Due date calculators provide an estimate, not an exact date. Only about 5% of babies are born on their exact due date. The most accurate method is an ultrasound in the first trimester, but even that can have a margin of error of about 1-2 weeks.

Yes, your due date may be adjusted if an early ultrasound shows a significant difference from your LMP dates, or if your healthcare provider notes discrepancies during physical exams. Most adjustments occur in the first trimester.

If you don't know your LMP date, you can still estimate your due date using the date of conception, if known, or by an early ultrasound. Your healthcare provider may also use physical exams to estimate gestational age.

Standard due date calculations assume a 28-day menstrual cycle with ovulation on day 14. If your cycles are longer or shorter, your due date may be adjusted. For example, women with 35-day cycles may ovulate later, pushing the due date later than LMP-based calculations.