Marital Status or Number of Children
It seems logical that your ability to manage a household full of the chaos of multiple kids would make you an ideal job candidate, since you can put those skills to work in an office environment that is sedate by comparison. The problem is, employers don't see things the same way. When they see an employee with kids, they see an employee who's going to take a lot of sick days to take care of them; who's going to ask to leave early to pick up Timmy from soccer practice, and who's going to need a very expensive health insurance plan.
It can work the other way, too. Employers might dislike people who don't have kids, or see a young married woman as someone who will undoubtedly be taking maternity leave within a year or so.
In the end, it's illegal for a company to discriminate based on a candidate's family situation. Don't give them the temptation to do so.