And the children of Israel were fruitful, and increased abundantly, and multiplied, and waxed exceeding mighty; and the land was filled with them. The children of Israel were fruitful - פרו paru, a general term, signifying that they were like healthy trees, bringing forth an abundance of fruit.
And increased - ישרץ yishretsu, they increased like fishes, as the original word implies. See Genesis 1:20 (note), and the note there.
Abundantly - ירבו yirbu, they multiplied; this is a separate term, and should not have been used as an adverb by our translators.
And waxed exceeding mighty - ויעצמו במאד מאד vaiyaatsmu bimod meod, and they became strong beyond measure - superlatively, superlatively - so that the land (Goshen) was filled with them. This astonishing increase was, under the providence of God, chiefly owing to two causes:
1. The Hebrew women were exceedingly fruitful, suffered very little in parturition, and probably often brought forth twins.
2. There appear to have been no premature deaths among them. Thus in about two hundred and fifteen years they were multiplied to upwards of 600,000, independently of old men, women, and children.