“...it is a trend that has been going on since 1950...”
Isn’t that about the time that birth control became available and fairly effective?
It did not come along until 1960 and we were already on a downward slope.
In fact in the early sixties we had a slight bump to the positive. Our biggest drop was in 1978.
Now if you are talking about condoms, those had been around since the 1920s. Actually before but they were widely available in the 1920s.
Birth control ie the pill became available in the sixties. Before that it was pretty much condoms, abortion, and in some countries sterilization campaigns. The historic pattern is that as economic prosperity rises, birth rates fall. By 1950, following the aftermath of WWII, there was a burst of global economic activity including trade, capital investment, and decolonization.