Pregnancy calculations can vary due to different methods used by apps and doctors. Apps generally calculate from the first day of your last menstrual period (LMP), while doctors may use ultrasound measurements. Ultrasound is more accurate for determining gestational age, as it measures the size of the baby. It's possible that your app is not accounting for these factors or using a different calculation method. To ensure accuracy, rely on your doctor's assessment and discuss any discrepancies with them.