Due date recalculator
Recalculate your estimated due date and compare LMP dating with ultrasound or IVF dates using common ACOG principles.
Due date recalculator
Use this free due date recalculator to estimate when your baby may arrive. Choose the dating information you have, enter the date, and review the result below. The result is an estimate, not a deadline or a diagnosis.
What a due date recalculator tells you
A due date recalculator estimates your expected date of delivery from information such as your last menstrual period, an ultrasound-based date, or an IVF transfer date. The conventional calculation starts from the first day of the last menstrual period and adds 280 days, or 40 weeks (ACOG, 2017).
The date is a practical reference for prenatal scheduling and measuring gestational age. It does not predict the exact day labor will begin, and it cannot account for every difference in ovulation, implantation, fetal growth, or cycle history.
Your prenatal clinician may use ultrasound findings and other clinical information to confirm or revise the estimate. Once an expected due date has been selected and documented, ACOG advises that later changes should be limited and discussed with the patient (ACOG, 2017).
How to use the recalculator
Select the input method that matches the information available to you. Enter dates carefully, especially the year, and then run the calculation. If the tool offers more than one method, calculate each separately rather than combining uncertain dates.
- Last menstrual period: Use the first day bleeding began, not the day the period ended. The standard estimate is 280 days from that date (ACOG, 2017).
- Ultrasound dating: Enter the due date or pregnancy dating recorded on your scan report. Do not try to interpret measurements from an ultrasound image yourself.
- IVF transfer: Use the transfer date and the embryo stage requested by the tool. Transfer dating does not depend on remembering a menstrual period.
Save or screenshot the result if you want to discuss it at an appointment. If it differs from the date in your medical record, ask which date your care team is using rather than switching your official date on your own.
LMP dating compared with an ultrasound date
Last menstrual period dating assumes the pregnancy can be placed on a conventional timeline from the first day of the last period. Under that convention, the expected due date is 280 days, or 40 weeks, after the LMP began (ACOG, 2017). The NHS uses the same 40-week starting point in its calculator (NHS, 2020).
LMP dating can be less dependable when periods are irregular, the date is uncertain, hormonal contraception was recently stopped, or bleeding was mistaken for a period. Variation in ovulation can also make a calendar estimate differ from fetal measurements.
Ultrasound dating uses fetal measurements and the timing of the scan. ACOG considers an accurate early ultrasound an important way to establish or confirm gestational age, while later measurements are less suitable for repeatedly changing the expected date because normal differences in fetal size become more relevant (ACOG, 2017).
A scan-based date does not automatically replace the LMP date. Clinicians compare the dates, consider how certain the LMP is, review when the scan occurred, and apply clinical redating criteria. I do not have a sourced numerical redating threshold in the supplied information, so this tool should not decide whether your medical record needs to change.
Due date methods at a glance
The best input is usually the date your clinician considers most reliable. This table shows what each method uses and where its limitations matter.
| Method | How the estimate is made | Main limitation |
|---|---|---|
| Last menstrual period | Add 280 days, or 40 weeks, to the first day of the LMP (ACOG, 2017). | It depends on remembering the correct date and may not reflect irregular ovulation. |
| Ultrasound date | Use the expected due date assigned from fetal measurements and scan timing. | A calculator cannot verify measurement quality or decide whether clinical redating is appropriate. |
| IVF day-3 transfer | Add 263 days to the transfer date (Calcipedia, 2026). | The embryo stage and transfer date must be entered correctly. |
| IVF day-5 transfer | Add 261 days to the transfer date (Calcipedia, 2026). | The embryo stage and transfer date must be entered correctly. |
If different methods produce different dates, that does not necessarily mean something is wrong. Bring the dates and their sources to your prenatal clinician, who can identify the expected date of delivery used for your care.
Irregular cycles, unknown periods, and IVF dating
If your cycles are irregular or you do not know when your last period began, an LMP calculation may create a precise-looking result from uncertain information. Do not guess simply to fill the field. Use a clinician-assigned ultrasound date when available, or ask your care team how the pregnancy is being dated.
