It is indeed very difficult to distinguish between the luteal phase and the first trimester of pregnancy, because the corresponding early pregnancy reactions do not occur even when you are newly pregnant. The only thing that can be distinguished is that the body temperature during the luteal phase is elevated, and this high temperature does not exceed 14 days, after 14 days it decreases, and basically about 24 hours after the basal body temperature decreases, menstruation will occur. Pregnancy is also associated with a high temperature, and this high temperature should be at least 15 days to indicate pregnancy, which is a small clinical difference between the two. If your period is usually regular, it is recommended to take a blood test for HCG as soon as possible to find out if you are pregnant.