I believe the declining birth rate is really from only two things, very closely related: firstly, starting in the 1970s, you have women taking jobs alongside their husbands to sustain the American lifestyle. This leaves them, on average, less time to raise their kids. The second thing is that women cannot move up the ladder if they have two or more kids without alienating them. This is especially true in the business world, a popular area for females, where hours can become excruciating. I went to high school near Wall Street, I'm familiar with a lot of the women working there, and specifically among the women with two or more kids, they're either stuck down the ladder (less hours, less pay) or they've not seen their kids as often as society wants them to (again, work hours).
Working mothers can't raise kids effectively, not unless they're working at home or they're working a job with normal hours. There's also a huge dynamic shift from one kid to two kids - raising one child can actually be done without damaging career prospects. It's just much easier, he/she won't get injured so easily, won't have disputes, the relationships are much easier to control. Now, two kids? Three? Five? The dynamic completely changes. They NEED a lot more work put into them. That's where you get the low hours or the full-time babysitter. In the business world, a woman is plain old done moving up the ladder once she has her second kid. It can be done, but her babysitter will know more about her kids than she will. I knew a woman with 4 kids who decided that moving up the ladder was more important than staying with her kids. Her kids didn't turn out well for it.
If we want to raise the birth rate, then we need to lower female employment. We need to encourage them to stay at home. But to afford their staying at home and to sustain our current lifestyle, their husbands need to be paid much more. Then because of our societal freedom, the women would continue getting a side job to make even more money, because we can't discriminate wages based on gender.
Me, I would love to be able to have a stay-at-home wife. I would love to be able to support a house by myself and raise a large family. For some reason, a lot of people think I will have 13 children - almost always exactly 13 children. But with our economy, it would be next to impossible to raise such a large family with only one person paying for everything. And if the wife works, then there's nobody to raise the children. I'm sure lots of people share this.... frustration.