The short answer
In any pregnancy, whoever the parents are, the chance of a serious genetic or congenital condition is about 2–3%. For first cousins it is roughly double: about 4–6%. In other words, an extra two children in every hundred — so between 94 and 96 out of 100 are not affected by these conditions. Nearly all of the extra risk comes from recessive conditions, and it is concentrated in the minority of families that actually carry one.
Where the doubling comes from
First cousins share a pair of grandparents, and roughly one eighth of their DNA is the same DNA, inherited from those shared ancestors.
That only creates a problem for recessive conditions — the ones that need two changed copies of the same gene before anything goes wrong. A rare gene change is rare because two copies of it almost never meet in the same person. Being related breaks that. If one of your shared grandparents carried one, it can travel down both sides of the family and arrive in the same child twice.
Everything else is unaffected. Dominant conditions, chromosome conditions such as Down syndrome, and the common conditions with many causes are no more likely for a cousin couple than for anyone else.
The average hides two very different families
This matters more than the headline figure.
The 4–6% is an average across all first-cousin couples, and the reality underneath it is nothing like even:
- Most cousin couples do not share a rare recessive change at all, and their real risk sits close to the ordinary background.
- A minority do share one — and for them the chance of that specific condition is 25% in every pregnancy, far above the average.
The average is the number to quote when you know nothing about a family. The moment you know something — an affected relative, a child who died young and undiagnosed, a condition that keeps appearing in the wider family — the average stops being the right answer, usually by a long way.
Family history costs nothing, and it is the single most powerful piece of evidence available before any test is ordered.
What pushes the number higher
| Feature | What it does |
|---|---|
| A child or relative already affected by a recessive condition | The biggest single factor. It means the gene change is definitely in the family, not just possible. |
| Cousin marriages repeated over several generations | The effect adds up. A cousin couple whose own parents were also cousins are more closely related than ordinary first cousins. |
| Double first cousins — sharing all four grandparents | Twice the shared DNA of ordinary first cousins, so twice the extra risk. |
| A community where one condition is common | Makes it more likely that the shared change is one of the common local ones — which also makes targeted testing very effective. |
| Older parents, previous losses, repeated miscarriage | Nothing to do with being related, but part of the same conversation. |
What to actually do
- Write the family history down properly, both sides. Three generations if you can get them, including children who died young and relatives remembered only as “always sickly”. This costs nothing and often changes the answer more than any test.
- Both of you have carrier screening. Because the extra risk is recessive, finding out which changes you each carry turns a population average into your own number. It is most useful before a pregnancy rather than during one.
- Ask for genetic counselling if anything turns up — a shared carrier result, an affected relative, or a condition known in your community. That is where reproductive options, including testing embryos created through IVF, get discussed.
Marriage between first cousins is common in many parts of the world and its legality varies from place to place. That is a legal and social question rather than a medical one, and it does not change any of the arithmetic above.
Common questions
- Is it safe for first cousins to have children?
- The great majority of these pregnancies produce healthy children. The increase is real and worth knowing about — roughly two extra cases in every hundred — and it is much smaller than the figure most people have in their head. Carrier testing can narrow it further.
- What about second cousins?
- Second cousins share far less DNA — about a thirty-second of it — and the extra risk to their children is four times smaller again. In practice it is close to that of any unrelated couple.
- Does it matter which side of the family we are cousins on?
- For most conditions, no — only how closely related you are. It can matter for X-linked conditions, because the X chromosome never passes from a father to a son.
- We already have a healthy child. Does that lower the risk for the next one?
- A little. A healthy child is weak evidence that you do not both carry the same change — but only weak, because 3 in 4 children of two carriers are unaffected anyway. Otherwise each pregnancy starts again.
- Should we get tested even though nobody in the family is affected?
- Carrier testing is usually offered to related couples whatever the family history, precisely because no family history is not reassuring here. Most children with a recessive condition are the first case their family has ever had.