The pregnancy calculator takes the first day of the last time you had your period as a reference, in such a way that if it has been 5 weeks since you had your period and you are pregnant, you will be 5 weeks.
As you can see, the pregnancy calculator is not very complicated, other than noting the date of your last menstruation, making yourself a calendar and taking note of the benchmarks week by week which are shown below. It will help you deal with the different pregnancy stages and encourage you to face the most difficult times.