Why is Sarah's pregnancy so remarkable?

Question

Genesis 17:17 ESV states:

Then Abraham fell upon his face, and laughed, and said in his heart, Shall a child be born unto him that is an hundred years old? and shall Sarah, that is ninety years old, bear?

The Bible says that Sarah was 90 years old when she had Issac, which seems remarkable--and by modern standards would be--but Sarah died at the age of 127, meaning 90 was something like...a Biblical 60? And Sarah died young(ish)--Abraham's father was 205 when he died--so really she might have only been 40-something, relatively speaking.

Abraham was 175 when he died, so for him, 100 years old was merely middle aged.

Sarah might be on the cusp of menopause at 90, but why was Abraham doubting his own fertility?

EDIT: I'm aware that Abraham--and even the narrator--thinks that both he and Sarah are old to conceive. I'm trying to reconcile this with all the other evidence that seems to contradict this conclusion.

Answer 1

The answer is in one verse, Genesis 18:11, NIV:

Abraham and Sarah were already very old, and Sarah was past the age of childbearing.

Other translations render the fact of menopause more directly, ESV:

Now Abraham and Sarah were old, advanced in years. The way of women had ceased to be with Sarah.

This verse is presented in the voice of the narrator of the story, not by one of the protagonists, so we are assumed to accept it as fact - that Sarah was post menopause and Abraham was old. The implication is clear.

That an elderly woman past menopause should conceive is (in Christian terms) nearly as remarkable as a virgin birth. That's the message the text is sending us.

Answer 2

You're looking at only one aspect—the age of death, and extrapolating from that single data point.

But we don't know if menopause, for example, comes after a certain percentage of life expectancy or after a certain number of decades. It is likely the latter because even now, women who have late menopause (over 55) only live an average of two years longer than women who go through menopause in their 40's [source]. Regardless, we are explicitly told that Sarai was post-menopause.

But Abram was concerned about the possibility of having a child because he had a valid reason to be that goes beyond the single metric of life expectancy.

He would have looked back at the experience of his recent forefathers:

• Arphaxad fathered Salah at 35
• Salah fathered Eber at 30
• Eber fathered Peleg at 34
• Peleg fathered Reu at 30
• Reu fathered Serug at 32
• Serug fathered Nahor at 30
• Nahor fathered Terah at 29
• Terah fathered Haran at 70 before he had Abraham
• Abraham fathered Isaac at 100

Almost all of them began having children in their early 30's.

Why Abraham's father Terah started so late is up for speculation, but it's not unreasonable to think that Abraham would have been concerned about not having any children by this point, even far past his father, who was already quite old before he had children.

He would have been a major outlier, and any man would have doubt in such a scenario.