week.
• However, it is only a screening test, not a diagnostic test like amniocentesis or CVS.

There are two types of tests that can be conducted during pregnancy for Down syndrome: a screening test using ultrasound (sonogram), which gives an indication that there is a higher risk of a disorder; and a diagnostic test, such as amniocentesis or chorionic villus sampling, which provides a firm diagnosis.