The short answer
A de novo gene change is one found in a child but not in either parent's blood test — it happened for the first time in the egg, the sperm or the very early embryo. For most families the chance of it happening again is low, and a figure of about 1–2% is often quoted. That is only a starting point. The real chance can be well under 1 in 1,000 for a genuine one-off, or considerably higher if a parent carries the change in some of their egg or sperm cells. The condition and the family's exact test results decide which.
What “de novo” actually means
Every child is born with a handful of gene changes that neither parent has. Almost all of them do nothing at all. A de novo change that matters is one that happened to land in a gene that explains a condition.
It may have happened:
- in the one egg or sperm that made this child;
- just after the egg and sperm joined;
- in a small group of a parent’s cells, too few for a blood test to see;
- in some of a parent’s egg or sperm cells, but nowhere else in their body.
All four can produce the same sentence on a report — “not detected in the parents” — but they give quite different chances for another pregnancy.
Find your situation in this table
| What the family’s tests show | What it usually means for another child |
|---|---|
| The change is in the child only | Usually low, but not automatically zero |
| The same change is clearly present in a parent | The ordinary inheritance rules apply — often 50% for a dominant condition |
| The change is present in a parent at a low level | Higher than the usual 1–2% estimate |
| Two affected children, both parents negative on blood tests | Germline mosaicism becomes a strong possibility |
| The change happened after the child’s earliest development | The chance for brothers and sisters may be close to that of any other family |
This is a guide to the reasoning, not a calculator. The gene involved and the type of change can both alter the interpretation.
Why people quote one or two per cent
Genetic services have long used a figure of about 1–2% after an apparently new dominant or X-linked change. It is deliberately cautious: it leaves room for the possibility that a parent carries the change in some egg or sperm cells.
It does not mean every family has exactly that risk. More sensitive testing now shows that many changes are genuinely one-off, with a repeat chance far below 1%. In a smaller group of families a parent does carry it in some cells, and there the chance can be several per cent or more.
The summary worth remembering: low is not zero, and a general percentage is not your result.
How healthy parents can have two affected children
A person can have groups of cells with slightly different DNA. This is called mosaicism.
If the change is present in some of a parent’s egg or sperm cells but absent from their blood, that parent is completely well, tests negative, and can still pass it on more than once. It explains the otherwise baffling family with two affected children, healthy parents and normal parental tests. The change did not necessarily happen twice — one parent was quietly carrying it all along.
When a parent carries it at a low level
If the change happened very early, while a parent was themselves an embryo, it can be present at low levels in their blood, skin and reproductive cells. That parent may have no symptoms at all, or a very mild version of the condition.
An ordinary test can easily miss a change present in only a small share of cells. Finding it raises the chance for another child, and can matter for the parent’s own health as well.
What it means for the affected child’s own future children
“De novo” describes how the change arrived in this generation. It says nothing about the next one.
If your child has a dominant change in all of their cells, each of their own children will usually have a 50% chance of inheriting it — the same as any other dominant condition. So the parents’ chance of another affected pregnancy can be very low while their child’s own future chance is much higher. These are two different questions and they deserve two different conversations.
Questions worth taking to an appointment
- Was the change actually tested for in both of us, or just assumed to be new?
- Could the test have picked up a change present in only a few cells?
- Is mosaicism known to be common in this particular condition?
- Does the result change our child’s own chance of passing it on?
- Is testing available before or during another pregnancy?
If another pregnancy is on the cards, a genetics service can explain whether testing during pregnancy, or testing embryos created through IVF, is possible. A small chance can still be tested for; whether to use that test is a personal decision, not a medical instruction.
Common questions
- Does de novo mean the condition is not genetic?
- No. The condition is caused by a gene change — "de novo" only means the change was new in the child rather than passed down from a parent.
- Did we do something to cause it?
- No. New gene changes happen in every pregnancy, in every family. Most do no harm at all; occasionally one lands in a gene where it matters. Nothing either parent did or failed to do caused it.
- Can my child pass it on to their own children?
- Often yes. If the change is present in their egg or sperm cells, a dominant change can usually be passed on with a 50% chance in each pregnancy, even though it was new in them.
- Why isn't the chance zero if both parents tested negative?
- Because a parent can carry the change in some of their egg or sperm cells without it showing up in a blood test. This is called germline mosaicism, and it is the reason a small number of families have more than one affected child.
- Can we get a more personal number than 1–2%?
- Sometimes. More sensitive testing of both parents, and occasionally testing a different sample, can move a family well above or well below the general estimate.