How each method is calculated
| Method | Calculation | Main limitation |
|---|---|---|
| Last menstrual period | About 280 days, adjusted for selected cycle length | Ovulation may not occur at the assumed time |
| Conception date | About 266 days after estimated conception | The exact date is often uncertain |
| IVF transfer | Transfer date plus 266 days minus embryo age | Clinic records should be used for official dating |
Why a clinical due date may differ
LMP dating assumes a predictable relationship between cycle start and ovulation. Irregular cycles, uncertain dates, recent hormonal contraception, breastfeeding, and other factors can make that assumption unreliable. Early ultrasound and documented IVF timing provide different information that a clinician evaluates using established criteria.
A due date is normally established and documented early in care. It should not be repeatedly changed based on online calculations. The cornerstone guide explains irregular cycles, LMP versus conception, and clinical ultrasound dating in one place.
What the timeline adds
The result does more than display one due date. It names the arithmetic used, shows an LMP-equivalent date, estimates conception, marks weeks 14 and 28, and places today on a 40-week visual timeline. That makes the output easier to audit and helps distinguish the gestational calendar from the conception interval.
Gestational age is conventionally counted from estimated LMP, roughly two weeks before conception in a typical cycle. That is why someone may be described as four weeks pregnant about two weeks after conception.
Safety and limitations
This tool cannot assess pregnancy health
It cannot confirm a pregnancy, evaluate bleeding or pain, interpret ultrasound, determine viability, or provide emergency guidance.
Use prenatal care for medical decisions and seek prompt help for concerning symptoms. The calculator does not store the entered date.
Implementation evidence
How this calculator is checked
LMP + 280 days with cycle adjustment, conception + 266 days, or IVF transfer + 266 days − embryo age.
Expected: Due date October 8, 2026
Expected: Due date September 19, 2026
Expected: Result blocked
These reference cases are exercised during the automated build. They verify arithmetic and input handling; they do not validate the estimate for an individual person. Read the full testing policy.
Frequently asked questions
How accurate is an online due-date calculator?
It correctly applies common dating arithmetic, but accuracy depends on whether the starting date represents the pregnancy correctly. Clinical dating can differ.
Can an ultrasound change the due date?
Yes. A clinician may use early ultrasound and dating guidelines when menstrual dates are uncertain or the measurements differ enough to meet clinical criteria.
What if my periods are irregular?
LMP may be a weaker starting point because ovulation may not match the usual assumption. Bring cycle history and any known dates to prenatal care.
How is a day-5 embryo transfer calculated?
The calculator adds 261 days to the transfer date, which is 266 days minus the embryo’s five days of development.
Sources
References and further reading
- American College of Obstetricians and Gynecologists — Methods for Estimating the Due Date
- Office on Women’s Health — Stages of Pregnancy
Method and sources reviewed: July 23, 2026