Sarah, the lawful wife of Abraham, did not conceive a child until she was will into old age.
In fact, the Bible writes explicitly what her condition was:
"Now Abraham and Sarah were old and well stricken in age; and it ceased to be with Sarah after the manner of women." (Genesis 18: 11)
What is the importance of recognizing the physical impossibility for Abraham and Sarah to have a child?
They could not have the child on their own. They needed the miracle-working grace of God:
"[Abraham] against hope believed in hope, that he might become the father of many nations; according to that which was spoken, So shall thy seed be." (Romans 4: 18)
It's all about believing what God said, not taking our circumstances or feelings as the final authority.
Now, back to Sarah. She judged God faithful to fulfill His promise.
We have a more sure certainty of God's promises in our life.
Why? We have Jesus!
"16For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. 17For God sent not his Son into the world to condemn the world; but that the world through him might be saved. " (John 3: 16-17)
He was promised to come from the beginning, after the fall of man
"14And the LORD God said unto the serpent, Because thou hast done this, thou art cursed above all cattle, and above every beast of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life:
"15And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel." (Genesis 3: 14-15)
"24But for us also, to whom it shall be imputed, if we believe on him that raised up Jesus our Lord from the dead; 25Who was delivered for our offences, and was raised again for our justification." (Romans 4: 24-25)
"Of which salvation the prophets have inquired and searched diligently, who prophesied of the grace that should come unto you: 11Searching what, or what manner of time the Spirit of Christ which was in them did signify, when it testified beforehand the sufferings of Christ, and the glory that should follow. 12Unto whom it was revealed, that not unto themselves, but unto us they did minister the things, which are now reported unto you by them that have preached the gospel unto you with the Holy Ghost sent down from heaven; which things the angels desire to look into." (1 Peter 1: 10-12)
We can trust that God is faithful, because He fulfilled the greatest promise, to send the Messiah, Jesus, to be our Savior.