The Awful Mathematics of Trisomy

Most of you have probably forgotten you even have me on your feed.  I promise normal service will be resumed shortly.  There are a couple of reasons that I’ve not been posting, but easily the most important is that I discovered my wife was pregnant.  It turns out there are one hundred things an expectant father needs to do, not least a great deal of shopping.  However, I’ve been meaning to write this for some time, and I hope someone will find it important.

If you don’t know about Trisomy, it’s one of the worst pieces of news expectant parents can get.  The most common kind, Down’s Syndrome, means that your child will suffer from severe mental impairment, have difficulty with fine motor skills, run a high chance of heart disease and will probably need to live in sheltered housing their entire life.  The less common versions are worse, with only 1% of babies born with Edwards’ Syndrome living to age 10.  Most have died 15 days after birth.  There’s no cure; these are genetic conditions.  The only thing that can be done to prevent it is to abort the fetus.  Even someone who is rabidly pro-choice would agree that they’d rather not have to make that dreadful choice themselves.

Now let’s talk about the maths.  In the hope this information is useful, I’m going to use the actual statistics for my child.  Here’s how it works: the baseline probably of Down’s Syndrome for my wife and I is 1 in 200.  That is way too high and there is nothing that can be done about that.  However, at 3 months, doctors can perform ultrasound scans and check for certain physical abnormalities that are characteristic of the syndrome.  This is an imperfect process, both because of human error and because not all children with the condition exhibit these symptoms.  However, the scan found nothing, and our chances were reduced to 1 in 4000.

However, here’s where the maths turn nasty.  There’s also a 1 in 8000 chance of my baby having a more rare form of trisomy.  One in 8000 sounds like a very small number indeed, but by splitting out the probabilities they’re hiding the combined number, which is something like 1 in 2600 that my child has some form of trisomy.*  These odds may sound small if you have healthy children or no children, but I can assure you that when it’s your kid, those odds aren’t really something you want to live with.
So, what could you do?  Well, there’s a direct genetic test that can establish beyond doubt whether there’s a genetic abnormality.  However, there’s two problems with it.  The first is that, if there is a problem, there’s still no cure.  Your only option would be an abortion, so unless you and your partner are comfortable with that there’s absolutely no point in doing the test.  The second is that the test itself carries a whopping 1% chance of aborting the fetus.  I didn’t like the odds 1 in 2600, I really don’t like the sound of 1 in 100.  So, in practice, you would never use this test unless you had already made a decision to abort and the 3 month scan revealed cause for concern.  Worse, the chance of the test going wrong and the chance of a genetic abormality are completely independent.  This means that if the test did go wrong, the chances would be something like 99.96% that the baby had been perfectly healthy.

Is There Anything That Can Be Done?

As you can imagine, we didn’t elect to take the test.  Now, everything that I’ve told you so far has been, ultimately, stuff you can’t do anything about.  It won’t even help you make a decision, because it’s ultimately not an intellectual thing.  However, you can in fact reduce the chance of your future child having a genetic abnormality.  You can have children early.

The base chance of trisomy or any other birth complication basically starts going up the moment you hit adulthood.  It’s overwhelmingly dependent on the age of the mother.  By the time the mother is over 35, the base probabilities are higher than you’d like to countenance.  Sadly, in the UK that’s pretty much the standard state of affairs amongst people in my socio-economic group.  Basically, we’ve all got it wrong.  If you’ve hit 30, it’s time to think very hard about whether or not you want to have kids.  If you’re not currently with someone with whom you can envisage having children, you should be thinking very hard indeed.  Even if you spent most of your twenties trying not to have a child, you might discover that it takes much longer than you’d expect (and with every passing year, your chances of successful conception go down as well).  Finally, it’s worth considering that there’s no evidence that having children earlier has any long-term impact on your career.  (Having children at all does, but by the age of 40 it won’t matter if you had them at 31 or 36.)

Having children is a big decision, and there are many things that can go wrong for them, including unprepared or absentee parents.  Still, I would encourage anyone who thinks they’ll have children “one day” to think seriously about changing “one day” to “soon” or “next year” (for more reasons than just this one).  My baby girl will be born in July, and there is a 100% chance I will love her completely.

*I’m not sure if different trisomy conditions exclude the possibility of another occurring, but it doesn’t make much of a difference to the maths either way.

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Julian Birch

Full time dad, does a bit of coding on the side.

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