From Last Period to D-Day: How Your Pregnancy Due Date Is Really Calculated
By the Super Simple Digital Tools Team · Updated June 2026
Your due date is one of the first numbers you learn in pregnancy, yet few people are told how it is produced. The figure comes from Naegele's rule, named after the German obstetrician Franz Naegele who formalized it in the early 1800s. The rule adds 280 days, exactly 40 weeks, to the first day of your last menstrual period. It has survived for two hundred years because it needs only one piece of information that almost everyone can recall: when your last period began.
The math behind the rule is simpler than it sounds. Take the first day of your last menstrual period, add seven days, count back three months, and then add a year. A period starting on January 1, for example, lands on a due date of October 8 the same year. The calculator on this page performs that arithmetic instantly and consistently, removing the risk of miscounting months or forgetting leap years when you try to do it by hand.
A key assumption hides inside the rule: it treats ovulation as happening on day 14 of a 28-day cycle, placing conception about two weeks after your period starts. That is why a calculator counts 280 days from the LMP but only 266 days from a known conception date; the 14-day gap is the same window viewed from two starting points. When your cycle is longer, shorter, or unpredictable, that built-in assumption is where LMP-based estimates can drift from your real timeline.
This is also why doctors lean on a first-trimester ultrasound. Between roughly weeks 8 and 13 a sonographer measures the crown-rump length of the fetus, which grows at a very predictable rate, and uses it to date the pregnancy directly rather than by calendar. For irregular cycles this scan is often more accurate than the LMP, and if it disagrees significantly with your calculated date, the ultrasound date usually becomes the official one.
The most useful mindset is to treat the due date as the center of a window rather than a fixed appointment. Only about 1 in 20 babies actually arrive on the predicted day, while the large majority are born within two weeks on either side, and any birth between 37 and 42 weeks is considered full term. Use the date to plan scans, leave, and preparations, but build in flexibility, because babies keep their own schedule.
- Use the very first day of bleeding from your last period, not the day it ended or the day you noticed spotting, since Naegele's rule counts from that exact start.
- If your cycle is regularly longer or shorter than 28 days, prefer a known conception or ovulation date, or ask your provider to adjust the estimate accordingly.
- Re-check your estimate after your first-trimester ultrasound; if the scan date differs notably, go with the ultrasound, which dates the fetus directly.
- Plan key milestones like the anatomy scan or maternity leave around a two-week window on each side of the due date rather than the single date itself.