There is an interesting post at http://www.aomin.org/aoblog/index.php?itemid=3428 that concludes that the Israelites had to average about 50 children per family per generation in order to reach the numbers in the census when they left Egypt. That astounding number would be correct if the census only counted four generations, but that's not all they counted. They counted all those who were living at the time, not just the members of the fourth generation. Genesis 15:16 says: "But in the fourth generation they shall come hither again...." Jacob went into Egypt with 70 males. The first generation after him was Levi, then Kohath, then Amram, then Moses was the fourth. But when the Israelites left Egypt Moses was 80 years old. He had children, most likely grandchildren, and probably great-grandchildren living at the time. The census counted all the males aged 20 and over who were able to go forth to war (Numbers 1), for a total of about 600,000. This didn't count those under 20, nor those too old to go to war, or infirm. If you assume that Israelites had about the same number of males and females, as we do, then their population was at least 1.2 million in this census, plus those under 20, plus those too old to go to war, or infirm. My conservative estimate is that the census counted Moses' generation (the fourth from Jacob), plus those of his children and grandchildren (the fifth and sixth generations). By this count, the Israelites would need to average only 5 children per family just to have over 1 million children *born* in the sixth generation. This doesn't even account for the living members of the fourth and fifth generations. Add them and you have at least 1.2 million. So there's no problem reaching a population of well over a million by the time the Israelites left Egypt, because the census counted not only the fourth generation of which Moses was a member, but all those who were alive at the time -- including the living members of the fifth, sixth, and possibly even seventh generations.