There might be some scientific factors that are lowering fertility, but I think money is a larger issue. It's less affordable to have children than it used to be and with cost of living so high people can barely afford to look after themselves let alone children.
This is the answer. If you want to increase the fertility rate then you have to make having kids more affordable. Back in the 50s & 60s a one income household was easily supporting dad, mom, and 2+ kids. Now, to live that same lifestyle both parents need to work but two working parents means you need to pay for childcare which is a HUUUUUUUUUUUUGE expense. Having one kid cuts down on that expense a lot.
Then you also have the career considerations of mom. The "workplace ecosystem" has not adapted or adjusted nearly enough to actually accommodate women who want to have children. Having a kid is a career killer for women in so many fields especially if you are still trying to establish a professional reputation. That's why so many people delay having a family. Women who have kids before they've securely established themselves professionally might as well give up on any sort of career advancement.
Bottom line, our economy is hostile towards families. This is why we need universal Pre-K/childcare and universal paid parental leave of at least 6 months. I guarantee you that once women know they don't need to commit career suicide to have a kid then the fertility rate will skyrocket.
