For women with regular periods (28-30), the simple algorithm for the due date is based on the first day of the last menstrual period (solar date), the month minus 3 (or plus 9), and the number of days plus 7. If the day of intercourse is clearly known, then the due date will be 38 weeks from that date. For pregnant women who cannot remember their last menstrual period or whose menstrual cycle is disordered, your doctor will need to further check your week of gestation and expected date of delivery at the time of delivery based on the date of intercourse, the first time you were found to have a positive urine HCG, the time of your first early pregnancy reaction, your blood HCG in early pregnancy and the ultrasound results.