there is such thing as "implantation bleeding" it happens when the embryo floats down into the uterus and implants into the uterine lining, causing anything from spottying to period like bleeding. You can also cramp during this time - it is your uterus expanding.
There are also things called chemical pregnancies - where there is enough HcG to cause a + pregnancy test, but for whatever reason the embryo doesn't implant and passes right through....the woman would have a heavier than normal period as usually never know. So 'technically' she could be pregnant and have a period.
Spotting or not, what you think is a period or not, a home pregnancy test that measure at least 40 HcG is 99% accurate as soon as your period is late
