When HCG levels are not elevated 15 days after ovulation, there may be some error if the measurement is done on your own, and you may not necessarily not be pregnant. However, if it is measured for a blood test, the probability of pregnancy is low. Pregnancy may occur if a woman has unprotected sex after ovulation. If pregnancy is successful, HCG levels in the blood and urine will be outside the normal range by 15 days after ovulation. If you use a pregnancy test on your own, there may be measurement errors due to quality problems or improper handling, and the accuracy may deviate to a certain extent. It is recommended to go to the hospital for blood HCG test, which is sensitive and accurate, with a relatively high accuracy rate.