Due Date Calculator: LMP, Conception and IVF

Pregnancy planning often starts with a date. This due date calculator can help you understand the calendar arithmetic behind that estimate, whether you enter your last menstrual period, an assumed fertilization date or an IVF transfer. The result is an estimated due date (EDD), rather than a prediction of the day labor will begin.

This guide explains the three input methods, the result cards, development milestones and how maternity-care dating differs from an online calculation. Keep the date established by your maternity team as your clinical reference. The calendar on this page has no ultrasound measurements, examination findings or fertility-clinic records to reconcile with your input.

How Is Your Due Date

The dating rules

ACOG describes menstrual dating as LMP plus 280 days, assuming a 28-day cycle with ovulation on day 14. For IVF, embryo age and transfer date determine dating: a day-5 transfer adds 261 days and a day-3 transfer adds 263. ACOG dating guidance

The calculator's assumed-fertilization option uses 266 days. That is the same calendar model with its assumed two-week offset removed: 280 − 14 = 266. Entering a date into this option does not establish that fertilization actually occurred on that date. The form calls it an assumption for this reason.

These are additions of calendar days, not additions of nine calendar months. Months have different lengths. Using a fixed number of days lets the calculation follow the same rule across short months, long months and changes of year.

A worked example

Suppose the first day of the last period was August 1, 2026. Adding 280 days gives May 8, 2027. Under this model, the assumed fertilization date is August 15, 2026. Counting 266 days from that assumed date also gives May 8. The two calculations agree because their starting points differ by exactly 14 days.

If August 1 instead represents an assumed fertilization date, the result is April 24, 2027. This is a different input, rather than a disagreement between equations. Before comparing two outputs, check which event the date represents. An IVF transfer is a third kind of event, and the embryo-age selection changes its offset again.

Pregnancy weeks and the date of birth are different questions

A gestational-age display measures elapsed calendar time from the starting point used by the model. It does not measure development or identify the start of labor. For the August 1 menstrual example, on October 5, 2026 there have been 65 elapsed days: nine complete weeks plus two days. That is an arithmetic example of 9 weeks, 2 days, not an observation about a particular pregnancy.

The EDD remains a specific date on the calendar. It is useful to record that date accurately, together with its method, while leaving birth timing to maternity assessment. Calling it an estimate does not turn it into a clinical statement that every date around it is equally suitable for delivery.

For your own notes, write the event and date together: “first day of last period: August 1,” rather than simply “pregnancy date: August 1.” This avoids an avoidable two-week difference when the same number is entered under a different method.

How to Use

Choose the event before entering the date

The three buttons at the top of the form are Last Period, Conception and IVF Transfer. Select the button that matches the information you actually have. The date-field label changes with that choice, so read the label before submitting. The conception button asks for an assumed fertilization date, rather than treating a remembered event as clinical confirmation.

For Last Period, enter the first day of the period used for dating. The form adds the menstrual offset in the preceding section. It has no cycle-length field and does not automatically adjust for a longer, shorter or irregular cycle. Do not look for an adjustment hidden in the result: the model uses the stated assumption.

For Conception, enter only the date you intend to use as the fertilization assumption. The calculator cannot verify that assumption from intercourse dates, symptoms, an ovulation prediction or a pregnancy test. Record where the date came from when discussing the result with your maternity team. This makes the uncertainty explicit instead of allowing the label to imply precision the form cannot provide.

For IVF Transfer, enter the actual transfer date, then select the embryo age recorded by the clinic. A day-5 blastocyst and a day-3 embryo have different offsets. If your records describe a different embryo age, or you are unsure which option applies, ask the clinic rather than choosing the nearest option. The page supports these two selections only.

Which method is best to use?

Use the date and event supported by your records; have uncertain dating assessed by your maternity team. An online form cannot decide between conflicting clinical records. For an IVF pregnancy, bring the transfer date and embryo-age record, rather than substituting a menstrual estimate for the treatment information.

Submit, read, then check the input again

Click Calculate Due Date after entering the date and, where applicable, embryo type. A future input date is rejected because this form describes an estimated current pregnancy. If an error appears, correct the field before treating any earlier output as current. Changing the method, date or embryo type clears the previous result.

The result should be checked against the exact date you entered. Dates such as 08/01 can be written differently in different countries; use the date control's displayed format and check the named month in the result. A correct equation applied to the wrong month will still produce a plausible-looking date.

Compare the supported methods with a fixed example

The following examples all use August 1, 2026 as the entered date, but give it a different meaning. They demonstrate the arithmetic and the importance of the field label:

These are invented scenarios, not a set of alternative dates for one patient. To compare an actual clinic date with a calculator, keep the same transfer record and embryo age in both. If they differ, note both dates and ask the clinic to explain the basis before replacing the established date.

