Pregnancy Due Date Calculator Guide: Estimate EDD, Gestational Age, Trimester, and IVF Dating
A pregnancy due date is more accurately called the estimated due date, or EDD. It provides a standardized reference point for measuring gestational age, scheduling prenatal testing, evaluating fetal growth, and describing whether a pregnancy is preterm, term, late term, or postterm.
ACOG states that the average pregnancy lasts approximately 280 days, or 40 weeks, counted from the first day of the last menstrual period. This convention means gestational age begins about two weeks before fertilization in a typical 28-day menstrual cycle.
That difference often causes confusion. A person described as six weeks pregnant usually conceived approximately four weeks earlier under ordinary menstrual dating. Gestational age and fetal age are therefore not the same measurement.
The traditional LMP calculation assumes a roughly 28-day menstrual cycle with ovulation around day 14. When cycles are substantially shorter, longer, or irregular, the basic LMP estimate can be less reliable unless cycle length or ovulation information is taken into account.
Early ultrasound provides another important dating source. ACOG, the American Institute of Ultrasound in Medicine, and the Society for Maternal-Fetal Medicine state that first-trimester ultrasound—through 13 weeks and 6 days—is the most accurate ultrasound method for establishing or confirming gestational age.
If the last menstrual period is uncertain or inconsistent with an early ultrasound beyond accepted clinical limits, the clinician may establish the EDD from the ultrasound instead. ACOG specifically notes that before 9 weeks of gestation, a discrepancy greater than five days can be a reason to change the estimated due date, with different discrepancy thresholds used later in pregnancy.
Once the best obstetric estimate has been established from the available menstrual, ultrasound, or assisted-reproduction data, ACOG recommends documenting it clearly. Later ultrasound examinations are generally used to assess fetal growth rather than repeatedly changing an already well-established due date.
Assisted reproductive technology requires a different method. ACOG states that pregnancies resulting from ART should use the ART-derived gestational age. For IVF, the age of the embryo and the date it was transferred determine the EDD.
For example, ACOG gives 261 days from the transfer date for a day-5 embryo and 263 days from the transfer date for a day-3 embryo. This reflects the fact that the embryo has already developed for several days before transfer.
The EDD should also not be interpreted as the date on which birth is expected to occur with certainty. ACOG states that only about one in twenty women gives birth on the exact estimated due date. The EDD is primarily a gestational-age anchor rather than a precise prediction of labor onset.
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 and beyond.
This calculator therefore reports more than one date. It shows the estimated due date, current gestational age, trimester, approximate conception timing where appropriate, and milestone dates while making clear which dating method produced the result.
How to Calculate a Pregnancy Due Date From LMP, Conception, IVF Transfer, or Ultrasound Dating
- Choose the dating method: Select last menstrual period, known conception or ovulation date, IVF embryo transfer, or established ultrasound/clinical due date.
- Use LMP when the menstrual date is reliable: Enter the first day—not the final day—of the last menstrual period.
- Enter cycle length when supported: If cycles are consistently longer or shorter than 28 days, cycle-length adjustment can provide an alternative estimate, but clinical ultrasound dating may be more reliable when cycles are irregular.
- Use conception mode only when timing is reasonably known: Sexual intercourse date is not automatically the conception date because sperm can survive for several days and ovulation timing can vary.
- For IVF, enter embryo age and transfer date: Choose day-3 or day-5 transfer, or another clinically documented embryo age if the calculator supports it.
- Use the established clinical EDD when one has already been assigned: If your prenatal clinician has established a due date from early ultrasound or ART dating, use that date rather than repeatedly recalculating from later scans.
- Review gestational age: The calculator expresses pregnancy progress in completed weeks and days rather than decimal weeks.
- Review trimester and term milestones: Use gestational-age boundaries rather than treating the EDD itself as the start of a single fixed delivery window.
Formula and variables
Standard pregnancy dating counts 280 days from the first day of the last menstrual period. When fertilization timing is known, approximately 266 days are counted from conception because conception ordinarily occurs about two gestational weeks after the LMP reference date. ACOG specifies ART-derived dating for IVF, including 261 days after transfer for a day-5 embryo and 263 days after transfer for a day-3 embryo.
