One of the mechanisms that oral contraceptives use is preventing implantation in the uterus. The hormones occasionally fail to prevent an ova from being produced. There is another hormone which prevents the uterine wall from accepting this possible fertilized ova.
I have heard that. But identifying allowing contraception as the precipitous watershed event in the life of a church is absurd, imho. It ignores the true root causes, which in the case of the vibrancy (or lack thereof) in a church is loss of focus on the good news.