Pregnancy Due Date Calculator
Estimate a due date from the first day of your last menstrual period, a known conception date, an IVF transfer, a clinical ultrasound report, or an already assigned due date. See gestational age, trimester, time remaining, and key week-by-week dates.
- 5 dating methods
- Weeks and days
- Pregnancy milestones
- Calendar-safe math
Calculate Your Estimated Due Date
Select the best information you have. For clinical decisions, use the estimated due date recorded by your obstetrician, doctor, or midwife.
Your result will appear here.
Pregnancy timeline
Waiting for inputKey pregnancy dates
| Milestone | Calendar date | Timing | Status on selected date |
|---|---|---|---|
| Calculate a result to view the timeline. | |||
Calculation steps
- Select a calculation method and enter a valid date.
- The calculator will convert your input into an estimated 40-week due date.
Privacy note: All date calculations run on this page. The calculator does not send or store the dates you enter.
How to Use This Pregnancy Due Date Calculator
- Select the dating method that matches your most reliable information.
- For LMP dating, enter the first day of your last menstrual period and your usual cycle length. Use this option only when the date is known and cycles are reasonably regular.
- For conception or ovulation dating, enter the known biological date. Do not substitute the date of intercourse because fertilization may occur later.
- For IVF, enter the embryo transfer date and choose whether a day-3 or day-5 embryo was transferred.
- For ultrasound dating, copy the scan date and the completed gestational weeks and days stated on the clinical report.
- If your maternity team has already assigned an EDD, choose the confirmed due-date option and enter it directly.
- Keep “Calculate gestational age on” set to today, or choose another calendar date.
- Select “Calculate Due Date.” Review the EDD, completed weeks and days, trimester, time remaining, milestone table, and formula steps.
What Is an Estimated Due Date?
An estimated due date, often shortened to EDD, is the calendar date at which gestational age reaches 40 weeks and 0 days. It gives prenatal teams a common timeline for appointments, screening windows, fetal growth assessment, and discussions about care. It does not predict the exact day labour will start.
Gestational age is counted from an LMP-equivalent starting date, about two weeks before conception in the standard model. This is why someone may be described as four weeks pregnant about two weeks after conception. The convention keeps obstetric dating consistent even though conception often cannot be observed directly.
The NHS states that pregnancy commonly lasts from 37 to 42 weeks from the first day of the last period. That five-week span does not make every date clinically identical. Modern obstetric terminology separates early term, full term, late term, and postterm periods, and individual care depends on the health of the pregnant person and fetus.
Pregnancy Due Date Formulas
Last menstrual period and Naegele’s rule
For a reliable LMP and a regular 28-day cycle, the standard calculation adds 280 days, or 40 weeks, to the first day of the last menstrual period.
Traditional Naegele’s rule expresses the same estimate as: add seven days, subtract three months, and add one year. Exact day-based code handles month lengths and leap years more safely. When a regular cycle is longer or shorter than 28 days, the tool makes a simple cycle adjustment:
This adjustment assumes ovulation shifts by the same number of days as the cycle length. It is not dependable for irregular cycles, uncertain bleeding dates, or cycles outside the supported 21-to-35-day range.
Known conception or ovulation date
Pregnancy averages about 266 days, or 38 weeks, from conception to the estimated due date. The calculator adds 266 days to a known conception or ovulation date. An intercourse date is less precise because sperm may remain capable of fertilization for several days.
IVF embryo transfer
Assisted-reproduction dating uses the transfer date and embryo age. A day-3 embryo has already completed three of the 266 post-conception days. A day-5 embryo has completed five.
Day-3 transfer: add 263 days
Day-5 transfer: add 261 days
Use the embryo age and official EDD supplied by the fertility clinic. Do not replace an ART-derived date with a period-based calculation.
Ultrasound report
If a clinical report states gestational age on the scan date, the remaining days to 40 weeks can be added to that date. The tool accepts completed weeks plus 0 to 6 additional days.
This option does not interpret crown-rump length, head measurements, or other fetal measurements. Those measurements require validated equipment, trained operators, and clinical interpretation.
Worked Due Date Example
Suppose the first day of the last menstrual period was May 5, 2026, and the usual cycle length was 30 days.
- Start with 280 days for a standard 28-day cycle.
- Calculate the cycle adjustment: 30 − 28 = 2 days.
- Add 282 days to May 5, 2026.
- The adjusted estimated due date is February 11, 2027.
- The LMP-equivalent dating start is two days later than the entered LMP because the cycle adjustment shifts the assumed ovulation timing.
The result remains an estimate. If an appropriate ultrasound establishes a different EDD, the maternity team decides whether the difference meets clinical redating criteria.
Comparison of Pregnancy Dating Methods
| Method | Best used when | Main calculation | Important limitation |
|---|---|---|---|
| LMP | The first day is certain and cycles are regular | LMP + 280 days, adjusted for cycle length | Assumes predictable ovulation and accurate recall |
| Conception or ovulation | The biological date is known from reliable tracking | Date + 266 days | Intercourse date is not the same as confirmed conception |
| IVF transfer | Pregnancy followed embryo transfer | Transfer + 266 days − embryo age | Clinic records should control the official EDD |
| Ultrasound report | A report gives gestational age on the scan date | Scan + remaining days to 280 | Accuracy falls as pregnancy advances |
| Confirmed EDD | A maternity professional has assigned the due date | Uses the recorded EDD directly | Do not change it without clinical review |
Gestational Age, Trimesters, and Term Ranges
The calculator reports completed gestational weeks and remaining days. For example, 18 weeks and 4 days means 18 full weeks have passed and the pregnancy is four days into the next week. It does not round to 19 weeks.
