Very few people will understand this language, it is not clear communication, you'll have to spell it out. I asked what are the "good" motives.
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There's no exhaustive list but in general this is what Pope Pius XII mentioned:
"Serious motives, such as those which not rarely arise from medical, eugenic, economic and social so-called “indications,” may exempt husband and wife from the obligatory, positive debt for a long period or even for the entire period of matrimonial life."
http://www.papalencyclicals.net/pius12/p12midwives.htm.
Serious risk of death to the mother seems an obvious one. Or, if the family would literally become homeless if another mouth needed to be fed. As was mentioned earlier, it was not the sort of things couples could just "do"-- they would need to consult with their pastor so he could gauge their genuine need and also their disposition. Putting off procreation because you need a new yacht or because you don't want to cut cable are obviously not legitimate reasons.