Ultrasound belongs to clinical dating, not this input form

In England, NHS maternity services offer a dating scan at 11–14 weeks and a second scan at 18–21 weeks; additional scans depend on circumstances. This is regional care information, not an appointment schedule generated by this website. NHS ultrasound information

The calculator has no field for crown–rump length, head measurements or an ultrasound report's gestational age. You cannot reproduce an ultrasound assessment by moving the input date until the result looks similar. Keep a clinically assigned date labeled as such in your records.

Understanding Your Results

Your estimated due date

The large date at the top and the Due date card show the same EDD in different formats. Check the year as well as the month and day, especially if the estimate falls in the next calendar year. The result does not say that labor is expected to begin on that exact day or that a birth before or after it is automatically healthy.

Estimated gestational age

The tool divides elapsed days from its menstrual starting point into whole weeks and remaining days. In the LMP method, that is the date entered. In the assumed-fertilization and IVF methods, the calculator first creates an equivalent menstrual starting point using its two-week offset.

For example, 65 elapsed days are 9 weeks, 2 days, while 70 elapsed days are 10 weeks, 0 days. The extra days matter when you compare written records. Nine weeks and six days is still nine completed weeks; it is not ten completed weeks even though it is close to the next week.

This age updates with the current date used when you calculate. The due date is based on the input and offset, so it does not move forward merely because you revisit the page tomorrow. If a screenshot was taken last week, its gestational-age display belongs to that calculation date.

The assumed-fertilization card

This card reports a date created from the chosen method. For the LMP example it is LMP plus 14 days; for IVF it is transfer minus the selected embryo age. Read it as the model's reference date. It is not a laboratory confirmation of the moment fertilization occurred.

Trimester and progress bar

This website labels an estimated age below 14 completed weeks as first trimester, 14 through 27 as second, and 28 onward as third. These are the display's boundaries. The progress bar divides completed weeks by 40, rounds the percentage and caps it at 100%. It does not measure fetal readiness, remaining risk or the probability of delivery.

At 20 completed weeks, that calculation gives 50%. At 30 completed weeks, it gives 75%. The bar uses completed weeks, so it does not incorporate the remaining days into the percentage. A bar at 100% is not a clinical reason to wait, induce labor or assume that a pregnancy has ended.

Term categories have precise boundaries

ACOG distinguishes the following gestational-age categories. They are separate from this website's trimester labels. ACOG term definitions

The table explains terminology; it does not create a delivery plan. In particular, 37 weeks is not synonymous with full term. Use your team's dating and recommendations when discussing timing.

Our pregnancy weight-gain calculator explains another reference calculation. Keep its measurements and source limits separate from the calendar output here: neither a date estimate nor a weight reference independently assesses pregnancy health.

The Three Trimesters

A timeline for understanding development

A due-date guide is more useful when it connects calendar weeks with development, without turning milestones into tests. The descriptions below come from the U.S. Office on Women's Health (OWH). They describe stages of development, rather than observations made by this calculator. OWH stages of pregnancy

Different educational resources group trimester weeks differently. OWH's headings use weeks 1–12, 13–28 and 29–40; the site's result uses the display boundaries explained above. To avoid confusing a heading convention with a medical finding, the developmental examples below are attached to specific weeks.

First Trimester

At four to five weeks, the brain and spinal cord have begun to form, the heart begins to form, and arm and leg buds appear. These statements describe early formation. They do not mean the organs are fully developed or that a date calculation can confirm their condition. OWH early development

At eight weeks, major organs and external structures have begun to form. Arms and legs are becoming longer, fingers and toes have started forming, and eyelids have formed. The important word is “begun”: development continues beyond this stage. A description of organ formation is different from an ultrasound assessment of a particular pregnancy. OWH eight-week stage

At 12 weeks, nerves and muscles begin working together and the fetus can make a fist. The eyelids close to protect the developing eyes. These are examples of a developmental sequence, not a checklist parents need to verify through symptoms at home. OWH 12-week stage

The first trimester can also bring tiredness, tender breasts, nausea with or without vomiting, changes in food preferences and constipation. OWH explains that experiences differ between people and between pregnancies. Having fewer symptoms does not give this calculator a way to judge development, and nausea is not a test of appropriate hormone progression. OWH maternal changes

If you are recording symptoms, keep that diary separate from the dating record. Write what happened and when, rather than using the EDD to label a symptom as reassuring. For severe symptoms, use the warning-sign guidance below. Calendar age alone should not decide whether you seek help.

Our hCG calculation page describes arithmetic between laboratory results. It does not assess viability or exclude an ectopic pregnancy. Using a date estimate and an hCG calculation together does not create that diagnostic capability.

