About the Due Date Calculator
A due date calculator estimates the expected date of delivery (EDD), the day a pregnancy is statistically most likely to result in birth. The standard calculation is based on Naegele's rule: 280 days (40 weeks) from the first day of the last menstrual period. The actual delivery date varies considerably — only a small fraction of births occur on the calculated due date.
The 280-day convention
Naegele's rule, formalized in the 19th century, computes EDD as LMP + 280 days. The convention assumes a 28-day cycle with ovulation on day 14, fertilization on day 14, and a true gestation of 266 days from fertilization. Despite its age and simple assumptions, it remains the global standard because it's predictive within useful tolerance and because the inputs (LMP date) are universally available.
An equivalent shortcut: take the first day of LMP, subtract 3 months, add 7 days. So an LMP of August 11 yields a due date of May 18 of the following year.
The actual distribution of birth dates
Births don't cluster on the due date itself. Roughly 4% of births occur on the due date; about 60% occur within a week of it; about 88% occur within the full-term range of 37–42 weeks. The mean appears to be slightly later than the calculated EDD for first-time mothers (often by 3–8 days in observational studies), and slightly earlier for women with prior pregnancies.
Beyond 41 weeks ("late-term") and 42 weeks ("post-term"), the risk profile shifts and induction is increasingly considered. Modern obstetric practice generally does not allow pregnancies to continue beyond 42 weeks given evidence of increased perinatal risk.
Ultrasound dating and EDD adjustment
First-trimester ultrasound, especially before 14 weeks, is the most accurate method for estimating gestational age. ACOG guidelines recommend that if the LMP-based EDD differs from the ultrasound estimate by more than 5–7 days (in early pregnancy) or larger thresholds at later gestational ages, the ultrasound date supersedes the LMP-based date.
Don't be unsettled by a few-day shift in EDD after the early ultrasound. It usually reflects ordinary variation in ovulation timing or LMP recall, not anything concerning. Larger discrepancies in the first trimester sometimes warrant follow-up evaluation.
Cycle-length adjustments
For women with consistently longer or shorter cycles than 28 days, Naegele's rule can be misleading. A 35-day cycle implies ovulation around day 21 instead of day 14 — making the LMP-based EDD about 7 days too early. Many EDD calculators allow cycle length as an input and adjust accordingly.
For women with irregular cycles, LMP-based dating is intrinsically uncertain and ultrasound dating is preferred from the start. IVF-conceived pregnancies are dated from the embryo transfer date plus an offset specific to the embryo's day of development at transfer — far more precise than LMP-based dating.
Formula
- LMP = First day of the last menstrual period
- EDD = Estimated due date
- Cycle length = Adjusted variant: EDD = LMP + 280 + (cycle length − 28) days
Worked examples
Standard 28-day cycle
LMP: March 5. EDD = March 5 + 280 days = December 10. Full-term range (37–42 weeks): November 19 – December 24.
Long 35-day cycle adjustment
Same LMP (March 5), but the woman's cycles are consistently 35 days. Adjusted EDD: March 5 + 280 + 7 = December 17. The unadjusted Naegele EDD (December 10) would be 7 days early.
Ultrasound dating override
LMP-based EDD: December 10 (8 weeks at first prenatal). Crown-rump length on ultrasound: 6 weeks 5 days — 9 days behind LMP dating. Per ACOG, EDD shifts to ~December 19. Going forward, all gestational-age tracking uses the new date.
Frequently asked questions
Will my baby be born on the due date?
Statistically unlikely — only about 4% of births occur on the calculated date itself. Roughly 60% occur within a week of it; the full-term range of 37–42 weeks captures ~88% of births. Plan for a window, not a date.
How is the due date calculated?
By adding 280 days (40 weeks) to the first day of the last menstrual period — Naegele's rule. Equivalently: subtract 3 months and add 7 days. The convention assumes a 28-day cycle; longer or shorter cycles shift the actual conception (and EDD) accordingly.
Why does my due date change at the ultrasound?
First-trimester ultrasound dating is more accurate than LMP-based dating, especially when the LMP isn't certain or cycles aren't regular. Per ACOG, when ultrasound and LMP disagree by more than a few days in early pregnancy, ultrasound becomes the basis for the official due date.
What happens if I go past my due date?
Most U.S. providers monitor closely after 41 weeks (with non-stress tests, biophysical profiles) and offer or recommend induction at 41–42 weeks. Going beyond 42 weeks is associated with increased perinatal risk and is generally avoided in modern obstetric practice.
How accurate is a due date from IVF?
Very accurate — IVF dates are based on the actual embryo transfer date plus a precise offset for the embryo's development stage at transfer. EDD precision in IVF pregnancies is typically within ±1 day, far better than LMP-based dating.
Can I influence when labor starts?
Once full-term, normal labor onset depends on a complex interaction of maternal and fetal hormones not directly under conscious control. Various claims (specific foods, exercise, intercourse) have weak or inconclusive evidence for inducing spontaneous labor at term. Medical induction (oxytocin, mechanical ripening) is the only well-supported way to start labor when needed.
Related calculators
Concepts
Sources & methodology
- American College of Obstetricians and Gynecologists — Methods for Estimating the Due Date — source