Several years ago, the questions became a serious issue for me, as I planned to get married. Because I had been told Crohn’s is genetic, I thought that meant I would most likely pass it down to any offspring. I decided to do some research to find out what, exactly, that likelihood was.
So I snuck into a medical library and downloaded every paper I could find about IBD genetics. That was more than ten years ago, so I have updated those results with anything more recent I could find. (Sources are identified with numbers in parenthesis — like this: (1) — and listed at the bottom of the post.)
True, Ulcerative Colitis and Crohn’s disease are partly caused by genetics — which means they can be passed down in families. But unlike strictly ‘genetic’ diseases, there is also an environmental component to the disease, the exact nature of which is still unknown (there are lots of theories).
It also helps to know that Crohn’s and UC are ‘complex’ and ‘polygenic’. ‘Complex’ means that the genetics of the diseases do not follow the same pattern of inheritance as simple genetic attributes, like eye color or Huntington’s disease.
‘Polygenic’ means that there are a number of genes — maybe 163 or more — linked to IBD (there are about 200,000 genes in human DNA total). That doesn’t mean you need all 163 genes, or that there are 163 different kinds of IBD.
What this means for would-be parents is that the odds of a parent passing IBD down to a child are fairly low — around 1 in 10. This was much lower than I expected, although the estimates vary somewhat. It’s also possible for two parents with IBD to have kids that are free of the diseases.
The rest of this is going to be number-intensive, so let’s start with baseline numbers: the probability of any given person in the U.S. developing Crohn’s is 1 in 25,000; for UC, it’s 1 in 1,000 (3). In percent terms, that’s .004% for CD and .01% for UC.
The biggest risk factor for developing one of the diseases is, of course, having an affected family member. Several studies suggest that heredity is more important in CD than in UC, usually by a few percentage points. (1, 4, 5, 6)
This doesn’t mean that a person needs an affected family member to get the disease; plenty of people are the first in their family, including me. A recent study from Denmark — using the entire Danish population — suggests that not more than 12% of IBD cases can be identified as ‘family cases’.(8)
It makes a difference which family member has the disease. If the relative is a sibling with CD, the risk ranges from 13% to 36%; for UC, it’s 7% to 17%. (1) The high end of those estimates may come from studies of identical twins, so for non-twins it’s probably closer to the low end.
So now for the main question: kids. A strong study from Belgium found that for first-degree relatives (i.e. siblings or children) of CD affected people, the risk is 4.8% for IBD in general and 3.9% for CD specifically. The risk for children was highest, at 10.4%. (5)
Based on the methodology and the large sample in this study, I think this is probably pretty accurate. The estimate of 10% — 1 in 10 — corresponds to what I have seen in other studies, as well. Since genetics is more important in UC than CD, the rate is lower for children when one parent has UC.
If both parents have IBD, the risk gets a bit higher; one study found that these children have a 33% — 1 in 3 — chance of developing the disease before age 28.(4)
Weirdly enough, the same study suggests there is a slightly higher risk — about 2 in 1000 — of developing IBD if your spouse already has or develops one of the diseases. Again, this points to the genetic/environmental interplay; it’s not that IBD is necessarily contagious, but that couples living together for long periods face the same environmental triggers.
Kids, or no?
Worry about passing the disease down is not the only reason that people with IBD choose not to have kids. There are other risks that IBD can pose in pregnancy, although those are pretty small, too. Would-be parents might also be worried about their energy levels, or their ability to support a family, or any other number of reasons.
It’s not my aim to criticize or coerce anybody who is reluctant to have kids because of IBD. Instead, I just want to show the odds are not as bad as some people (namely, me) assume.
As I say, 1 in 10 was a lot lower than I would have guessed. For me, it moved having kids from a ‘bad’ decision to an ‘okay’ decision — and made it a lot less scary.
(1) Ahmad, T. et al. Review article: the genetics of inflammatory bowel disease. Aliment Pharmacol Ther 2001; 15: 731-748.
(3) Haigh, B. Inflammatory bowel disease and innate immune response genes: the challenge of complex polygenic disorders for the clinical geneticist. Clin Genet 2006; 69: 315-318
(4) Laharie, D. et al. Inflammatory bowel disease in spouses and their offspring. Gastroenterology 2001; 120: 816-819
(5) Peeters, M. et al. Familial aggregation in Crohn’s Disease: increased age-adjusted risk and concordance in clinical characteristics. Gastroenterology 1996; 111: 597-603
(6) Orholm, M. et al. Familial occurrence of inflammatory bowel disease. New England Journal of Medicine Jan 10, 1991; 324:2, 84-88
(7) “Crohn’s Disease”. First Consult. (An online medical database accessed through the library).
(8) Moller, FT et al. “Familial risk of inflammatory bowel disease: a population-based cohort study 1977-2011”. American Journal of Gastroenterology 110:4 (April 2015); free abstract online.
Photograph “Baby Kyle” by Flickr user Julia Rosario Photography used under Creative Commons license.