A diagnostic procedure for pregnant women to determine whether or not their foetuses have gross chromosomal anomalies. It involves sampling the liquid immediately surrounding a foetus within the amnion. This amniotic fluid is extracted through the mother's abdominal and uterine walls with a hypodermic needle. The amniotic fluid mostly contains foetal urine but also has millions of foetal skin cells that can be cultured to produce a karyotype. There is 99+% accuracy in diagnosing Down syndrome and most other gross chromosomal aberrations including neural tube defects such as spina bifida.