However if your menstrual cycle is more irregular then it is difficult to say when you are likely to be most fertile.
Nonetheless, having your period late does not automatically mean that you must be pregnant. About.com lists ten reasons that your period might be late. Possible reasons include:
- Changes in medication
- Changes in schedules
- Being overweight or underweight
For more detail about possible reasons for your period being late, visit pregnancy.about.com/od/amipregnant/a/missedperiod.htm
Of course the best way to reassure yourself is to take a pregnancy test. If the test turns out to be negative, then it is normally advised that you wait for another week until re-testing. If after a week the test is still negative and your period is still missing, then you should call your doctor or midwife and ask for a physical exam as soon as possible. There is medication that is available to help bring about a late period.