Amniocentesis can detect a wide range of genetic and chromosomal findings, and the specific tests run on the fluid sample depend on what the family and care team are looking for.

Chromosome number and structure. A test called karyotyping examines the full set of chromosomes to look for an extra or missing chromosome, such as the change that causes Down syndrome, or larger structural rearrangements.

Smaller missing or extra pieces of chromosomes. A more detailed test called chromosomal microarray analysis can detect tiny deletions or duplications of genetic material that are far too small to see with standard karyotyping. Because this technology can find changes that karyotyping alone would miss, current guidelines recommend that microarray testing be offered to anyone having an invasive diagnostic test.

Specific single-gene conditions. When a family is known to be at risk for a particular condition caused by a change in one gene, such as an inherited metabolic disorder or a condition like cystic fibrosis, targeted molecular testing can look specifically for that genetic change in the pregnancy.

Neural tube defects and certain infections. Amniotic fluid can also be tested for markers associated with open neural tube defects, such as spina bifida, and in some situations for evidence of certain infections affecting the pregnancy.

Amniocentesis is considered highly accurate for the conditions it is designed to detect, with results generally regarded as close to one hundred percent reliable for chromosome number and structure once a sample has grown successfully in the laboratory. That said, no test detects everything. Amniocentesis identifies genetic and chromosomal conditions specifically tested for; it does not evaluate every possible birth difference, developmental outcome, or physical feature, and normal results do not guarantee an unaffected or complication-free pregnancy. Before testing, a genetic counselor can help clarify exactly which conditions are being tested for and what a normal or abnormal result would and would not tell the family.