LMP EDD = first day of LMP + 280 days; Conception EDD ≈ conception date + 266 days; IVF day-5 transfer EDD = transfer date + 261 days; IVF day-3 transfer EDD = transfer date + 263 days- LMP — Last menstrual period
- The first day of the last menstrual period used as the standard starting point for gestational age.
- EDD — Estimated due date
- The estimated calendar date corresponding to 40 weeks of gestational age under the selected dating method.
- CD — Known conception date
- A known or closely estimated fertilization or conception date when available.
- ET — Embryo transfer date
- The IVF embryo-transfer date used with embryo age to calculate ART-derived gestational age.
- GA — Gestational age
- Pregnancy age measured in completed weeks and days from the established obstetric dating reference.
Scenario 1: Due Date From a Known Last Menstrual Period
The first day of the last menstrual period is January 1, 2026. The calculator uses the standard 280-day gestational dating convention.
- First day of LMP
- January 1, 2026
- Standard gestational duration
- 280 days
- Start with January 1, 2026 as gestational day 0.
- Add 280 calendar days.
- The estimated due date is approximately October 8, 2026.
- Estimated conception under a standard 28-day-cycle assumption would be approximately two weeks after the LMP date.
Result: The estimated due date is October 8, 2026 under standard LMP-based dating.
This is an estimated delivery date and a gestational-age reference point. Early ultrasound or known ART dating can confirm or revise the obstetric estimate when clinically appropriate.
Understanding your results
Estimated due date
This is the date corresponding to approximately 40 weeks of gestation under the selected dating method.
It is not a guarantee that labor or delivery will occur on that date.
Current gestational age
This is expressed in completed weeks plus additional days.
For example, 12 weeks and 4 days means 12 complete weeks have passed and the pregnancy is four days into the next week.
Estimated conception date
In LMP mode this is an approximation based on ordinary ovulation timing.
It should not be presented as a precisely known biological event unless fertilization timing is actually known.
Trimester
ACOG defines the first trimester through 13 weeks 6 days, second trimester from 14 weeks 0 days through 27 weeks 6 days, and third trimester beginning at 28 weeks.
The EDD itself corresponds approximately to 40 weeks.
Term classification
Early term begins at 37 weeks, full term at 39 weeks, late term at 41 weeks, and postterm at 42 weeks under ACOG definitions.
These categories describe gestational timing rather than predicting when spontaneous labor will occur.
Assumptions
- LMP mode assumes the entered date is the first day of the last menstrual period.
- The basic LMP method uses a 280-day gestational duration.
- Conception-date mode assumes the entered date is reasonably close to actual fertilization.
- IVF mode uses known embryo age and transfer date.
- The calculator uses calendar-date arithmetic rather than timestamp arithmetic.
- The result is an estimated due date rather than a guaranteed delivery date.
- A clinician-established EDD should take precedence over repeated consumer recalculation when pregnancy dating has already been formally established.
Limitations
- LMP dating can be inaccurate when the menstrual date is uncertain, cycles are irregular, cycle length differs substantially from 28 days, ovulation timing is atypical, or bleeding was mistaken for menstruation.
- ACOG states that first-trimester ultrasound is the most accurate ultrasound method for establishing or confirming gestational age.
- Later ultrasound has a wider dating margin of error than early ultrasound and generally should not repeatedly change a well-established EDD.
- ACOG recommends ART-derived dating when pregnancy results from assisted reproductive technology.
- Sexual intercourse date is not necessarily conception date.
- Ovulation can occur earlier or later than day 14 even in apparently regular cycles.
- The calculator cannot diagnose fetal growth restriction, pregnancy viability, miscarriage, ectopic pregnancy, preterm labor, or another obstetric condition.
- Only about one in twenty women gives birth on the exact estimated due date, according to ACOG.
- Multiple pregnancy, medical complications, prior obstetric history, fetal conditions, or maternal conditions can affect recommended delivery timing independently of the calculated EDD.
- The estimated due date should not be used to schedule elective delivery without clinical guidance.
- ACOG recommends avoiding nonmedically indicated delivery before 39 weeks in appropriate uncomplicated pregnancies.
- The calculator does not determine whether induction or cesarean delivery is medically indicated.
Common mistakes
- Entering the last day of the menstrual period instead of the first day.
