How is Due Date Calculated?
Most pregnancies last around 40 weeks (or 280 days) from the first day of your last menstrual period (LMP). This calculator uses Naegele's Rule, which is the standard method used by doctors.
What if my cycle isn't 28 days?
The standard calculation assumes a regular 28-day cycle. If your cycle is longer (e.g., 30 days), your ovulation likely occurred later, so your due date is pushed back. Our calculator automatically adjusts for this.
How Naegele's Rule Works
The method is simple: take the first day of your last menstrual period, add 7 days, subtract 3 months, and add 1 year. For example, an LMP starting March 10 gives a due date of December 17 the following year — which is mathematically the same as adding 280 days (40 weeks) to the LMP.
Why LMP Instead of Conception Date?
Most due-date calculations count from the last menstrual period rather than the actual conception date, even though conception typically occurs about two weeks later. The reason is practical: the LMP is far easier to pinpoint reliably than the exact day of conception.
An Estimate, Not a Deadline
Only about 5% of babies are actually born on their calculated due date. A normal delivery window spans roughly 38 to 42 weeks, so treat the date as the center of a two-week-wide range rather than a fixed appointment. The estimate still matters for scheduling prenatal care and tracking fetal development milestones.