Due Date Calculator (from LMP)
Estimate your baby’s due date from your last menstrual period using Naegele’s rule, adjusted for cycle length, with trimester milestones.
How it works
Naegele’s rule: due date = LMP + 280 days (40 weeks) · Cycle adjustment: add (cycle length − 28) days
Naegele’s rule estimates the due date as 280 days — 40 weeks — from the first day of your last menstrual period, which is why gestational age is counted from before conception. Because ovulation drifts with cycle length, a cycle longer or shorter than 28 days shifts the due date by the same number of days. Only about 4% of babies arrive exactly on the estimated date; an early-pregnancy ultrasound gives the most accurate dating and often refines this estimate.
Worked example
For a last period beginning 1 June with a 28-day cycle, the due date is 1 June + 280 days = 8 March the following year. A 30-day cycle would push it two days later, to 10 March.
Sources: Naegele’s rule
Frequently asked questions
Why is the due date measured from my last period?
Conventionally, pregnancy is dated from the first day of the last menstrual period because that date is usually known, whereas the exact conception day often is not. This makes a full-term pregnancy 40 weeks by convention, about two weeks longer than the time since conception.
Does cycle length change the due date?
Yes. Naegele’s rule assumes a 28-day cycle with ovulation on day 14. If your cycles run longer or shorter, ovulation and the due date shift by the difference, which this calculator applies automatically.
How accurate is an LMP due date?
It is a solid estimate but only a few percent of births land on the exact date. A first-trimester ultrasound measures the baby directly and is the most accurate way to confirm or adjust dating.
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Educational estimate using standard formulas and population averages — not medical advice. Talk to a qualified clinician before making decisions about diet, training, or pregnancy.