When does embryo implantation start to be calculated from?

Patient's question:

No

Doctor's answer:

The number of days pregnant, which is calculated from the first day of the last menstrual period. Although the actual time of pregnancy is calculated from the implantation after the sperm and egg combine, it is generally impossible to determine the day of fertilization, as the time of intercourse on the same day may not be the time of fertilization.