Which prenatal testing option provides a diagnostic karyotype?

Amniocentesis is a prenatal testing option that provides a diagnostic karyotype, which is essential for identifying chromosomal abnormalities in the fetus. This procedure involves the extraction of amniotic fluid surrounding the fetus, which contains fetal cells. These cells can be cultured in the laboratory, allowing for a detailed analysis of the fetal chromosomes. The results from amniocentesis can definitively diagnose conditions such as trisomy 21 (Down syndrome), trisomy 18, and other chromosomal disorders.

The advantage of amniocentesis over other testing methods is its ability to yield a definitive karyotype, which is critical for accurate diagnosis. Other options, such as cell-free DNA screening and serum screening, provide risk assessment rather than definitive results, as they cannot confirm the presence of abnormalities but only indicate the likelihood based on statistical analysis. Ultrasound screening is primarily used for assessing fetal anatomy and development, but it also does not provide genetic information or karyotype analysis. Thus, when seeking a definitive diagnostic tool for chromosomal abnormalities, amniocentesis is the appropriate choice.