- Assuming conception occurred on the first day of the LMP.
- Calling gestational age fetal age.
- Assuming every cycle is exactly 28 days.
- Assuming ovulation always occurs on day 14.
- Using intercourse date as exact conception date.
- Using LMP dating for an IVF pregnancy when embryo age and transfer date are known.
- Adding 280 days to the IVF transfer date.
- Using the same formula for day-3 and day-5 embryo transfers.
- Changing the due date every time a later ultrasound gives a slightly different size estimate.
- Treating the EDD as a guaranteed birthday.
- Calling every delivery from 37 through 42 weeks simply full term.
Practical use cases
Scenario 2: LMP-based pregnancy dating
A user knows the first day of the last menstrual period and had reasonably regular cycles.
The standard 280-day calculation provides an initial EDD estimate.
Scenario 3: Known conception date
Fertilization timing is reasonably known through ovulation monitoring or another clinical context.
The calculator adds approximately 266 days from conception rather than using an assumed LMP date.
Scenario 4: Day-5 IVF embryo transfer
An embryo was transferred after five days of development.
ACOG specifies an EDD 261 days after the day-5 transfer date.
Scenario 5: Day-3 IVF embryo transfer
A three-day embryo was transferred.
ACOG specifies an EDD 263 days after the transfer date.
Scenario 6: LMP and early ultrasound disagree
The menstrual estimate and first-trimester ultrasound produce different dates.
The calculator can display both, but clinical redating should follow the obstetric dating criteria applied by the prenatal clinician.
Planning and decision guide
Pregnancy dating begins from the LMP convention
ACOG states that average singleton gestation lasts 40 weeks, or 280 days, from the first day of the last menstrual period to the estimated due date.
This convention predates modern ability to identify conception precisely and remains the standard gestational-age framework.
Scenario 7: Four weeks pregnant does not mean conception occurred four weeks ago
Under standard gestational dating, the first approximately two weeks precede fertilization.
The gestational-age clock begins from LMP rather than conception.
Conception age is usually about two weeks less than gestational age
In a typical 28-day cycle with ovulation near day 14, fertilization occurs approximately two weeks after the LMP reference date.
This is an approximation, not a universal biological schedule.
Naegele-style dating assumes ordinary cycle timing
The familiar clinical shortcut adds about one year, subtracts three months, and adds seven days to the LMP date, which is mathematically equivalent to an approximately 280-day pregnancy.
Direct date arithmetic avoids confusion across leap years and different month lengths.
Cycle length can shift an LMP estimate
A consistently longer cycle often implies later ovulation, while a shorter cycle can imply earlier ovulation.
An optional consumer adjustment can add the difference between actual average cycle length and 28 days to the standard LMP EDD.
Scenario 8: 32-day cycle
The cycle is four days longer than the standard 28-day assumption.
A simple cycle-adjusted estimate would move the LMP-derived EDD approximately four days later, although early ultrasound remains clinically important.
Cycle adjustment should remain optional
Menstrual cycle length is not a perfect proxy for ovulation date.
Irregular cycles in particular should not be forced through a simple arithmetic correction.
Early ultrasound is the strongest ultrasound dating method
ACOG states that ultrasound through 13 weeks 6 days is the most accurate method to establish or confirm gestational age.
Early crown-rump length measurements are therefore important when menstrual dating is uncertain or inconsistent.
Scenario 9: LMP uncertain
The user cannot reliably identify the last menstrual period.
A first-trimester ultrasound can provide a stronger clinical basis for assigning gestational age than guessing an LMP date.
Ultrasound dating has gestation-specific discrepancy thresholds
ACOG does not recommend changing an EDD for every small ultrasound difference.
The amount of discrepancy needed for redating depends on how early the ultrasound is performed.
Before 9 weeks, a discrepancy greater than five days can justify redating
ACOG gives more than five days as an appropriate reason for changing the EDD when ultrasound is performed before 9 weeks, particularly when clinical assessment also raises questions about LMP reliability.
Later pregnancy uses different thresholds.
Do not implement one universal ultrasound-redating threshold
The allowable discrepancy increases as pregnancy progresses because ultrasound dating becomes less precise.
