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. Read Due Dates With Irregular Periods when LMP is uncertain.
What the progress display means
Gestational age is conventionally counted from the 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. Trimester boundaries are broad clinical labels, and different sources sometimes use slightly different week cutoffs.
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.
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
Last reviewed: July 13, 2026