The truth is, there is no scientific evidence that fertilization can actually produce symptoms in the woman right after conception occurs. However, no study or expert has ever stated that it is either possible or impossible to know. Some women believe they knew the exact moment, while others do not realize. Sensitivity to hormonal changes could be the explanation behind those cases where the woman believes she "felt something".
See this for more: What Are the Earliest Symptoms of Pregnancy Before Missed Period?
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