system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does Downs Syndrome occur more than other trisomy diseases?

I get that for many chromosomes it’s probably just cause the pregnancy is unviable with a trisomy of that chromosome. However, there are a few other ones, like trisomy 18 and 13, where it is survivable to the point of being detected yet the number of pregnancies with these syndromes are significantly less than Downs Syndrome. Is there some reason why the 21st chromosome appears to be more suspectible to having an extra copy?
gpt
I do not believe that 21 is more susceptible to trisomy, but it is more compatible with life than others. Most pregnancies with any trisomy will not make it to term. People born with trisomy 13 or 18 will rarely survive the first year of life due to the laundry list of complications. 

Hope this helps!