When it comes to estimating a baby's arrival, pregnancy calculators use the first day of your last menstrual period as a starting point, adding about 280 days or 40 weeks to predict the due date. In my practice, I explain to patients that this is an estimate, not an exact science, because every pregnancy is unique. Factors like irregular cycles, ovulation timing, and fetal growth can affect when labor actually begins. Ultrasound measurements in the first trimester help refine this estimate, giving a more personalized due date. I always emphasize that while due dates help us plan and monitor progress, most babies arrive within a two-week window before or after the calculated date. It's important for expectant parents to understand that the due date is a guideline to prepare, not a precise deadline for delivery.