Why was the birth of Isaac to Abraham and Sarah a miracle?

A:Genesis 17:17 says that Sarah was 90 years old when God said to Abraham that she would conceive and bear a son. On this account, she would have been perhaps 90 or 91 when Isaac was born. Abraham was a hundred years old when God made this promise, and still 100 years old when Isaac was born.

These seem improbable ages for them to have a son, and in the absence of another explanation would seem to have been a miracle, as were the great ages to which they are said to have lived. That other explanation is that the Book of Genesis attributes to people ages - like 90 and 100 - that are easy to remember for oral story-telling, or which are based on the number 17. If we look at the lifespans of Abraham, Isaac and Jacob, The Bible says that:

1. Abraham lived to 175 (5 X 5 X 7)
2. Isaac lived to 180 (6 X 6 X 5)
3. Jacob lived to 147 (7 X 7 X 3)

• Each lifespan involves a perfect square (5, 6, then 7 in a numeric series),
• the third factor also forms a series (7, 5, 3)
• in each case the sum of the factors is 17.

If we look at the lifespan of Sarah, we find that Genesis says she lived to 127 years, which is the sum of these consecutive square numbers plus 17 (127 = 52 +62 + 72 + 17).

These ages are so well-contrived that we should accept that even if Abraham and Sarah were real, historical people, they would not have lived to these great ages, nor would they have really had a child at exactly 100 and 90 years old, respectively. The miracle is in the story-telling.