What can cause a late period? And how long can a late period be?

There are many possible causes: Pregnancy or not ovulating (releasing an egg) can make your period late by weeks or months. Common causes of not ovulating include polycystic ovarian syndrome (PCOS), underactive thyroid, excess prolactin hormone, aging of the ovary (diminished ovarian reserve), or low hormone levels (hypogonadism) from heavy exercise, eating disorders, or stress. You can see an Ob/Gyn for tests, including a blood pregnancy test; see a Reproductive Endocrinology and Infertility (REI) MD if you’re trying to conceive.