Imagine a woman gets pregnant on day X. What is the shortest interval (like a day, a week, a month) after which she can detect she is pregnant?
Example: Let's say that she keeps track of her menstruations and the next one should occur on 2018-05-01. On 2018-04-29 she has sex without protection and she doesn't take birth control pills. On 2018-05-02 she hasn't got her period. Is it reasonable to assume that she is pregnant based on these facts?
Note: I need this information for a fiction book. It doesn't need to be 100 % accurate, just believable.