it would depend on the severity of the problem, if it would mean the child would have a low quality of life eg. in pain the whole time and would only live a short time then maybe, also the thought of who would be able to care for the child, you have to be able to support an unhealthy baby which would mean a large emotional strain and would force you to give up work, become a full time carer and not just its mother. your example of downs syndrome though, there was an 80% chance that I was going to be a downs child but my mother didn't abort me and luckily im perfectly healthy. And,a downs child isn't the hardest person to take care of, in fact many go to college and get jobs.
overall the choice for me would be 'what the quality if life is going to be like' and 'can i support this child, both emotionally and financially'