Chances of Another Child with Williams Syndrome?

If you have recently learned that your child has Williams syndrome—or if you have the condition yourself—you may be wondering what this means for your future family planning. For the vast majority of parents who do not have the condition themselves, the chances of having another child with Williams syndrome are very low, typically less than 1% ^[1].

Why Is the Risk So Low for Most Parents?

Williams syndrome is caused by a missing piece of genetic material (a microdeletion) on chromosome 7, which usually includes the ELN (elastin) gene ^[2][1]. In almost all cases, this missing genetic material is a de novo mutation ^[1]. De novo means “new” — the change happened entirely by chance in the egg or sperm before conception, or very early in the baby’s development. It is an isolated, random event.

Because this genetic change is new to the child, the parents typically do not carry the deletion in their blood or general body cells. The reason the recurrence risk is stated as “less than 1%” rather than “zero” is to account for a rare possibility called germline mosaicism. This means a small number of a parent’s egg or sperm cells might carry the genetic change, even though the rest of their body does not. Still, the overall risk of the exact same event happening again in a future pregnancy is extremely small ^[1].

When a Parent Has Williams Syndrome

The situation is different if a parent actually has Williams syndrome. In this case, the condition follows an autosomal dominant inheritance pattern ^[3]. This means that a person with Williams syndrome has a 50% chance of passing the condition on to each of their children ^[3]. This 50% chance applies to every pregnancy, regardless of the child’s sex.

Important Safety Note for Women with Williams Syndrome: Because Williams syndrome often involves cardiovascular issues like narrowed blood vessels (such as supravalvular aortic stenosis), pregnancy can place a dangerous strain on the heart. Women with Williams syndrome should always consult with a cardiologist before becoming pregnant to ensure it is physically safe.

The Value of Genetic Counseling

Whether you are an unaffected parent or an adult with the condition, meeting with a genetic counselor can be highly reassuring and empowering ^[4]. A genetic counselor or medical geneticist can:

• Explain specific lab results, such as a chromosomal microarray or FISH test, which are genetic tests used to look closely at chromosomes ^[5].
• Order blood tests for unaffected parents. This is not because they are expected to have the standard microdeletion, but to check for rare, hidden chromosomal variations and confirm the genetic change in the child is truly a new (de novo) event ^[4].
• Discuss specific testing options for future pregnancies. This may include prenatal screening or pre-pregnancy options like in vitro fertilization (IVF) with genetic testing, allowing families to make informed choices ^[5].