Well, as far as mistaking implantation bleeding for a period...that depends alot on the woman and the amount of bleeding she experiences during a period. You don't necessarily menstruate the same amount of fluid during each period, and sometimes you might experience a very very light period for one reason or another...so it is possible to mistake one for the other in some instances.
And as far as periods during pregnancy...the difinitive answer should be that no, you can't have a period and be pregnant at the same time. When you menstruate, the lining of the uterus is shed. That lining is what would nurture the fertilized egg. So you can't shed that lining and still be pregnant. Check out On the Rag for more on that.
And as far as advising a pregnancy test anyway...well, basically you can't go wrong with a pregnancy test. Taking a test won't make you pregnant, but it sure will put your mind at ease, especially if you're worried that you might be mistaking implantation bleeding for a light menstrual flow.
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