There are actually many things that can delay your period that have nothing to do with being pregnant. For instance, many medications list delayed periods as a side effect, especially psychiatric drugs that can elevate your levels of prolactin. (This hormone is produced in low levels by your body at all times, but it becomes elevated during and right after pregnancy to help you produce breast milk.) "Usually women get told [about the possibility] because it’s a relatively common side effect" of these medications, explains Raquel B. Dardik, MD at the NYU Langone Medical Center. "So it’s not usually much of a surprise."
However, other culprits might seem to come out of nowhere.
Before we get to those, we need to quickly mention a few caveats: in most of the cases to follow, we're talking about just one late period. If this has happened to you three or more times in a row, that's a sign that you should check in with your doctor to make sure you're not dealing with irregular periods, which can be caused by more pressing matters like polycystic ovary syndrome (PCOS) or fibroids (benign uterine tumors). Also, if your period is over a week late and you know you've had unprotected sex this cycle, it's time to take a pregnancy test.