I find it interesting how many of the comments here come from the bubble that most posters (including me) live in. The vast majority of Americans are not working in companies offering any maternity leave above what is required by law, and paternity leave isn't a phrase most Americans have ever heard in their life. Similarly, blaming shrinking birth rates on women becoming more educated and engaging in higher-level roles in the workforce is silly when it only applies to a small percentage of women in America. While it may be contributing in a small way to birth rates, it simply cannot be the primary cause. Most women in America are not educated and are not working in career-oriented jobs. Neither are most men. Around 40% of American adults have a college degree. When you consider how many of those degrees are completely useless when it comes to finding a job, you can probably cut the number down to around 20% of Americans holding a degree in something that is a net positive on their financial outlook. If you add in industry certifications maybe you can get that number up to 30%. Women earn slightly more degrees than men, but it's close enough to half to say around 15% of the population accounts for this theory of women working up the career ladder accounting for declines in birth rates. That just isn't statistically significant enough to be a root cause. I think it's too easy for most of us to forget that the majority of the country doesn't live like us, doesn't earn like us, isn't educated like us, and does not make decisions based on the same criteria that we do.
Based on absolutely nothing of substance, my theory for the birthrate issue is that life sucks right now for so many. People are miserable and don't have a lot of hope for the future. Happy people have more children.