Genesis 21:1 And the LORD visited Sarah as He had said, and the LORD did for Sarah as He had spoken.
2 For Sarah conceived and bore Abraham a son in his old age, at the set time of which God had spoken to him.
3 And Abraham called the name of his son who was born to him—whom Sarah bore to him—Isaac.
This is not the norm, but, God does what He wants, outside of customs of men. Sarah is visited by the LORD. He promised not only Abraham, but her also. She finally concieved and gave her husband a boy, named Isaac (laughter). They both laughed at the idea of bearing children at their age. At the exact time God said she would. God isnt about vague promises. He is very detailed and specific. Literal and in the realm of impossibility to mens abilities. They trust the LORD 25 yrs for Isaac, I can only imagine the excitement they felt, vindication, and for the first time, a proud feeling of the name God gave him. father of many.