Family, Baby & Pets Tools

Pregnancy Due Date Calculator opens this category because Naegele's rule adds 280 days to the last menstrual period and gives the right answer for a 28-day cycle. It gives a subtly wrong answer for every other cycle length, because ovulation shifts with the cycle and the rule does not adjust for it automatically. A 32-day cycle moves the due date four days later; a 24-day cycle moves it four days earlier. Neither correction is applied by a bare calendar widget that adds 40 weeks. This tool takes the cycle length into account, adjusts the estimate, and shows the arithmetic it used.

Three input methods cover the real ways people arrive at a dating estimate. The LMP method is the default: the first day of the last menstrual period is the date most people already know. The conception method works backwards from a known conception date, subtracting the standard 14-day ovulation offset to recover the equivalent LMP. The ultrasound method accepts the gestational age printed on the scan report and back-calculates from the scan date, which is the most accurate approach in the first trimester before 14 weeks. The milestone timeline that follows the result marks seven checkpoints: end of the first trimester, anatomy scan window, viability threshold, third trimester, full term, due date and the 42-week post-term boundary.