There are lots of factors that can lead to a late period such as stress. When did you have sex during her cycle? There is only one day in the month that she can get pregnant and it's called ovulation. Keep in mind that any sperm that may have entered her vagina can survive for up to a week and wait for ovulation to occur. Generally ovulation occurs during the 14th day of her cycle. However if you used protection properly and checked it, I wouldn't worry, as the odds are against you. If it would help calm the two of you then you can take a home pregnancy test around now and it will be accurate. Stressing about being pregnant can be the cause of a late period so just relax.