IVF dating is different because the timing of fertilization and embryo development is known more closely. On the transfer date, a day-3 embryo is conventionally dated as a pregnancy of 2 weeks and 3 days, while a day-5 embryo is dated as 2 weeks and 5 days (London Pregnancy Clinic, 2024).
That convention produces an estimated due date 263 days after a day-3 transfer and 261 days after a day-5 transfer (Calcipedia, 2026). Confirm whether your clinic recorded a cleavage-stage or blastocyst transfer before choosing an option. Frozen and fresh transfers use the relevant transfer date, but your fertility clinic's documented date should remain the primary reference.
How to interpret the recalculated date
Treat the result as an estimate around which prenatal care is organized, not as an appointment your body must keep. Labor can begin before or after an estimated date, and the supplied sources do not provide a figure for the percentage of births occurring exactly on that date.
A changed result also does not show whether pregnancy growth is normal. Only a qualified clinician with access to your history, examinations, and scan reports can assess that. Contact your care team if the recalculated date is substantially different from your record, if you are unsure which date to use, or if you have concerns about symptoms or fetal movement.
Keep using the expected due date documented by your clinician for appointments and pregnancy-week calculations unless they officially revise it. If you are near the end of pregnancy, follow the labor and urgent-care instructions given by your own hospital or birth team rather than relying on a calendar alone.
Frequently asked questions
How do I recalculate my due date?
Enter the first day of your last menstrual period, a clinician-assigned ultrasound date, or the requested IVF transfer details. Standard LMP dating adds 280 days, or 40 weeks, to the first day of the period (ACOG, 2017). If the result differs from your medical record, ask your clinician which date should guide your care.
Which due date is more accurate, LMP or ultrasound?
An accurate early ultrasound may be more useful when the LMP is uncertain or cycles are irregular. ACOG recommends comparing menstrual information with ultrasound findings when establishing gestational age (ACOG, 2017). Accuracy depends on scan timing and quality, so a calculator cannot determine whether an ultrasound date should replace the date in your record.
Can my due date change after an ultrasound?
Yes, a clinician may revise the due date when ultrasound dating and menstrual dating differ enough to meet clinical criteria. The decision also considers scan timing and confidence in the LMP. ACOG advises that later changes to an established expected due date should be limited and documented (ACOG, 2017).
How is a due date calculated with irregular periods?
An ultrasound-based date is often more useful when irregular periods make the LMP uncertain. The conventional LMP method still adds 280 days, or 40 weeks, to the first day of the last period (ACOG, 2017), but that calculation may not reflect when ovulation occurred. Ask your clinician which date has been selected for your pregnancy record.
What if I do not know the date of my last period?
Use a clinician-assigned ultrasound date if one is available. Guessing an LMP can produce a misleadingly precise result, particularly when bleeding was unusual or cycles varied. Your clinician can combine ultrasound findings with your history to establish an expected due date; this calculator cannot reconstruct an unknown menstrual date.
How do I calculate an IVF due date?
Use the embryo transfer date and select the correct embryo stage. A day-3 transfer is estimated by adding 263 days, while a day-5 transfer is estimated by adding 261 days (Calcipedia, 2026). Confirm the stage with your fertility clinic because choosing the wrong transfer type changes the result.
Is a calculated due date exact?
No, a calculated due date is an estimate used to date the pregnancy and organize care. It cannot predict the exact start of labor. The supplied sources do not provide a figure for how often birth occurs on the estimated date, so no exact probability is presented here.
How due dates are estimated
A due date is an estimate calculated as 280 days, or 40 weeks, from the first day of the last menstrual period.
ACOG guidance says first-trimester ultrasound through 13 6/7 weeks is the most accurate way to establish or confirm gestational age.
Once your clinician establishes the best due date, it should generally not be revised using later scans.
| Timing | Classification |
|---|---|
| 37-38 weeks | Early term |
| 39-40 weeks | Full term |
| 41 weeks | Late term |
| 42 weeks | Postterm |