Second Trimester

At 16 weeks, muscle tissue and bone continue to develop, skin begins to form, and sucking movements occur. Meconium develops in the intestinal tract. These descriptions explain what is developing at this point; the date input has not measured any of it. OWH 16-week stage

At 20 weeks, OWH describes increasing activity, with slight fluttering that may be felt. Fine hair called lanugo and a waxy coating called vernix cover the developing skin. Eyebrows, eyelashes and nails have formed, and the fetus can hear and swallow. This general account is not a prescribed movement count. OWH 20-week stage

At 24 weeks, bone marrow begins making blood cells, taste buds form, and fingerprints and footprints have formed. OWH describes lungs that have formed but do not yet function, together with developing hand and startle reflexes and regular sleep and waking. This is not a numerical prediction of survival after a premature birth. OWH 24-week stage

For the pregnant person, nausea and fatigue may ease during the second trimester, while new changes can include body aches, stretch marks and skin changes. OWH presents these as possible experiences, not a promise of a symptom-free “golden period.” A person who feels unwell should not dismiss it because an educational timeline says symptoms often improve. OWH second-trimester changes

A developmental description also differs from a screening result. Your care team can explain the purpose and findings of any scan or test offered. This calculator does not produce an anatomy report, a test result, an appointment reminder or a statement that screening has been completed.

Third Trimester

At 32 weeks, bones are formed but remain soft; the eyes can open and close and detect changes in light. OWH describes practice breathing movements while the lungs are not fully formed, and storage of minerals such as iron and calcium. These details illustrate continuing development rather than a guarantee of readiness for birth. OWH 32-week stage

At 36 weeks, the vernix coating becomes thicker and body fat increases. These are descriptive milestones. The calculator does not measure body fat, lung maturity or estimated fetal weight, and it does not decide whether growth is appropriate. OWH 36-week stage

Later pregnancy may involve heartburn and trouble sleeping. However, a list of possible discomforts must not be used to dismiss breathing problems or other warning signs. CDC advises immediate medical care for the urgent signs described in the next section. OWH later changes; CDC warning signs

If movements stop or slow compared with the usual pattern, seek medical attention immediately. Do not interpret a later gestational age or less space as reassurance. CDC emphasizes that a change in movement is what matters and that no single movement count applies to everyone. CDC fetal-movement warning

Milestones at a glance

Use the timeline to understand the language of prenatal information. If a report raises a concern, ask about that specific finding rather than comparing it with a general milestone table. The tables here supply background; the maternity record supplies information about your pregnancy.

Why Due Dates Change

Establishing a date and changing it are different steps

ACOG recommends determining, discussing and documenting the EDD from the available dating information. Later changes should be reserved for rare circumstances and documented. First-trimester crown–rump-length dating, through 13 weeks 6 days, is the most accurate ultrasound method, with accuracy of approximately ±5–7 days. ACOG dating assessment

That accuracy range describes a dating method, not a promised birth window and not an accuracy rating for this website. The form performs day addition. It has no measurements from which to reproduce ultrasound dating, and there is no universal ±two-week accuracy guarantee for its result.

When the menstrual input is uncertain

Start by identifying what is known: the date used, whether it really represents the first day of a period, and whether the record is certain or an estimate. The form does not have a field for confidence. An uncertain number can therefore produce an exact-looking date with no visible uncertainty interval.

A helpful record might say, “I entered August 1 because that is the date in my period tracker,” or “I entered August 1 as an approximate date; I am not certain.” Those notes describe the evidence behind the input. They are more informative to your care team than changing the number until the calculator matches an expected month.

When a scan and calculator differ

Bring both records to the appointment. Check the date of the scan, the gestational age recorded on the report, the input method used here and the EDD already in your maternity record. The clinician can explain which information establishes dating and which information describes later measurements.

ACOG's redating criteria depend on gestational age and clinical circumstances; there is no single discrepancy threshold for every scan. ART-derived dating uses treatment information. ACOG clinical dating considerations

Do not average two due dates or assume that the newest scan automatically replaces the established one. The website cannot evaluate whether a difference represents input error, dating uncertainty, a measurement difference or a finding requiring assessment.

A practical record for a date discussion

These are suggested organizational notes, not a clinical scoring system. They help you ask a focused question: “Which date should I use for my care, and what information was it based on?” If the team changes the date, record the explanation and the new established EDD instead of keeping multiple unlabeled dates in different apps.

When to Consult

Arrange a dating discussion when the records are unclear

Ask your maternity team about an uncertain menstrual date, conflicting estimates, an IVF record you do not understand or a result that differs from the established EDD. Bring the relevant dates rather than using a calculator result as evidence that the clinical record is wrong.

