Check the interaction between fertility and per capita income in Gabon and Congo-Brazzaville in this Gapminder chart.
Did growth in income per capita drive the increased fertility before 1970 ? Was the post-1976 decrease in fertility caused by falling income or was it the result of policy decisions ?
Notice that Gabon had, by far, the lowest fertility rate in 1950 on the continent and the highest per capita income.
I’m also intrigued by the 1978-1984 bump in congolese income per capita. Obviously it wasn’t the oil since Gabon didn’t experience the same thing (though Gabon has a bigger capacity back then). And politically, those were interesting times.