How the due date calculator works
Starting from the first day of your last menstrual period, the calculator applies Naegele's rule by adding 280 days. It then adjusts for cycle length, estimates the likely conception window around ovulation, and marks when the second and third trimesters begin so you can see the pregnancy timeline at a glance.
Why ultrasound dating may differ
Last-period dating assumes regular cycles and ovulation on day 14, which is not true for everyone. A first-trimester ultrasound measures the baby directly and is generally the most accurate way to date a pregnancy, so your provider may revise the estimate.