Sentence:  This means that such factors as the health of the parents , particularly the mother , their ability to provide their children with the necessities of life , the degree of population density of a country and the <WSD>shortage</WSD> of housing facilities may legitimately be taken into consideration in determining the number of offspring . 
Meaning of the word marked with <WSD> tags:  the property of being an amount by which something is less than expected or required