Sleep deprivation and exhaustion, usually. I'm not sure anyone who has not raised children can really comprehend the constant focus it takes when they are infants. It's a 24/7 job to keep them alive in the early years, and even as they age, there is a constant fear factor when doing things we take for granted as adults. Throw in all the distractions and other responsibilities of the modern world, and it's pretty daunting. I have to remind my 8-year old to poop regularly.
Having a special needs child is even more of a stress. Imagine, that parent has spent the last 11 years being completely focused on keeping the child alive. It's tragic that such a small mistake has such devastating consequences.