Amniocentesis and chorionic villus sampling explained

Both can examine fetal genetic material, but amniocentesis removes a small amount of fluid surrounding the baby, while chorionic villus sampling, often called CVS, takes placental tissue. The choice depends on pregnancy timing, the condition being investigated, and which sample can answer the clinical question.

A clinician performs either procedure with ultrasound guidance. CVS can generally be performed earlier in pregnancy and samples cells from the placenta. Amniocentesis is performed later and samples amniotic fluid, which contains fetal cells. Amniotic fluid can also be assessed for substances relevant to certain fetal conditions that a placental sample cannot evaluate.

These are diagnostic procedures, unlike screening tests that estimate whether a condition may be present. A diagnostic result can still be unclear or require confirmation when the laboratory finds an unusual or uncertain genetic change.

Review the reason for testing with your obstetrician or fetal medicine specialist. Ask which laboratory analysis is planned, what it can and cannot identify, when results may return, and whether genetic counselling would help. Tell the team about medicines, allergies, blood group concerns, bleeding problems, and any recent pregnancy symptoms.

Follow the clinic's instructions about eating, drinking, bladder preparation, transport, and activity afterward. During either test, report pain or feeling faint immediately. Mild cramping or a small amount of spotting can occur afterward, but the care team should explain what they expect in your particular case.

Contact your maternity unit promptly after either procedure if you have fluid leaking from the vagina, bleeding that is more than light spotting, worsening or persistent abdominal pain, fever, chills, contractions, or you feel significantly unwell. If bleeding is heavy, pain is severe, or you feel faint, seek urgent care. Use the specific contact instructions given by your procedure team because they know the pregnancy and how the sample was taken.

Useful questions include: Why is diagnostic testing being offered now? Which procedure fits the timing of my pregnancy? Will the planned analysis answer our main question? Could placental mosaicism affect a CVS result? Might an amniocentesis still be needed after CVS? Ask how results will be explained and who will support you in considering their implications.