Did Sarah snicker?
The apostle Paul doesn't think so.
Sure, Sarah laughed within herself and she denied she did, perhaps because she was trying to be polite, or perhaps because she was afraid that she might get yelled at.
But Abraham laughed as well and for the same reason, namely out of gladness for becoming a father (Genesis 17:17).
Being childless was a great grief, especially in those days and especially for women. But Abraham and Sarah had received a big promise some time before: they were going to be parents for sure, even the arch-parents of a great nation.
To mark that promise, Abraham had circumcised himself, his son Ishmael and the entire village that he ruled. And to put that in perspective: previously, Abraham waged a war with 318 men that were born in his house (Genesis 14:14). Add to them their elderly fathers and an untold number of foreign-born men and you have an idea of what an operation this would have been. Abraham must have been an incredibly persuasive man to get this all done. No man will surrender a piece of his penis without having been presented with an exceptionally good reason, and no woman will have her husband incapacitated for quite some time (Genesis 34:25) for just an airily inkling.
Sarah, being Mrs. Mayor, was most likely busy for months afterwards appeasing the women folk, explaining what was going on and the whole enterprise was certainly impossible to not pay heed to or even forget.
In addition to that, Sarai was re-named Sarah, so that every time someone called her, she was reminded of the fact that God promised her a child. And when she overhears a visiting stranger assure her husband that she will be a mother same time next year, her heart leaps for joy and she laughs, just like Abraham.
The stranger's response is full of encouragement and kindness: "Go ahead. Be glad. Be openly happy."
Centuries later, Paul writes, "By faith even Sarah herself received ability to conceive, even beyond the proper time of life, since she considered Him faithful who had promised." (Hebrews 11:11)
Sarah's faith existed before she conceived of Isaac, not after, and she laughed because of that faith.