Sentence:  The number of children per woman depends, as has been said, on biological and social factors which determine: (1) the frequency of births during a woman's <WSD>fecund</WSD> period, and (2) the portion of the <WSD>fecund</WSD> period--between puberty and menopause--effectively utilized for reproduction. 
Meaning of the word marked with <WSD> tags:  Highly fertile; able to produce offspring.