I wouldn't even ask if a woman was considering pregnancy. I simply wouldn't hire a young woman if there's a chance that over the next five years, she's going to be gone 20% of the time (two children at 6mo a pop over five yeras) and still getting paid. That's insane. It means that woman is making 25% more than her male counterpart. Talk about sexism...
1 - Unless you're running a fast food joint where 99% of the employees are minimum wage burger flippers, then chances are you're looking for employees with particular
skills. In that case, as an employer, it's in your interests to offer employment conditions favourable to people seeking said jobs. Shock horror, the person that is best suited for the job might be a woman of child-bearing age.
2 - Most people want to settle down and have families. While this in itself is not necessarily a mark of maturity, IMO couples who plan carefully on this score are more likely to be mature people that you as an employer are interested in hiring.
3 - As a continuation in time from point 2, people who settle down and have families are probably more likely to be happier people, and happier people are generally considered to be more productive people. Which is good for your business in itself; and it promotes a healthier, less stressful work environment.
4 - Work environments that are predominantly populated by one gender tend to lead to stressful situations due to the lack of balance. Predominantly female environments tend to result in more egotistical behaviour and bitchiness, predominantly male ones also tend to result in more egotistical behaviour and aggressiveness.