The definition of pregnant is "abounding in fancy, wit, or resourcefulness," "rich in significance or implication," "containing unborn young within the body," or "having possibilities of development or consequence." Milton is now metaphorically speaking depicting a rape.
Milton chooses to make "these" which he is obtaining from the deep pregnant with "infernal flame." Infernal by definition is, "of or relating to a nether world of the dead," or "of or relating to hell." Milton could have easily chosen another word in place of pregnant, but this allows the reader to envision darkness and evil growing and maturing over a duration of time. Time, effort and patience is involved in pregnancy. This shows that the evil power that will come forth is going to be awesome in its destructive power. Milton's use of the both pregnant and infernal together immediately makes me think of the curse of Eve for eating from the tree of knowledge.
Woman was supposedly cursed with the pain of childbirth for her giving into temptation by the serpent. Milton was a firm believer in tradition when it came to writing. In every epic the use of tradition is a must especially when trying to justifying the ways of God to man.
The product of this birth will be the "implements of mischief." which is a direct reference to the hell dogs that come from Death raping Sin.