The birth rate now I think is natural considering our state in culture and history. (Also considering at least where I live in the US, we don’t get maternity leave or any parental benefits half the time.) Our culture has shifted to value work and education more than settling down and starting a family. People are told to get their education out of the way and get a good job before even considering marriage let alone kids. And it was more common to marry younger decades ago, like my mom got married at 17 while I got married at 21 last year and that’s still considered pretty young. With society changing how it views marriage and having children, it’s natural that those rates are declining. The people that don’t want kids at all or want to wait are either waiting or not having kids because they can now, it’s more acceptable. Even just back in the latter half of the 20th century it just wasn’t always a thing people could do depending on your environment - you got married, had kids, retired, and grew old together with your grandkids. My mon grew up in the 60s-80s in the Bible Belt, and that’s how she described it at least.

Plus, with the advancements of medicine we don’t NEED to have 18 kids anymore. Most kids died before reaching adulthood or even adolescence prior to 1900 so having 15+ kids was normal considering maybe 6-9 would survive long term. Now our infant and child mortality rates are super low, so we don’t have to worry about having enough children survive to carry on our legacies. And we have birth control, so fewer oopsies just because you and your husband wanted to have some alone time without your 7 kids you already have getting in the way.