If this site later adds a redating engine, it should implement the full ACOG gestational-age table rather than one hardcoded five-day rule.
Once an EDD is established, repeated changes should be rare
ACOG recommends determining and documenting gestational age and EDD as soon as reliable LMP and/or first accurate ultrasound information is available.
ACOG’s patient guidance notes that once a due date has been selected, later ultrasound examinations generally do not keep changing it.
Scenario 10: Third-trimester baby measures one week ahead
That may reflect fetal size rather than incorrect pregnancy dating.
The established EDD should not automatically be moved forward simply because a later ultrasound estimates a larger fetus.
IVF dating should use embryo age and transfer date
ACOG explicitly recommends ART-derived gestational age when pregnancy results from assisted reproductive technology.
This avoids imposing an artificial menstrual date when fertilization and embryo-development timing are already known.
Day-5 embryo transfer uses 261 days
ACOG states that the EDD for a day-5 embryo is 261 days from embryo replacement or transfer.
The embryo has already completed five days of development at transfer.
Day-3 embryo transfer uses 263 days
ACOG gives 263 days after transfer for a day-3 embryo.
The two-day difference between day-3 and day-5 calculations reflects embryo age at transfer.
Scenario 11: Same transfer date, different embryo age
A day-3 and day-5 embryo transferred on the same calendar date do not receive identical EDDs.
The older embryo corresponds to an EDD two days earlier.
Known fertilization date takes priority over estimated menstrual timing
ACOG’s obstetric data definitions identify known fertilization, such as assisted reproductive technology, as an accepted basis for the best EDD.
A generic LMP estimate adds no useful information when fertilization timing is already clinically known.
The EDD is a guide for pregnancy progress
ACOG states that the due date is used to check pregnancy progress and track fetal growth.
It also anchors the timing of prenatal tests and gestational-age-dependent decisions.
Only about 1 in 20 births occurs on the exact due date
ACOG patient guidance states that only about one in twenty women gives birth on the EDD.
The calculator should therefore avoid countdown language implying the exact birth date is known.
Scenario 12: Due date arrives without labor
Reaching 40 weeks does not automatically mean the pregnancy is abnormal.
The pregnancy remains within the full-term category through 40 weeks 6 days.
Term pregnancy is subdivided into clinically meaningful ranges
ACOG and SMFM recommend using early term, full term, late term, and postterm rather than treating the entire 37–42-week interval as biologically equivalent.
Neonatal outcomes differ even within the broad term period.
Early term is 37 weeks 0 days through 38 weeks 6 days
This is term gestation but earlier than the full-term window.
It should not be labeled preterm solely because it occurs before 39 weeks.
Full term is 39 weeks 0 days through 40 weeks 6 days
ACOG identifies this as the full-term interval.
The estimated due date falls within this range at 40 weeks exactly.
Late term is 41 weeks 0 days through 41 weeks 6 days
Pregnancy beyond the EDD can remain within a recognized term category.
Clinical monitoring and management decisions belong with the prenatal care team.
Postterm begins at 42 weeks
ACOG defines postterm pregnancy as 42 weeks 0 days and beyond.
A due-date calculator should identify this gestational threshold without recommending an induction plan.
Trimesters are also based on gestational age
ACOG currently describes the first trimester as LMP through 13 weeks 6 days, the second trimester as 14 weeks 0 days through 27 weeks 6 days, and the third trimester beginning at 28 weeks.
The calculator can display the current trimester automatically.
Scenario 13: 27 weeks 6 days versus 28 weeks
At 27 weeks 6 days, the pregnancy remains in the second trimester under ACOG definitions.
At 28 weeks 0 days, the third trimester begins.
Gestational age should be shown as weeks plus days
Obstetric practice uses notation such as 12 4/7 weeks rather than decimal values such as 12.57 weeks.
Consumer output such as “12 weeks, 4 days” preserves the same concept more clearly.
Do not round gestational age up
A pregnancy at 11 weeks 6 days has not completed 12 weeks.
Showing “12 weeks” can place a user on the wrong side of a clinical timing threshold.
Scenario 14: 13 weeks 6 days
This remains within ACOG’s first-trimester dating interval.
The following day, 14 weeks 0 days, begins the second trimester.
