This test is done at 14–18 weeks of pregnancy. The fluid surrounding the fetus, called amniotic fluid, contains skin cells from the developing fetus. A sample of this fluid is taken by inserting a needle through the abdomen into the uterus, and then sent to a laboratory to be studied for the presence of the HD mutation in the fetal cells.

No anaesthetic is required. When the test is carried out by an obstetrician experienced in this technique, the risk of a miscarriage related to the test is about 0.5% (1 in 200 pregnancies).