How the due date is calculated
Naegele's rule estimates a full-term pregnancy as 280 days (40 weeks) from the first day of your last menstrual period.
- LMP — first day of your last menstrual period
More detail
Why 280 days
Naegele's rule assumes a 28-day cycle with ovulation on day 14, and counts a full-term pregnancy as 40 weeks from the LMP (about 38 weeks from conception). It's a widely used estimate, not a guarantee — only about 1 in 20 babies arrive exactly on the due date.
Tip. If your cycle is longer or shorter than 28 days, or you know your conception date, an ultrasound-based estimate from your provider will be more accurate than this rule of thumb.
Frequently asked questions
How accurate is Naegele's rule?
It's a widely used estimate, not a guarantee — only about 1 in 20 babies arrive exactly on the calculated due date. Most births happen within a week or two either side.
What if my cycle isn't 28 days?
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle runs longer or shorter, the estimate skews off — your provider can adjust it, or a first-trimester ultrasound will be more accurate.
How is gestational age different from due date?
Gestational age (shown above as weeks + days) counts how far along the pregnancy is right now, measured from the LMP. The due date is the projected end point, 280 days after the LMP.
Can I use my conception date instead of LMP?
This calculator uses LMP, the standard clinical starting point. If you know your conception date instead, add 266 days (38 weeks) to it for a rough equivalent — or ask your provider to recalculate from that date.