Sarah Palin's reproductive judgment

The big story with Sarah Palin's son Trig is how she had him even after she knew he would have Down's Syndrome. My question is different. How responsible is it for a woman who already has 4 children to decide to have another child at the age of 43, knowing the risks of things like Down's Syndrome? And, if it was an accidental pregnancy, how does a 43-year old woman with 4 children let such an accident happen? I know that no birth control is 100%, but do you really see Sarah Palin using contraception anyway? There's a 1% chance that it was pure accident, and a 99% chance that it was just plain bad judgment.