How does amniocentesis test fetal cells for chromosome abnormalities?

Amniocentesis is an invasive prenatal diagnostic procedure used to identify chromosomal abnormalities in a fetus. During the process, a hollow needle is inserted through the mother's abdominal wall and uterus into the amniotic sac to collect a small sample of amniotic fluid.

This fluid contains living fetal cells that have been sloughed off by the developing fetus. Once these cells are recovered, they are induced to grow in a laboratory setting. Scientists then analyze the cultured cells to determine the karyotype, which allows for the assessment of chromosome number, composition, and structure.12

Because these cells are embryonic in origin, they provide a reliable representation of the fetus's genetic constitution, making the procedure a standard method for detecting chromosomal anomalies, such as those associated with Down syndrome. While generally considered safe, the procedure carries risks, including infection or pregnancy loss, and is typically performed between the 15th and 17th weeks of gestation.23