The formulas explained
How pregnancy due dates are calculated
Due-date formulas are simple. Deciding which starting date deserves confidence is the difficult part.
Checked against authoritative sources
Evidence reviewed 6 August 2026 · 9 minute read
General education, not individual medical advice.
The short answer
The standard LMP estimate adds 280 days to the first day of the last menstrual period. A cycle adjustment shifts that estimate by the difference from 28 days; IVF uses transfer date plus 261 days for a day-5 embryo or 263 for day 3.
The LMP convention: add 280 days
By convention, the EDD is 280 calendar days after the first day of the last menstrual period. This is equivalent to 40 weeks of gestational age.
The formula assumes a regular 28-day cycle and ovulation around day 14. It cannot see whether ovulation happened earlier or later.
Sources for this section: American College of Obstetricians and Gynecologists, American College of Obstetricians and Gynecologists.
Cycle-length adjustment
A common calculator adjustment adds the difference between the usual cycle length and 28 days. A 31-day cycle shifts the estimate three days later; a 25-day cycle shifts it three days earlier.
The NHS calculator accepts cycle length, but this remains a calendar estimate. Variable or irregular cycles are not repaired by entering an average number.
Sources for this section: NHS, American College of Obstetricians and Gynecologists.
Why this tool skips conception-date input
Outside assisted reproduction, an exact conception date is rarely known. Intercourse, ovulation, and fertilization can occur on different dates, so an apparently precise input may rest on a weak assumption.
The calculator therefore uses the conventional LMP method or documented IVF transfer details. It cannot infer conception or paternity from a due date.
Sources for this section: American College of Obstetricians and Gynecologists.
IVF dating uses embryo age and transfer date
ACOG recommends using ART-derived gestational age. For a day-5 embryo, the EDD is 261 days after transfer; for a day-3 embryo, it is 263 days after transfer.
Do not average an IVF result with an LMP result or use a cycle-length adjustment. The transfer date and embryo age provide the dating basis.
Sources for this section: American College of Obstetricians and Gynecologists.
Early ultrasound can establish or confirm dating
First-trimester ultrasound through 13+6 is the most accurate method to establish or confirm gestational age. A clinician compares that result with the history and applies established redating guidance.
Later scans are not invitations to keep changing the EDD based on each new measurement. ACOG says later changes should be reserved for rare circumstances and documented.
Sources for this section: American College of Obstetricians and Gynecologists.
Read the result as an estimate
The calculation returns a precise date because calendars require one. Biological timing is not equally precise. The useful result is a shared reference point for care, not a prediction of labour beginning that day.
Once a clinician establishes the due date, use it consistently. Bring any uncertainty about LMP, bleeding, cycles, or IVF details to the care team.
Sources for this section: American College of Obstetricians and Gynecologists, NHS.
Three worked examples
If the LMP began on 1 January and the usual cycle is 28 days, the convention adds 280 calendar days. If the usual cycle is 31 days, the adjusted estimate moves three days later. The shift reflects a cycle assumption; it does not prove ovulation occurred on a particular day.
If a day-5 embryo was transferred on 1 January, the IVF convention adds 261 days. A day-3 transfer on the same date adds 263. Those ART results should stand on their own rather than being averaged with LMP or cycle dates.
- Use calendar dates, not 24-hour millisecond blocks.
- Keep the input method visible beside the result.
- Save the EDD documented by the care team once it is established.
- Do not back-calculate an exact conception date from the EDD.
Sources for this section: American College of Obstetricians and Gynecologists.
Common questions
What is Naegele's rule?
It is the calendar convention that estimates an EDD 280 days from the first day of the LMP, traditionally described as adding one year, subtracting three months, and adding seven days.
How accurate is a due-date calculator?
It applies the formula accurately to the input. The clinical accuracy depends on whether that input and method fit the pregnancy.
Does a longer cycle mean a later due date?
A cycle-adjusted estimate shifts later by the number of days above 28, but irregular cycles require clinical dating rather than a simple average.
Sources and review notes
We favour public-health guidance, clinical bodies, official statistics, and original research. Source links are provided so you can inspect the wording and boundaries yourself.
- 01Methods for Estimating the Due DateAmerican College of Obstetricians and Gynecologists · May 2017; reaffirmed 2025
- 02Pregnancy due date calculatorNHS
- 03When Pregnancy Goes Past Your Due DateAmerican College of Obstetricians and Gynecologists