I have recently come to the (re)realization that bishops are the authoritative teaching body of the Church. As such, it is their responsibility to properly and effectively teach such sticky subjects as the regulation of births. However, those teachings must be in accord with the Bishop of Rome and magisterium of the Church, so I still think there is merit in exploring the relevant papal documents. Let us then continue by hearing the thoughts of Pope Pius XII.
I had thought that Pius XII had written an encyclical about contraception. As it turns out, the only statements he made about the subject were in in various allocutions (addresses) to associations of doctors and the like. These don't carry nearly the same weight as encyclicals and are certainly not infallible. An exploration of the doctrinal authority of papal allocutions can be found here, but I cannot vouch for its accuracy. Nevertheless, Paul VI quotes from these addresses extensively in Humane Vitae, thus lending some of theauthority of an encyclical. I searched for the texts of these addresses and only found the 1951 Address to Midwives on the Nature of Their Profession and the 1958 Address to Officers and Representatives of the Associations for Large Families-of Rome and of Italy. If anyone knows where I might find the rest of them, I'd be indebted. Anyhow, here's the address to midwives.