This page labels the first trimester as 0 weeks 0 days through 13 weeks 6 days, the second trimester as 14 weeks 0 days through 27 weeks 6 days, and the third trimester as 28 weeks 0 days onward.
| Obstetric range | Gestational age | How the calculator labels it |
|---|---|---|
| Preterm | Before 37 weeks 0 days | Third trimester, before term |
| Early term | 37 weeks 0 days to 38 weeks 6 days | Early term |
| Full term | 39 weeks 0 days to 40 weeks 6 days | Full term |
| Late term | 41 weeks 0 days to 41 weeks 6 days | Late term |
| Postterm | 42 weeks 0 days or later | Postterm |
These ranges describe gestational timing. They do not tell you when delivery should occur. Induction, caesarean birth, monitoring, and other care decisions require an individualized medical assessment.
How Accurate Is a Due Date Calculator?
A calculator is exact at calendar arithmetic but limited by the input. Period-based dating assumes the LMP is remembered correctly, bleeding represented a true menstrual period, and ovulation timing roughly followed the stated cycle length. Recent hormonal contraception, breastfeeding, irregular cycles, postpartum changes, or bleeding in early pregnancy may weaken those assumptions.
First-trimester crown-rump-length ultrasound is the most accurate ultrasound dating approach. ACOG guidance describes accuracy of about 5 to 7 days through 13 weeks 6 days. Approximate accuracy widens later in pregnancy, reaching roughly 21 to 30 days for third-trimester ultrasound dating. A late size difference may reflect fetal growth rather than an incorrect due date, so a later scan should not be used to casually move an established EDD.
Once a due date is selected from reliable early information, subsequent changes should be uncommon, discussed with the patient, and documented. A pregnancy without an ultrasound that confirms or revises the EDD before 22 weeks 0 days is considered suboptimally dated under ACOG guidance.
When the LMP Method Needs Extra Caution
- You do not know the first day of the last menstrual period.
- Your cycles vary substantially from month to month.
- The bleeding may have been spotting rather than a usual period.
- You recently stopped hormonal contraception.
- You were breastfeeding, recently pregnant, or had not resumed regular cycles.
- You conceived through IVF or another assisted-reproduction method.
- An early ultrasound gives a different gestational age.
In these situations, enter a clinician-assigned EDD when available. Do not average two different due dates or repeatedly change the date based on each new scan.
Common Due Date Calculator Mistakes
- Entering the last day of a period instead of the first day.
- Using an intercourse date as if it were a confirmed conception date.
- Entering an average cycle length when cycles are irregular.
- Selecting day 3 instead of day 5 for an IVF transfer, or the reverse.
- Entering rounded ultrasound weeks while leaving out the additional days.
- Assuming a due date predicts the exact day of birth.
- Changing a clinician-established EDD without medical review.
- Reading “18 weeks, 4 days” as 18.4 decimal weeks.
Calculation Method and Review Sources
The 280-day LMP estimate, cycle-length input, and general 37-to-42-week guidance were reviewed against the NHS due date guidance. The dating hierarchy, ultrasound accuracy, ART calculation, and guidance on retaining an established EDD were checked against ACOG Committee Opinion 700, hosted by SMFM, and its PubMed record.
The definition of preterm birth was checked against the World Health Organization preterm birth fact sheet. Term ranges were reviewed against the NICHD term pregnancy definitions. Urgent symptom wording was reviewed against the CDC urgent maternal warning signs. Sources inform the educational method; they do not endorse this website or replace prenatal care.
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Frequently Asked Questions
How is a pregnancy due date calculated from LMP?
Add 280 days, or 40 weeks, to the first day of the last menstrual period. For a regular cycle between 21 and 35 days, this tool also adds the difference between the cycle length and 28 days.
Why does pregnancy dating start before conception?
Obstetric gestational age traditionally starts on the first day of the LMP, about two weeks before conception in a typical 28-day cycle. This shared convention supports consistent clinical scheduling.
Is a due date the exact date my baby will be born?
No. It is the date gestational age reaches 40 weeks under the selected dating method. Labour may begin before or after the EDD.
Should I use LMP or ultrasound for my due date?
Use the EDD assigned by your maternity professional. Reliable first-trimester ultrasound is especially useful when the LMP is unknown, uncertain, or inconsistent with cycle history.
How does cycle length change the estimated due date?
For a regular 30-day cycle, the calculator adds two days to the standard LMP estimate. For a regular 26-day cycle, it subtracts two days. The adjustment is not reliable for irregular cycles.
How is an IVF due date calculated?
Add 263 days to a day-3 embryo transfer or 261 days to a day-5 transfer. The fertility clinic’s official ART-derived EDD should remain the primary date.
What if I do not know my last period date?
Do not guess if the uncertainty is meaningful. Contact a doctor or midwife for pregnancy assessment and clinical dating, which may include an appropriately timed ultrasound.
Does the calculator work for twins or triplets?
The basic gestational dating method is the same, but multiple pregnancies often need different monitoring and delivery planning. Follow the schedule and EDD recorded by the specialist maternity team.
Why did my clinician change my due date?
An early ultrasound, assisted-reproduction records, uncertain LMP, or a clinically significant date difference may support redating. The reason and final EDD should be documented in your medical record.
Health and Calculation Disclaimer
This calculator provides educational date estimates only. It does not confirm pregnancy, fetal age, fetal health, or when labour will begin, and it is not medical or financial advice. Use the EDD assigned by your maternity team. Seek urgent medical care for severe or concerning symptoms, including heavy bleeding, fluid leakage, severe pain, chest pain, trouble breathing, severe headache, vision changes, fever of 38°C (100.4°F) or higher, or reduced fetal movement.