15 March 2016

Will my next baby have trisomy 18?

Another one of those top 10 questions. Your baby has or had T18, you are fortunately pregnant again and you want to know what is the chance is that this new baby will have T18 or even another trisomy.

This is isn't a clear cut question. First of all it depends on what type of T18 your baby has/had. If it was partial T18, you and your partner should have been tested to see if you passed it on and if you did, you should have been counselled regarding the risk of T18 to future children. If your child had PT18 and you haven't been offered this, then ask for a referral to genetic counselling.

However assuming your child had full or mosaic T18. The first thing to say is that it is very unlikely you will have another child with trisomy 18. The usual quoted risk is 1% above the baseline age related risk. If you use this, risk by age is as follows (please note that if viewing on a mobile phone, you may have to turn it sideways to see the complete table):

Age at delivery Baseline risk of trisomy 18 % Risk if previous T18 pregnancy %
20 1 in 4584
0.022% 1 in 4539
25 1 in 4053 0.025% 1 in 4013 0.025%
30 1 in 2727 0.037% 1 in 2700 0.037%
35 1 in 1152 0.087% 1 in 1141 0.088%
40 1 in 336 0.298% 1 in 333 0.301%
45 1 in 84 1.190% 1 in 83 1.202%

The full list for every age from 15 to 50 years is available here.

Obviously, this chart shows the trend you would expect, with older mothers having more babies with trisomy 18 and the risk steadily increasing after age 35. However, the risk for mothers who had a previous baby with trisomy 18 is different by very little than for mothers of a similar age without that history. Very few of us worry much about a less than 0.5% chance of a poor outcome, and if you are 40 or younger, statistically your chance of having a child with trisomy 18 is less than 0.5% regardless of your history.

But are there any factors other than age and history that predispose you to having another pregnancy with  trisomy 18?

Studies looking at repeat pregnancies with trisomies have varied widely in their quoted rates of repeat. It has been suggested that some women may be prone to errors in early cell division which predispose them to trisomic pregnancies. One study quoted a woman who had three consecutive pregnancies: one with trisomy 13, one with trisomy 21 and one with trisomy 18. However, it is unlikely that this is very common, or it would be reflected in a much higher overall repeat rate in the general population.

If you have a history of multiple pregnancy losses as well as a pregnancy with trisomy, or have had two trisomic pregnancies, there is a small possibility that you might be in this group. But as pregnancy loss is very common (1 in 4 pregnancies) and trisomies also relatively common - especially in older mothers - it is likely that one or two pregnancy losses and a trisomy pregnancy, or even two trisomic pregnancies, are purely coincidental and not likely to repeat.

The truth is that nobody can tell you if your next baby will also have trisomy 18. There are no tests that can be done unless you are opting for pre implantation genetic diagnosis, and you would only have cause to ask for that if you had multiple trisomy pregnancies - and it carries its own problems. You can be reassured that the risk is low, but in the end, you have to take a chance.

'Risk for T18 if previous T18 pregnancy' table

'Risk for T18 if previous T18 pregnancy' table (by age)

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