Separate routine dating questions from symptoms needing prompt care. A due date that looks plausible cannot exclude a complication. The calculator does not examine bleeding, pain, blood pressure, temperature, fetal movement or other clinical information. A displayed trimester and progress bar should therefore never determine whether a symptom deserves attention.

Urgent maternal warning signs

CDC advises immediate medical care for urgent maternal warning signs during pregnancy and in the year after birth. Its examples include:

• A headache that will not go away or gets worse, including a sudden severe headache, or changes in vision.
• Fainting or ongoing dizziness; trouble breathing; chest pain or a fast-beating heart with concerning symptoms.
• Fever of 100.4°F (38°C) or higher.
• Severe belly pain that does not go away, or severe nausea and vomiting that prevent keeping fluids down.
• Extreme swelling of the hands or face.
• Vaginal bleeding more than spotting, fluid leakage during pregnancy or unusual bad-smelling discharge.
• A baby's movement stopping or slowing compared with the usual pattern.

These examples are not a complete list of every possible problem. Contact an appropriate medical service immediately for an urgent sign, and tell them that you are pregnant or were pregnant within the past year. Use local emergency services if emergency help is needed. CDC urgent maternal warning signs

If something feels wrong even though it is not on a short list, CDC advises talking to a healthcare provider and asking questions. Do not wait for the calculator to reach a certain week before reporting a concern. Symptoms require their own assessment. CDC guidance on seeking care

Questions for your next appointment

You can use the following prompts to organize a routine discussion:

1. What is the EDD recorded for my pregnancy, and which information established it?
2. Does an uncertainty in my period or treatment dates need clarification?
3. Which appointments or tests have actually been arranged, and where are their dates recorded?
4. Who should I contact for urgent concerns, including outside ordinary clinic hours?
5. What plan should I follow as I approach or pass the established EDD?

These prompts are site-authored organizational suggestions. They are not a mandatory care protocol or a substitute for your team's instructions. This page does not assign a universal induction week, weekly-visit schedule or home method for starting labor. Keep the clinical plan and contact details where you can find them without needing to rerun a calculator.

Frequently Asked Questions

1. How accurate is a due date calculator?

This calculator adds a fixed number of calendar days to the selected input. The result is only as applicable as the date and assumptions entered. It has no verified universal error margin for predicting birth. The ultrasound accuracy range discussed above belongs to clinical dating, not to this online calculation.

2. Why is my ultrasound due date different from my LMP due date?

The form uses a fixed menstrual model and cannot interpret a scan. Check that the input represents the first day of the intended period, then ask your maternity team to explain the comparison with the report. A difference alone does not let this website determine a cause or conclude that everything is reassuring.

3. How do I calculate my due date with IVF?

Select IVF Transfer, enter the transfer date and choose the embryo age recorded by your clinic. The offsets are shown in the first table. For the worked August 1, 2026 transfer example, the results are April 19, 2027 for day 5 and April 21, 2027 for day 3. These are arithmetic examples; use the clinic's established dating for care.

4. Can my due date change during pregnancy?

Your maternity team can explain whether the clinical record needs a change and why. The dating section above describes ACOG's approach to establishing and documenting the EDD. This calculator does not decide whether a scan warrants redating. Changing a field here only changes the local estimate.

5. What if I don't know my last menstrual period?

Do not present a guessed date as certain. Tell your maternity team what information is available and what is uncertain. The assumed-fertilization option also requires an explicit assumption; it cannot recover an unknown date from symptoms. If you have IVF records, bring those to the discussion rather than trying to reconstruct a period date.

When saving a result, include the method, input and calculation date. That provides a clear record of what the form did. Keep the clinical EDD separately labeled so a screenshot does not accidentally replace the date used by your care team.

Sources & Medical References

1. ACOG: Methods for Estimating the Due Date — menstrual assumptions, ART offsets, first-trimester dating and establishing or revising the clinical EDD; Committee Opinion 700, reaffirmed 2025.
2. ACOG: Definition of Term Pregnancy — early-term, full-term, late-term and postterm boundaries.
3. U.S. Office on Women's Health: Stages of pregnancy — maternal changes and the week-specific developmental background; updated September 26, 2025.
4. NHS: Ultrasound scans in pregnancy — England's offered scan schedule and the role of the dating scan.
5. CDC: Urgent Maternal Warning Signs — urgent symptoms during pregnancy and the year after birth.

The calendar examples, input instructions, progress-bar explanation and suggested record sheets describe this website's arithmetic or organizational suggestions. They are not clinical findings or recommendations attributed to the listed authorities. Regional care information is identified where it is used.