Prenatal diagnosis

Prenatal diagnosis identifies, confirms, or excludes selected fetal genetic and structural conditions before birth. Testing may be offered because of an abnormal screening result, a finding on ultrasound, a known parental variant, a previous affected pregnancy, or a family history of an inherited disorder.¹ It can examine chromosomes, specific genes, or fetal anatomy. The results may guide pregnancy care, permit preparation for specialized neonatal treatment, and inform reproductive decisions; they do not predict every aspect of a child’s future health.

Diagnostic testing must be distinguished from screening. Cell-free DNA testing, serum tests, and many ultrasound measurements are screening methods: a high-risk result generally requires confirmation, while a low-risk result cannot eliminate all risk. Counseling explains the condition being investigated, possible uncertain findings, alternatives, and the implications of each result.

Chorionic-villus sampling and amniocentesis are the principal invasive diagnostic procedures. Chorionic-villus sampling, usually performed in the late first or early second trimester, removes a small placental sample through the abdomen or cervix; amniocentesis withdraws amniotic fluid, commonly from about 15 weeks onward.¹ Laboratory analysis can include chromosome analysis, microarray testing, or targeted molecular testing chosen for the clinical question.

Ultrasound is central to diagnosis of structural anomalies and helps guide invasive procedures, while fetal magnetic resonance imaging can add detail for selected brain, chest, or abdominal findings. Rarely, fetal blood sampling is used when information cannot be obtained adequately from other specimens. Invasive procedures carry a small risk of complications, including pregnancy loss, so the expected benefit is weighed against that risk by the patient and care team.

Results may be positive, negative, uncertain, or inconclusive, and each category requires clinical interpretation. A targeted negative result substantially reduces the likelihood of the condition tested for but does not rule out unrelated genetic or structural disorders. Chromosomal microarray may identify a copy-number change whose clinical significance is uncertain, while mosaic results can reflect differences between placental and fetal cells.²

Genetic counseling places laboratory findings alongside ultrasound, family history, and the parents’ values. Options can include continued pregnancy with altered monitoring, delivery planning at a specialist center, fetal therapy in selected conditions, adoption or parenting preparation, or ending the pregnancy where legally available. Decisions are personal and time-sensitive; clinicians should present balanced information without treating a test result as a directive.

Prenatal diagnosis can reveal conditions that are confined to the placenta, difficult to classify, or milder than their name suggests. Chorionic-villus sampling samples placental tissue, so a finding called confined placental mosaicism may require amniocentesis or other follow-up before conclusions about the fetus are drawn.¹ Conversely, some disorders arise after the sampled cells were collected or are not included in the requested test.

Rapid aneuploidy testing can provide an early answer about common chromosome-number differences, but it does not replace a broader analysis when one is indicated. A fetal structural anomaly may also result from a non-genetic cause, and a normal genetic test may leave its cause unresolved. The growing use of sequencing has expanded diagnosis for fetuses with unexplained anomalies, while also increasing the chance of uncertain or incidental findings that require specialist review.

Prenatal screening

Testing that estimates the chance of a fetal condition without usually establishing a diagnosis.

Chorionic-villus sampling

An invasive procedure that obtains placental tissue for genetic or chromosomal analysis.

Amniocentesis

An invasive procedure that withdraws amniotic fluid containing fetal cells for laboratory testing.

Chromosomal microarray

A laboratory method that detects gains and losses of chromosomal DNA, including changes too small for a standard karyotype.

Variant of uncertain significance

A genetic finding whose relationship to disease cannot currently be classified confidently.