Genetic amniocentesis is an invasive prenatal diagnostic technique used to identify fetal chromosomal abnormalities (such as Down syndrome), prenatally diagnosable genetic diseases, and most neural tube defects (spina bifida) by evaluating levels of amniotic fluid AFP. Amniocentesis is usually performed between 15 and 21 weeks of gestation but can be performed later in gestation if needed.
A small amount of amniotic fluid (20-30 mL; 4-6 teaspoons) is withdrawn from the sac around the baby under direct ultrasound guidance using a thin needle. Fetal cells within the fluid are grown in tissue culture, and then chromosomes are analyzed by specialized techniques. In addition, amniotic fluid AFP is assessed for risk of neural tube defects.
Chromosomal microarray (CMA) is a newer technique used to detect chromosomal abnormalities (microdeletions and duplications) that are not detected by traditional methods. The collected amniotic fluid that is replenished over the ensuing few hours. Complications are rare (~1 in 300 to 500) and include amniotic fluid leakage, vaginal bleeding or spotting, cramping, and miscarriage.