Starting in your 30s, especially later 30s makes this extremely difficult. It takes months of perfectly hitting ovulation windows to get pregnant, and then you generally want at least ~9 months between children, so you're looking at ~2 years per child, all while fertility starts to rapidly decline as you head into your 40s.
The long and short is that a sustainable population is going to require the majority to start having children in their 20s. That figure going down has nothing to do with teen pregnancy.