In one line
Invasive prenatal diagnosis is the deliberate sampling of feto-placental tissue — amniocentesis from 15+0 weeks, chorionic villus sampling from 11+0 to 13+6 weeks, fetal blood sampling later — to obtain a diagnostic genetic or haematological answer that screening cannot give; the consultant skill is no longer the needle but the counselling: who genuinely needs it now that the procedure-related miscarriage risk is roughly 0.1–0.3% rather than the 1% historically quoted, and what the right laboratory test is for the question being asked.
Mechanism & pathophysiology
Each technique samples a different tissue, and what it samples determines both the genetic answer it can give and its characteristic failure mode.
Amniocentesis aspirates amniotic fluid, which contains amniocytes — desquamated fetal cells from skin, the respiratory and urinary tracts, and the amnion. These are genuinely fetal in origin, so amniocentesis is the reference standard against which the others are judged for accuracy: a result reflects the fetal karyotype directly. The trade-off is timing. Before about 15 weeks the amnion has not fully fused with the chorion and the cell yield is poor, so the fluid is sparse and the membranes tent away from the needle — the reason early amniocentesis was abandoned (it raised both loss and talipes, the lesson behind the 15-week floor).
Chorionic villus sampling aspirates trophoblast and mesenchymal core from the developing placenta (chorion frondosum). Its great advantage is gestation: a diagnosis in the first trimester, when termination is simpler, safer and more private. Its characteristic pitfall is biological — the placenta is not always genetically identical to the fetus. The early embryo segregates into trophoblast and inner cell mass within the first few cell divisions, and a post-zygotic mitotic error can produce an aneuploid cell line confined to the placenta while the fetus is normal (or, less often, the reverse). This is confined placental mosaicism, found in roughly 1–2% of pregnancies sampled by CVS, and it is the dominant explanation when CVS mosaicism is later not confirmed: about 86% of mosaic CVS results are confined to the placenta rather than true fetal mosaicism. Direct (cytotrophoblast) preparations reflect this layer most; the cultured mesenchymal core correlates better with the fetus, which is why a discrepant or mosaic CVS result is resolved by a follow-up amniocentesis, not by repeating the CVS.
Fetal blood sampling (cordocentesis, usually at the placental cord insertion) draws fetal blood directly. It bypasses the placental-mosaicism problem and, uniquely, gives real-time fetal haematology, blood gas, platelet count and the ability to treat in the same sitting — the needle that diagnoses fetal anaemia is the needle that transfuses it.
The laboratory method matters as much as the sample, and the two have decoupled over the last decade.
- QF-PCR (quantitative fluorescent PCR) reads short-tandem-repeat dosage on chromosomes 21, 18, 13, X and Y, giving a rapid aneuploidy result in under 48 hours with near-complete concordance with karyotype for those chromosomes. It is the standard first-line rapid test on amniotic fluid or villi where the question is "is this one of the common trisomies?" — it does not see other or structural abnormalities.
- Full karyotype (cultured cells) is retained specifically to detect balanced rearrangements (reciprocal translocations, inversions), which neither microarray platform reliably sees. Triploidy, by contrast, is caught by QF-PCR and by SNP-based microarray through their allele ratios, so karyotype is not the only route to it.
- Chromosomal microarray scans the whole genome for copy-number gains and losses at far higher resolution than karyotype, on uncultured cells, and is the test of choice when there is a fetal structural anomaly — but it is blind to balanced rearrangements (no net copy-number change); a pure array-CGH platform also misses triploidy, though a SNP-based microarray detects triploidy through its allele ratios. It will turn up variants of uncertain significance that demand careful counselling.
- FISH gives an interphase aneuploidy answer on uncultured cells in hours, now largely supplanted by QF-PCR for cost.
- Beyond the chromosome, single-gene testing (targeted mutation analysis, exome sequencing) is used when the indication is a Mendelian disorder in the family.
The practical synthesis: choose the sample for the gestation and the indication, then choose the laboratory test for the question — QF-PCR for a high-risk aneuploidy screen result, microarray when an anomaly is on the scan, karyotype when a balanced translocation must not be missed.
Two further mechanistic points separate the consultant from the registrar. First, fetal blood sampling can also diagnose fetal infection and immune status directly — a fetal full blood count, reticulocytes and direct Coombs in alloimmunisation, or fetal-blood PCR/IgM for an intrauterine infection (parvovirus B19, cytomegalovirus) when amniotic-fluid PCR is equivocal — because it samples the fetal compartment itself rather than a surrogate. Second, the same uterine breach opens a bidirectional transplacental route, and the two risks run in opposite directions: fetal red cells crossing out into the maternal circulation (a feto-maternal haemorrhage) drive the rhesus-sensitisation risk (why anti-D is mandatory), while maternal blood and virus crossing in to the fetal compartment drive the blood-borne-virus inoculation risk (why maternal viraemia must be controlled first). The needle is bidirectional, and loss, sensitisation and vertical transmission are three separate consequences of that one breach rather than a single mechanism.
Assessment
The decision to put a needle into the uterus is a counselling decision first. Establish why a diagnostic test is being considered, because the indication sets both the urgency and the right tissue and assay.
- Define the indication precisely. The common drivers are: a high-risk combined or quadruple screen; a high-chance cfDNA/NIPT result; a fetal structural anomaly or soft-marker constellation on ultrasound; a parental balanced translocation or a known single-gene disorder with a defined recurrence risk; and, for fetal blood sampling specifically, suspected fetal anaemia (red-cell alloimmunisation, parvovirus B19), thrombocytopenia, or fetal infection requiring direct sampling.
- Distinguish screening from diagnosis explicitly in the counselling. A high-chance NIPT result is not a diagnosis. The NEXT trial put a number on why this matters: in a routine first-trimester population the positive predictive value of cfDNA for trisomy 21 was 80.9% — excellent for a screen, but it still means roughly one in five "high-chance" results is a false positive, and the PPV falls further for the rarer trisomies and in younger, lower-prior-risk women. A woman must understand that an invasive test is what confirms before any irreversible decision is taken.
- Pre-procedure work-up. Confirm viability, gestation and number on ultrasound; document placental site and the safest needle trajectory; check maternal rhesus group and antibody screen; review blood-borne virus status — HIV, hepatitis B, hepatitis C — because these change whether and when you proceed. In a multiple pregnancy, map and label each sac before sampling (chorionicity, membrane geometry, a reliable way to tell the twins apart) — a mislabelled result in twins is a catastrophic error.
- Counsel the actual numbers. Quote the procedure-related miscarriage risk honestly: on the best contemporary meta-analytic data it is around 0.1–0.3% above background for both amniocentesis and CVS in skilled hands — an order of magnitude lower than the "1% / 2%" still in old consent forms. Counsel the test's limitations (what QF-PCR will and will not see; the chance of a mosaic or uncertain result), the small failure/repeat rate, and the turnaround time.
- Interpretation is part of assessment. A normal QF-PCR does not exclude a microdeletion; a "low-level mosaic" on CVS usually means confined placental mosaicism rather than an affected fetus and is resolved by amniocentesis; a variant of uncertain significance on microarray is a counselling problem, not a result to be acted on alone.