I think it's clear that the reduction in teen pregnancy is indeed a big contributor to the decreasing fertility rate. I would guess the reason this doesn't get brought up in discussions about how to _increase_ the fertility rate is that reversing the trend on teen pregnancy is just really not a palatable solution to many people. Although there are some, usually on the religious right, who advocate for banning contraception, teaching abstinence-only sex education, etc., which would most likely have the effect of reversing the teen pregnancy trend.