Another possible problem is the following: In the last 50 years, the total tax burden of the average family has roughly doubled. This has caused both parents to work to maintain their lifestyles.
At the same time, the government safety net has taken the place of the family safety net.
Therefore, cut the size of government and the resulting tax burden half.
If only we could elect a GOP President and Congress. /oh, wait, we already tried that. Things got worse.
While government policies supportive of families (especially two-income families, the ones that are doing less reproducing) will indeed enlarge family size, it can only go so far. Industrialization of society has changed the basic reason behind having a flock of kids.
"Be fruitful and multiply," works for an agricultural society, where each mouth to feed came with a pair of hands, and a pair of feet to bring in the harvest, and where a substantial number of offspring are going to die prior to reaching reproductive age, but that rule just doesn't make sense in the modern world.