Due Date Calculator

How to use the due date calculator

Choose the input that best matches what you know. Use the first day of the last menstrual period, not the last day of bleeding, when the period was normal and the date is certain. Add the usual cycle length if the calculator accepts it. Use an established ultrasound due date when a clinician has already dated the pregnancy, and use transfer date plus embryo age after in vitro fertilization.

Treat the result as an estimated date of delivery, or EDD. It marks 40 weeks 0 days on the obstetric calendar. It does not predict the day labor will begin.

Record the input method beside the date because an LMP estimate, an early crown-rump length measurement, and an embryo transfer date do not carry the same uncertainty. Worked example: an LMP of January 8, 2026 gives an unadjusted EDD of October 15, 2026 because January 8 plus 280 days is October 15. If cycles usually last 32 days, a cycle-adjusted estimate adds four more days, producing October 19. A clinician may still retain or revise that date after reviewing the first accurate ultrasound.

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LMP, cycle-length, and conception formulas

Naegele's rule is EDD = LMP + 280 days. The calendar shortcut is LMP + 7 days, minus 3 months, plus 1 year. Both versions assume a 28-day cycle and ovulation near cycle day 14. They also assume the reported bleed was a true menstrual period rather than implantation-related or other early-pregnancy bleeding.

A common cycle correction is EDD = LMP + 280 days + (usual cycle length - 28 days). For a reliable 24-day cycle, subtract four days from the unadjusted date. For a reliable 35-day cycle, add seven days. This correction models later or earlier ovulation, but the luteal phase is not exactly 14 days in every person.

When conception or ovulation is documented, EDD = conception date + 266 days. Intercourse date is not a conception date because sperm can remain capable of fertilization for several days.

A positive ovulation test identifies an LH surge, not the exact hour of ovulation, so an early ultrasound can still provide a stronger clinical anchor.

[1][3][4]

How ultrasound changes dating confidence

ACOG identifies first-trimester crown-rump length, through 13 weeks 6 days, as the most accurate ultrasound method for establishing or confirming gestational age. Its expected accuracy is about plus or minus 5 to 7 days. The sonographer measures the embryo in a neutral position, and clinical equipment converts the measurement with a validated reference equation.

Accuracy widens as pregnancy advances. Composite head, abdominal, and femur measurements are accurate to about 7 to 10 days from 14 weeks through 21 weeks 6 days, and about 10 to 14 days from 22 weeks through 27 weeks 6 days. Third-trimester biometric dating is least reliable, with an error range around 21 to 30 days.

A late size-date difference can represent fetal growth restriction or accelerated growth rather than a wrong due date. Clinicians therefore preserve a reliable early EDD instead of repeatedly moving the date to match later scans.

Mean gestational sac diameter is not recommended for assigning the final due date when crown-rump length is available.

[1][5][6]

Clinical ultrasound redating thresholds

A difference between a calculator and an ultrasound does not automatically move the official EDD. ACOG recommends redating only when the discrepancy exceeds a gestational-age-specific threshold and the scan is the first dependable examination. Before 9 weeks, more than 5 days supports redating. From 9 weeks through 13 weeks 6 days, the threshold is more than 7 days.

Later thresholds are wider because biologic size variation has increased. The official date normally stays fixed after a reliable first-trimester scan.

Any change should be discussed and documented in the medical record so prenatal tests, growth charts, and delivery planning continue to use one obstetric clock.

[1][5]

Irregular cycles and uncertain bleeding dates

LMP dating weakens when cycles vary, the first day is uncertain, ovulation is irregular, hormonal contraception recently stopped, or bleeding occurred after conception. Polycystic ovary syndrome, breastfeeding, perimenopause, thyroid disorders, illness, and major weight change can shift ovulation far from cycle day 14.

Do not force an average cycle length into the formula when recent cycles ranged widely. Save the calculator result as a provisional estimate and arrange early prenatal care. If LMP is unknown or unreliable, ACOG favors the earliest accurate crown-rump length measurement, ideally no later than 13 weeks 6 days.

Bring period logs, positive pregnancy-test dates, ovulation-test results, fertility treatment records, and the date of any unusual bleeding to the visit. These details help a clinician judge whether an LMP is biologically plausible.

They do not replace ultrasound when the evidence conflicts.

[1][2][7]

IVF, insemination, and known ovulation

Pregnancies conceived through assisted reproductive technology use procedure-derived dating. ACOG specifies that a day-5 embryo transfer has an EDD 261 days after transfer and a day-3 transfer has an EDD 263 days after transfer. The equivalent gestational age on transfer day is 2 weeks plus the embryo age, so a day-5 transfer starts at 2 weeks 5 days.

