In Genesis 25:1-2 we see:
"Abraham had taken another wife, whose name was Keturah. She bore him Zimran, Jokshan, Medan, Midian, Ishbak and Shuah".
But even before that in Genesis 17 it says:
'Abraham fell facedown; he laughed and said to himself, “Will a son be born to a man a hundred years old? Will Sarah bear a child at the age of ninety?” '
Then in Hebrews 11:12 we read:
"And so from this one man, and he as good as dead, came descendants as numerous as the stars in the sky and as countless as the sand on the seashore. "
If the birth of Isaac is considered a miracle when Abraham was as good as dead, how is the birth of children for him in Keturah explained? A normal reading implies that the marriage with Keturah subsequent children happens after the birth of Isaac.
All quotations from the NIV.