It is counted from the last menstrual period to be able to determine more accurately the time of pregnancy, because the woman knows exactly when she gets her period but there is no way of knowing when fertilization actually occurs.
The truth is that it is somewhat strange to talk about weeks of pregnancy when fertilization has not occurred. Therefore, there are those who start counting from the moment they sense that fertilization has taken place, based on the fact that in a regular cycle, fertilization takes place about two weeks after the menstrual period starts.
These variations are the reason why sometimes 9 months of pregnancy and sometimes 10 months of pregnancy are referred to. However, pregnancy is usually considered to be 40 weeks from the date of the last menstrual period.