Beyond this, and verging on Future History territory, the only way I can imagine fertility rates being boosted will be to expand life expectancy and fertility windows generally. In the modern world, people are increasingly having to postpone childbearing well into their 30s or even 40s, thanks to the requirements of their lives. By that point, the window for prospective parents is limited.
Imagine, if you would, a setting where thanks to some biomedical intervention people not only live longer and healthier lives (into their 90s, say) but their fertility windows are made to stretch into their 50s. This would give people lots of extra time to not only have children later in life but to have long healthy lifespans with them.