Invasive prenatal diagnosis: CVS, amniocentesis, and fetal blood sampling

Invasive prenatal diagnosis obtains placental cells, amniotic fluid, or fetal blood so a laboratory can answer a defined clinical question. The procedure is only one part of the decision. The team must first choose the right sample, test, timing, and interpretation for the ultrasound finding, screening result, family history, or suspected infection.

Chorionic villus sampling (CVS) collects placental tissue and can usually provide earlier genetic diagnosis. Amniocentesis collects amniotic fluid, commonly from 15 weeks onward, for chromosomal, molecular, biochemical, or infection testing. Cordocentesis obtains fetal blood and is reserved for questions that require direct fetal haematology, infection assessment in selected contexts, or preparation for treatment such as transfusion.

Screening is not diagnosis

Cell-free DNA and serum screening estimate the chance of selected conditions; they do not establish a fetal diagnosis. A high-chance result should be interpreted with ultrasound and counselling, followed by an offer of diagnostic testing when a more certain answer is desired. A normal screen also cannot exclude every genetic or structural condition.

Choosing the laboratory test

The sample alone does not determine what will be found. Karyotype, chromosomal microarray, targeted gene testing, gene panels, exome sequencing, PCR for infection, and fetal blood tests answer different questions. Broader testing can produce uncertain or incidental findings, so pre-test counselling should address possible result types, turnaround time, limitations, parental samples, and how the result could change pregnancy or neonatal management.

Procedure risks and aftercare

Ultrasound guidance is used to select a safe path while avoiding the fetus and relevant placental structures. Risks vary with the procedure, gestation, anatomy, number of needle insertions, operator, and pregnancy circumstances. They may include bleeding, fluid leakage, infection, sensitisation, and pregnancy loss.

After the procedure, follow the centre’s written advice. Fever, heavy bleeding, significant fluid leakage, severe pain, contractions, or reduced fetal movements requires prompt clinical contact or urgent assessment.

The best test is not automatically the broadest or earliest one. It is the test most likely to answer the family’s clinical question at a time when the information can still support an informed decision.