A due-date calculator is not a fetal-growth calculator
Gestational age tells how far pregnancy has progressed.
It does not determine whether fetal size is appropriate for that age.
The due date should not be adjusted from fundal height or symptoms alone
Abdominal size, fetal movement, nausea, or other symptoms cannot establish pregnancy dating with the precision of reliable LMP, ART information, or early ultrasound.
The calculator should not infer a new EDD from symptom patterns.
Multiple gestation can affect delivery planning without changing basic dating
Twins or higher-order multiples still have an established gestational age and EDD.
Recommended delivery timing can differ because of obstetric risk and therefore should not be generated by the generic due-date calculator.
Scenario 15: Twin pregnancy
The pregnancy may be dated to 40 weeks in the same gestational framework.
Clinical delivery planning can occur earlier based on chorionicity, fetal status, maternal health, and other obstetric considerations.
Medical indications can change delivery timing
ACOG notes that medical conditions can justify delivery before 39 weeks even though nonmedically indicated early delivery is discouraged.
The calculator should therefore avoid saying that every pregnancy should continue automatically to its EDD.
Scenario 16: Medically indicated early delivery
The EDD does not change.
The obstetric team may nevertheless recommend delivery earlier because the due date and recommended delivery date are different concepts.
The Blood Pressure Category Calculator should remain separate
Pregnancy dating cannot determine whether maternal blood pressure is normal.
Use the Blood Pressure Category Calculator for blood-pressure interpretation, including pregnancy-specific severe thresholds.
The EDD should not be used to self-diagnose preterm labor
Symptoms such as regular contractions, fluid leakage, bleeding, or concerning pain require obstetric assessment rather than date arithmetic.
The calculator can report gestational age but cannot determine whether labor is occurring.
Date arithmetic should use date-only values
A due date is a calendar date, not a UTC timestamp.
Using date-only arithmetic avoids time-zone bugs that can accidentally shift the displayed EDD by one day.
Scenario 17: User travels across time zones
The estimated due date should remain the same named calendar date in the medical record.
It should not move merely because the browser is now in another time zone.
Leap years should be handled through calendar arithmetic
Adding 280 actual calendar days automatically accounts for February 29 when applicable.
Hardcoded month arithmetic can easily introduce one-day errors.
The strongest result identifies the dating source
Display “LMP-based estimate,” “conception-based estimate,” “day-5 IVF transfer estimate,” or “clinician-established EDD.”
That is more medically meaningful than returning one unlabeled date.
Frequently asked questions
How is a pregnancy due date calculated?
ACOG states that the average pregnancy is 280 days, or 40 weeks, from the first day of the last menstrual period.
Do I count from the first or last day of my period?
Use the first day of the last menstrual period.
Why is pregnancy counted before conception?
Gestational age traditionally begins from the first day of the LMP, which is usually about two weeks before fertilization in a typical 28-day cycle.
Is pregnancy 40 weeks from conception?
No. It is approximately 40 weeks from LMP or about 38 weeks, approximately 266 days, from conception.
How many days is pregnancy?
Average gestation is approximately 280 days from LMP according to ACOG.
Can I calculate my due date from conception?
Yes when conception timing is reasonably known. Approximately 266 days are counted from fertilization.
Can intercourse date tell me my exact conception date?
No. Sperm can survive for several days and ovulation timing varies, so intercourse date is not necessarily fertilization date.
Does cycle length affect my due date?
It can affect an LMP-based estimate because the standard calculation assumes roughly a 28-day cycle and ovulation around day 14.
What if my periods are irregular?
LMP dating may be less reliable. ACOG states that first-trimester ultrasound is the most accurate ultrasound method for establishing or confirming gestational age.
Is ultrasound more accurate than my last period?
Early ultrasound can be more reliable when LMP is uncertain or inconsistent. ACOG identifies first-trimester ultrasound through 13 weeks 6 days as the most accurate ultrasound dating method.
Can ultrasound change my due date?
Yes when the discrepancy meets clinical redating criteria, especially early in pregnancy. Small differences do not automatically require a new EDD.
Can every ultrasound change my due date?
No. ACOG patient guidance states that once a due date has been selected, later ultrasound examinations generally do not repeatedly change it.
What is the most accurate time for dating ultrasound?
