The driver of birthrates is really quite simple: when people are given a choice, most choose to have fewer (or no) babies.
I don't think it's quite that simple, most due to the evidence provided by TSMC. Children born to TSMC employees in Taiwan accounted for approximately 1.8% of all babies born nationwide in 2023, even though TSMC employees make up only about 0.3% of Taiwan's population. The usual reasons why women in OECD countries have low birth rates are turned on their head here. TSMC employees are highly educated, and well paid. So while the women in Chad probably no choice if the wanted to be sexually active, these TSMC employees could make any choice they wanted, and they chose to have children.
What stands out is TSMC spends a lot of money in supporting it's mothers. For example, creches are located right beside the workspaces, they provide child care and social structures. This corporate culture that is very supportive of women and motherhood does not come cheap, and it's difficult to see large direct benefits to the bottom line. I've never seen it in Western companies, that are drive purely by the bottom line. It's amazing to see it in TSMC.
In contrast, the highly patriarchal Japanese society is well known for the reverse. Japan consistently ranks near the bottom of the World Economic Forum’s Global Gender Gap Index (118th out of 148 in 2025). The gender pay gap in Japan is approximately 22%, the worst in the G7. Women are frequently pushed into "non-regular" (part-time or contract) work after having children, making it difficult to return to a career track. It's not entirely a surprise that a society that does not support women had a chronic birth rate issue.
But the light has finally switched on. The government is almost in panic mode now about the demographic time bomb. In 2023 (or so?) the government significantly increased the Childbirth Lump-Sum Allowance to ¥500,000 to cover delivery costs. They also expanded monthly child allowances and eliminated income caps to ensure more families receive help. I'd be surprised if this effort stops until it sees some success, but they are trying manufacture societal change and that will take a decade or two.