Sentence:  Pope Pius 12 , declared in 1951 that it is possible to be exempt from the normal obligation of parenthood for a long time and even for the whole duration of married life , if there are serious reasons , such as those <WSD>often</WSD> mentioned in the so-called medical , eugenic , economic and social `` indications '' . 
Meaning of the word marked with <WSD> tags:  many times at short intervals