Prenatal genetic screening and diagnosis: what each test does, and when

A pregnancy comes with a sequence of genetic tests on a calendar set by the medical world. NIPT at 10 weeks, anatomy scan at 18 to 20, amniocentesis if there is a finding. Knowing which test does what – and which results actually change anything – helps you read your reports without panic and ask better questions when one comes back ambiguous.

NIPT at 10 weeks plus

Non-Invasive Prenatal Testing analyses small fragments of placental DNA found in the mother’s blood. It screens primarily for trisomy 21, 18, and 13, with sex chromosome and microdeletion options. It is highly sensitive for the common trisomies (above 99 percent) but is a screen, not a diagnosis. A high-risk NIPT result requires confirmation by amniocentesis or CVS before any major decision.

The combined first-trimester screen

Where NIPT is unavailable or too expensive, the combined screen pairs nuchal translucency on ultrasound at 11 to 13 weeks with maternal blood markers (PAPP-A, free beta-hCG). It detects roughly 85 to 90 percent of Down syndrome cases – lower sensitivity than NIPT, but it costs less and gives information that NIPT does not (the nuchal measurement also flags some structural conditions).

The anatomy scan at 18 to 20 weeks

The single most informative imaging study of pregnancy. A skilled sonographer examines the fetal heart, brain, spine, kidneys, abdomen, and limbs in detail. Major structural anomalies are detected in roughly 70 to 90 percent of cases at this scan. Findings can change everything: from “no concerns” to “we need a fetal cardiology consult and possibly amniocentesis.”

Diagnostic testing: CVS and amniocentesis

Where screening or imaging raises concern, definitive testing is available. Chorionic Villus Sampling (CVS) at 10 to 13 weeks samples placental tissue. Amniocentesis at 15 weeks plus samples amniotic fluid. Both can give definitive chromosomal and single-gene results, and both carry small but real procedural miscarriage risks (around 0.1 to 0.3 percent in experienced hands).

The decision is yours

Every test in this sequence is offered, not required. Some couples test all the way through; some decline NIPT; some accept screening but decline diagnostic confirmation. There is no universally correct path. There is a path that is right for your values and circumstances, ideally decided before you are sitting with an unexpected result.

Questions worth asking your doctor

• Which prenatal tests do you recommend for us, given my age and history?
• For each, what is being measured, and what would a positive result actually mean?
• If a screen comes back high-risk, what would the next step be?
• What are our options if a diagnostic test confirms a finding?

This essay is educational. Every patient’s situation is different – the right plan is shaped in conversation with a fertility specialist who knows the full picture.