Does Pregnancy Start The First Day Of Your Last Period. By convention, pregnancies are dated in weeks starting from the first day of a woman's last menstrual period (lmp). Your pregnancy due date is calculated by adding 40 weeks (i.e., 280 days) to the first day of your last menstrual period (lmp).

Note that your menstrual period and ovulation are counted as the first two weeks of pregnancy. Healthcare providers use the lmp method to determine your estimated due date. If you have spotting or light bleeding one day, then no bleeding at all the next day, it was likely breakthrough bleeding, which can occur in some women leading up to their period.