- Though we typically think of a menstrual cycle lasting about 28 to 30 days, it can actually vary quite a bit. The menstrual cycle is split into two phases, the follicular phase and the luteal phase. The follicular phase is the time from when your period begins until ovulation; during this time, the egg is being prepared for release. The luteal phase is the time from ovulation until when your period begins. The follicular phase has no set time limit, while the luteal phase is almost always between 12 and 16 days long. Thus, unless you know exactly on which day you ovulated, it can be difficult to calculate if your period is late. For example, if your follicular phase lasted 21 days, then it could be as long as 37 days between periods, which, if you're expecting a 28-day cycle, could seem as though you're over a week late.
- Sometimes a late period might actually be a missed period. Though most of us associate missed periods with pregnancy, if you have a month in which you don't ovulate, it's possible to simply skip your period that month. Since ovulation is the event that sets the second phase of the menstrual cycle in motion, if you don't ovulate there isn't a hormonal trigger for your period to begin. Eventually, your levels of estrogen may drop low enough to cause "withdrawal" or "breakthrough" bleeding, but this may be much later than you've expected.
- It is possible to have a very long menstrual cycle due to delayed ovulation, which means an extended follicular phase. In her book "Taking Charge of Your Fertility," Toni Weschler notes a number of life stressors which can delay ovulation. These stressors can include: mental and physical stress, illness, medicines, strenuous exercise and significant weight changes.
- In a condition known as Luteinized Unruptured Follicle Syndrome (LUFS), your body reacts to all the different hormones and stages of the menstrual cycle as though everything has occurred normally. However, when ovulation is set to occur, the egg doesn't pass through the ovarian wall. This follicle forms a cyst which tends to resolve on its own. In the meantime, though, your period is late because your body hasn't received the signal for progesterone levels to drop, thereby beginning your period.
- Unfortunately, no form of birth control is completely fail-proof. While some methods have a higher rate of failure than others, all methods have the potential to fail (see Resources below). If your period is late while you're on birth control, there is always the possibility that you may be pregnant. It's advisable to take a home pregnancy test to be sure, as some forms of birth control can be harmful during pregnancy.
