False negative pregnancy tests? How likely are they?

Some people say that they have period like bleeding each month when pregnant and could easily be mistake for a period and still have a perfectly fine baby.

Is this extremely rare?? It worries me that I could still have periods and potentially be pregnant still

Also is it likely to receive false negative home pregnancy tests after 4 weeks of pregnancy(when period is due) like if you take a test each week after period was due does that ensure your not pregnant if they are all negative.