"Fecund" and its synonyms "fertile," "fruitful," and "prolific" all mean producing or capable of producing offspring or fruit, but each carries that sense a little differently.

FERTILE implies the power to reproduce or to assist in reproduction and growth, while

FECUND emphasizes abundance or rapidity in bearing fruit or offspring.

FRUITFUL adds the implication that the results attained are desirable or useful

PROLIFIC stresses rapidity of spreading or multiplying.

(From Merriam-Webster OnLine)