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.

  • $\begingroup$ Is it reasonable...? I don't think so, based on this limited data. A lot of variation is normal in the length of a cycle, and OCP can themselves alter the cycle's periodicity, at least in the close aftermath. $\endgroup$ – Satwik Pasani Jun 11 '18 at 8:15
  • $\begingroup$ What if the woman gathered a lot of data and knows (based on past data) that her period comes with the precision of a Swiss train? $\endgroup$ – Franz Drollig Jun 11 '18 at 11:36
  • $\begingroup$ It is then, only speculation in my opinion. For the purpose of a sci-fi setup it might be sufficiently plausible, but not otherwise. Plus, the fertility window of menstrual cycles is limited, and late in the cycle, the chances of a successful fertilization drop. A better premise might be to make her realize her pregnancy during the cycle before she has missed a period, using some biomarker or hormone or something. $\endgroup$ – Satwik Pasani Jun 11 '18 at 12:24
  • $\begingroup$ It's not a sci-fi setting. Maybe I expressed myself incorrectly. After what time after conception can a woman know that she is pregnant? One week, two weeks, a month, two months, more? $\endgroup$ – Franz Drollig Jun 11 '18 at 12:45

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