Considering that the clearest sign is a missed period, we can say that normal pregnancy symptoms start at week 4, as women usually get their next period about 4 weeks from the beginning of their last period. However, some women are able to notice some symptoms right after conception.
It should be noted, however, that if you are feeling something at 2 weeks, it is likely caused by your usual menstrual cycle, as at this point you are ovulating and about to conceive, but fertilization has not happened yet. At 3 weeks, most women notice what we know as implantation spotting, or start feeling fatigue, food aversions, frequent urination, etc.
Suggested for you: Am I Pregnant? – 10 Early Clues & Symptoms Before a Missed Period.