For a day-5 embryo transferred on February 20, 2026, add 261 days to obtain November 8, 2026. Do not substitute the medication-cycle bleed or the retrieval date if the fertility clinic has already issued an EDD. Frozen transfers follow the documented embryo age and transfer date.

Ovulation induction or intrauterine insemination provides more timing information than LMP alone, but fertilization is not directly observed. The treating clinic may combine trigger, insemination, and early ultrasound information.

Keep its assigned EDD in the obstetric chart unless the clinicians identify a documented error.

[1][8]

Twins, multiples, and later growth scans

Twins have one pregnancy due date even when later measurements differ. Dating should be established early, when crown-rump length is most useful, and chorionicity should also be documented because shared versus separate placentas changes surveillance and delivery planning.

An EDD of 40 weeks still defines gestational age in a twin pregnancy. It does not mean clinicians expect a twin pregnancy to continue to 40 weeks.

Timing of birth depends on chorionicity, complications, fetal growth, prior obstetric history, and the care team's current guidance. If one fetus becomes smaller, moving that fetus's date forward can hide growth restriction. Clinicians compare serial growth against the established pregnancy date. A calculator cannot interpret discordant twin growth or decide when surveillance or delivery is indicated.

[1][6][9]

What the due date means near term

ACOG defines early term as 37 weeks 0 days through 38 weeks 6 days, full term as 39 weeks 0 days through 40 weeks 6 days, late term as 41 weeks 0 days through 41 weeks 6 days, and postterm as 42 weeks 0 days or later. The EDD sits inside the full-term interval.

These categories guide risk discussions and delivery timing. They do not convert the estimate into a deadline. Medical indications can make earlier delivery safer, while uncomplicated pregnancies may continue beyond the EDD under a clinician's surveillance plan.

Call the maternity unit for regular painful contractions, fluid leakage, vaginal bleeding, severe headache, vision changes, chest pain, or reduced fetal movement.

A date calculator cannot determine whether labor has started or whether a symptom needs urgent evaluation.

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Uncertainty windows and suboptimal dating

A due date combines a biological estimate with a calendar calculation. Even precise embryo dating does not predict spontaneous delivery day. Studies comparing dating methods find that only a small minority of births occur on the exact EDD, while many occur within days on either side.

ACOG calls a pregnancy suboptimally dated when no ultrasound confirmed or revised the EDD before 22 weeks 0 days. In that situation, elective delivery has no role. Clinicians use the best clinical estimate, may repeat ultrasound after 3 to 4 weeks to assess interval growth, and may begin late-pregnancy surveillance earlier because the pregnancy could be further advanced than believed.

Do not average two competing due dates. Preserve each source and let the obstetric clinician select the best estimate.

Repeatedly changing dates can distort growth percentiles and screening windows, especially when the later discrepancy comes from abnormal fetal growth.

[5][6][12]

Clinical use and calculator limits

Gestational dating determines when clinicians interpret genetic screening, anatomy scans, fetal growth, preterm interventions, and late-term surveillance. CDC birth records use the best obstetric estimate rather than LMP alone because recall errors, irregular cycles, and early bleeding can misclassify gestational age.

Use this calculator to prepare for care, not to schedule medication, testing, induction, or travel without clinical advice. It cannot assess pregnancy location, viability, fetal growth, or the cause of bleeding.

Early pregnancy pain, shoulder pain, fainting, or heavy bleeding needs urgent assessment because ectopic pregnancy and hemorrhage cannot be excluded by a plausible due date. The official EDD belongs in the prenatal record with its source. Once a clinician establishes it from reliable data, use that same date in apps, forms, and later calculations. Ask whether a proposed redating meets the applicable threshold before replacing an earlier date.

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How it works

Naegele's rule: Due date = LMP + 280 days (40 weeks). Only ~5% of babies arrive on the due date.

Frequently asked questions

• How accurate is the due date?LMP-based dating is ±2 weeks. First-trimester ultrasound is more accurate for dating.

Related calculators

References

1. ACOG. Methods for Estimating the Due Date
2. NIH/NLM. Estimated Date of Delivery
3. ACOG. Optimizing Natural Fertility
4. Wilcox et al.. Timing of Sexual Intercourse in Relation to Ovulation
5. ACOG. Management of Suboptimally Dated Pregnancies
6. NIH/NLM. Sonography Assessment of Gestational Age
7. Wegienka and Baird. A Comparison of Recalled Date of Last Menstrual Period With Prospectively Recorded Dates
8. Tunón et al.. Gestational Age in Pregnancies Conceived After In Vitro Fertilization
9. ACOG. Role of Ultrasound in Twin Pregnancy
10. ACOG. Definition of Term Pregnancy
11. CDC. Urgent Maternal Warning Signs
12. Savitz et al.. Comparison of Pregnancy Dating by Last Menstrual Period, Ultrasound Scanning, and Their Combination