This depends on whether knowing the result would change anything for the child during childhood. Professional guidelines, including those from the American Society of Human Genetics and the European Society of Human Genetics, follow one main principle: test children when there is a medical benefit in childhood, and otherwise wait until they can make their own choice.

• Conditions where childhood testing is recommended Some conditions need screening or treatment in childhood. Familial hypercholesterolemia is the clearest example. The European Atherosclerosis Society recommends testing children from around 5 years of age and, for those affected, starting cholesterol-lowering medication from around 8 to 10 years of age. Starting early lowers the total cholesterol exposure over a lifetime and greatly reduces the risk of early heart disease. Familial adenomatous polyposis, a hereditary condition that causes hundreds of bowel polyps, is another example, since bowel checks usually begin around 10 to 12 years of age. Some inherited heart rhythm conditions also fall into this group.
• Adult-onset conditions For conditions such as BRCA1, BRCA2, and Lynch syndrome, screening does not usually start until adulthood. Guidelines generally recommend waiting until the child is an adult and can decide for themselves. This protects the child’s own right to choose whether they want this information, and when.
• Carrier status for recessive or X-linked conditions Being a carrier of beta-thalassemia or Duchenne muscular dystrophy does not affect a child’s own health in most cases, but it matters for their future pregnancies. Carrier testing is usually offered in the late teenage years or adulthood, ideally before marriage or pregnancy planning, when the young person can take part in the decision.

Parents often feel a strong urge to “just know”. That feeling is completely understandable. A genetic counselor can talk through the timing that fits your family’s condition, your child’s age and maturity, and what the result would change.