A period can be late due to illness, stress, medication, schedule or weight changes and pregnancy, according to About.com. Issues such as polycystic ovary syndrome or an ovarian cyst can also cause late or missed periods.
According to Web MD, while pregnancy is the most common reason for a late or skipped period, it is not uncommon for women to have a delayed period for different reasons. For women who are breastfeeding, it is not unusual to have irregular menstrual cycles while breastfeeding. Hormonal problems are often the reason for late or skipped periods as well. When hormonal imbalances occur, it can throw off the menstrual cycle.