Two layers before birth: screen, then confirm
Prenatal work follows the same screen-then-confirm logic as the rest of medicine. The first layer is a non-invasive prenatal screen: a simple maternal blood draw that reads fragments of placental DNA floating in the mother's blood to estimate the chance of common chromosome problems like trisomy 21. It is safe and sensitive — but it is a screen, so a high-risk result needs confirming.
The second layer is true prenatal diagnosis, which samples fetal cells directly. Chorionic villus sampling (CVS) takes a tiny piece of placenta around weeks 10–13; amniocentesis draws a little amniotic fluid from about week 15 onward. From those cells a lab can build a karyotype — a full picture of the chromosomes — to confirm an aneuploidy such as Down syndrome, or run targeted gene tests for a condition already known to run in the family. These are diagnostic, not screens; they carry a small procedure-related risk, which is exactly why they are offered, not imposed.
The heel-prick: catching treatable conditions in time
Newborn screening is one of public health's quiet triumphs. A few drops of blood from a baby's heel, taken in the first days of life, are tested for a panel of rare but serious conditions. The panel is chosen by one strict rule: only conditions where finding it early changes the outcome. The classic example is phenylketonuria (PKU) — a baby who cannot break down the amino acid phenylalanine will suffer brain damage on a normal diet, but grows up healthy on a special low-phenylalanine diet started in the first weeks. The test costs pennies; the diet prevents a lifelong disability.
- A heel-prick collects a few blood spots onto a card in the first 24–72 hours of life.
- A lab screens the spots for a panel of metabolic, endocrine, and blood disorders.
- Most results are normal; a flagged result triggers a prompt confirmatory diagnostic test, not a diagnosis on its own.
- Confirmed cases start treatment — a diet, a hormone, monitoring — before symptoms ever begin.