Most pregnancies last around 40 weeks (or 38 weeks from conception), so typically the best way to estimate your due date is to count 40 weeks, or 280 days, from the first day of your last menstrual period (LMP). Another way to do it is to subtract three months from the first day of your last period and add seven days.
Is pregnancy calculated from conception or implantation?
The length of pregnancy -- typically 40 weeks -- remains the same regardless of how doctors define the beginning of pregnancy, because the 40 weeks are not counted from the time of fertilization or implantation, but from the time of the mothers last menstrual period.
When do you start counting pregnancy?
Pregnancy is counted as 40 weeks, starting from the first day of the mothers last menstrual period. Your estimated date to birth is only to give you a guide.