How early do most women notice pregnancy, and how rare are late discoveries?

No my wife isn't pregnant zbut I have heard stories of women who have no idea they are pregnant until late in the pregnancy. Sometimes until month 7 or even month 9.

How common is that kind of thing? If it's not common, how early in the pregnancy can she notice? What are some signs to pick up on?
How early do most women notice pregnancy, and how rare are late discoveries?
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