ACOG states that first-trimester ultrasound, through 13 weeks 6 days, is the most accurate ultrasound method for establishing or confirming gestational age.
How is an IVF due date calculated?
ACOG recommends using the embryo age and transfer date rather than an assumed LMP.
What is the due-date formula for a day-5 embryo transfer?
ACOG states that the EDD is 261 days after the day-5 embryo transfer.
What is the due-date formula for a day-3 embryo transfer?
ACOG states that the EDD is 263 days after the day-3 embryo transfer.
Should I use LMP for IVF pregnancy?
No when ART dating is known. ACOG recommends ART-derived gestational age for pregnancies resulting from assisted reproductive technology.
What is gestational age?
Gestational age is pregnancy age measured from the established obstetric dating reference, traditionally the first day of the LMP.
What is fetal age?
Fetal or conception age refers more closely to time since fertilization and is usually about two weeks less than gestational age in ordinary menstrual dating.
How many weeks pregnant am I?
Subtract the established pregnancy dating start from the current date and express the result as completed weeks plus remaining days.
When does the second trimester start?
ACOG describes the second trimester as beginning at 14 weeks 0 days.
When does the third trimester start?
ACOG describes the third trimester as beginning at 28 weeks 0 days.
When is a pregnancy early term?
ACOG defines early term as 37 weeks 0 days through 38 weeks 6 days.
When is pregnancy full term?
Full term is 39 weeks 0 days through 40 weeks 6 days.
When is pregnancy late term?
Late term is 41 weeks 0 days through 41 weeks 6 days.
When is pregnancy postterm?
Postterm begins at 42 weeks 0 days.
How accurate is a due date?
It is an estimate used for gestational dating. ACOG notes that only about one in twenty women gives birth on the exact EDD.
What percentage of babies arrive on the due date?
ACOG states that only about 1 in 20 women gives birth on the exact estimated due date.
Is it normal to still be pregnant on my due date?
Yes. The EDD is an estimate, and 40 weeks remains within ACOG’s full-term interval through 40 weeks 6 days.
Does passing my due date mean I am postterm?
No. ACOG defines late term as 41 weeks 0 days through 41 weeks 6 days and postterm beginning at 42 weeks.
Can twins have the same 40-week due date calculation?
Gestational dating still establishes an EDD, but recommended delivery timing for multiple pregnancies can differ and requires obstetric guidance.
Can a medical condition change when I should deliver?
Yes. The EDD can remain unchanged while a clinician recommends earlier delivery for a medical indication.
Can I use my due date to decide when to induce labor?
No. Induction timing depends on gestational age, maternal and fetal health, pregnancy history, and clinical guidance.
Should an uncomplicated pregnancy be electively delivered before 39 weeks?
ACOG discourages nonmedically indicated delivery before 39 weeks in appropriate pregnancies.
Can this calculator tell if I am in labor?
No. It calculates dates and gestational age; it cannot diagnose labor or pregnancy complications.
Can I use this calculator if I am bleeding or having pain?
The calculator cannot evaluate bleeding, pain, fluid leakage, contractions, or other pregnancy symptoms. Those symptoms may require obstetric assessment.
Why is my app due date different from my doctor’s due date?
Your clinician may have used early ultrasound or ART-derived dating rather than a simple LMP calculation. The clinically established EDD should generally be used for pregnancy care.
How accurate is this due-date calculator?
The date arithmetic is exact for the method entered. Biological accuracy depends on the reliability of LMP, conception timing, embryo-transfer information, ultrasound dating, and clinical interpretation.
Sources and review
- Methods for Estimating the Due Date — American College of Obstetricians and Gynecologists. Accessed 2026-09-01.
- When Pregnancy Goes Past Your Due Date — American College of Obstetricians and Gynecologists. Accessed 2026-09-01.
- How Your Fetus Grows During Pregnancy — American College of Obstetricians and Gynecologists. Accessed 2026-09-01.
- Definition of Term Pregnancy — American College of Obstetricians and Gynecologists. Accessed 2026-09-01.
- reVITALize: Obstetrics Data Definitions — American College of Obstetricians and Gynecologists. Accessed 2026-09-01.
Reviewed 2026-09-01 by Dr Akawak Ejigu, DBA.