Pregnancy Due Date Calculator

Q: What is a pregnancy due date and how is it calculated?

A pregnancy due date — formally called the estimated due date (EDD) — is the projected date on which labour is most likely to begin, typically 40 weeks (280 days) from the first day of the last menstrual period (LMP). The most widely used method is Naegele's rule, which remains endorsed by ACOG (American College of Obstetricians and Gynecologists) today. It works by adding 280 days to the LMP, or equivalently subtracting 3 months and adding 7 days. Because ovulation typically occurs around day 14 of a 28-day cycle, LMP-based dating is a clinically practical standard even though conception does not technically occur on day 1.

Q: How accurate is an online pregnancy due date calculator?

An LMP-based due date calculator provides a useful starting estimate, but only around 4% of babies are born on their exact due date. Research published in BJOG: An International Journal of Obstetrics and Gynaecology found that most spontaneous labours occur between 39 and 41 weeks. A first-trimester ultrasound performed between 10 and 13 weeks gestation remains the gold standard for confirming gestational age, with an accuracy window of ±5 to 7 days. This calculator is best treated as an initial planning tool to guide conversations with your midwife or obstetrician.

Q: What formula does the pregnancy due date calculator use?

WellCal's pregnancy due date calculator uses Naegele's rule as its foundation: EDD = LMP + 280 days. When a custom cycle length is entered, the calculator adjusts by subtracting 14 from your cycle length and adding the difference to the standard 280-day estimate. For example, a 35-day cycle shifts the EDD forward by 7 days compared to a standard 28-day cycle calculation, reflecting the later ovulation that occurs in longer cycles. This cycle-adjusted approach more accurately represents when fertilisation was most likely to have occurred.

Q: Does my menstrual cycle length affect my estimated due date?

Yes, significantly. Standard Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is consistently longer or shorter than 28 days, the standard LMP method will over- or underestimate your EDD. For a 35-day cycle, ovulation likely occurs around day 21, meaning the EDD should be adjusted 7 days later than the standard result. Entering your actual average cycle length into this calculator produces a more personalised estimate. For highly irregular cycles, a dating ultrasound in the first trimester is the most reliable method of establishing gestational age.

Q: What is considered a full-term pregnancy?

According to ACOG, a full-term pregnancy spans 39 weeks 0 days to 40 weeks 6 days. Early-term is defined as 37 to 38 weeks 6 days, late-term as 41 weeks 0 days to 41 weeks 6 days, and post-term as 42 weeks or beyond. Babies born at full term generally have the best outcomes in terms of lung maturity, feeding ability, and neurological development. Delivery before 37 completed weeks is classified as preterm, which may require additional neonatal support and monitoring for the newborn.

Q: How does due date calculation differ for first-time versus subsequent pregnancies?

The mathematical formula is identical for all pregnancies, but statistically, first-time mothers (nulliparous women) tend to deliver slightly later than women who have previously given birth (multiparous women). A large study in BJOG found that nulliparous women reach spontaneous labour at a mean of around 40 weeks and 5 days, versus approximately 40 weeks and 3 days for multiparous women. This modest difference is clinically noted; your care provider will factor your full obstetric history into any monitoring or induction decisions as your due date approaches.

Q: What should I do after getting my estimated due date?

Your estimated due date is the foundation of your antenatal care plan, not a guaranteed delivery date. The first priority is to book a first prenatal appointment with a midwife or obstetrician — ideally before 10 weeks gestation. Your care provider will confirm your EDD via ultrasound, screen for risk factors, and establish your schedule of antenatal visits. In the meantime, begin or continue prenatal supplementation (400–800 mcg folic acid daily is recommended by the NHS and CDC), and consider supporting your overall health through good nutrition, adequate hydration, and quality sleep during this critical period.

Q: Can I use this due date calculator if I conceived through IVF?

Yes, but with an important adjustment. For IVF pregnancies, due dates are calculated from the embryo transfer date rather than LMP, since conception timing is precisely known. For a day-5 blastocyst transfer, the EDD is calculated as 261 days (37 weeks) from the transfer date; for a day-3 embryo transfer, it is 263 days. LMP-based calculation is generally not used for IVF because it introduces avoidable inaccuracy. Always confirm your EDD with your reproductive endocrinologist or fertility clinic, as they will have access to your precise transfer records.

Pregnancy Due Date Calculator

Estimate your due date and track pregnancy progress

First day of last menstrual period

Average cycle length (days)

Due date Oct 22, 2025

Weeks pregnant 12 weeks

Trimester 2nd

Days left 196 days

Pregnancy progress 30%

1st

2nd

3rd

Week 1 Week 13 Week 27 Week 40

How it's calculated

LMP date: Jan 15, 2025
Cycle length: 28 days
Conception estimate: Jan 29, 2025
Due date formula: LMP + 280 days + (cycle-28) days

🤰 You are in your second trimester. Continue regular prenatal care and healthy habits.

What Is a Pregnancy Due Date Calculator & How Does It Work?

A pregnancy due date calculator estimates the most likely date your baby will be born — formally known as the estimated due date (EDD) or expected date of delivery (EDD). It is based on the clinical assumption that a typical pregnancy lasts 40 weeks (280 days) from the first day of your last menstrual period (LMP). This standard is endorsed by the American College of Obstetricians and Gynecologists (ACOG), the NHS, and the World Health Organization (WHO) as the foundation for antenatal care planning worldwide.

The underlying method is Naegele's rule, named after German obstetrician Franz Karl Naegele. The formula is straightforward: EDD = First Day of LMP + 280 days. In practice, this is often applied as: subtract 3 months from your LMP date, then add 7 days and 1 year. WellCal's calculator also applies a cycle-length adjustment: because Naegele's rule assumes a 28-day cycle with ovulation on day 14, entering your actual average cycle length shifts the EDD accordingly — adding or subtracting days to account for earlier or later ovulation. This adjustment is recommended by both the Mayo Clinic and ACOG for women with cycles that differ significantly from 28 days.

Once you have your estimated due date, WellCal offers a suite of complementary tools to support your health throughout pregnancy. You may want to use our Calorie Calculator to understand your increased energy needs, our Water Intake Calculator to stay well-hydrated during each trimester, and our Ovulation Calculator if you are still in the conception phase or planning a future pregnancy. These tools work together to help you maintain the best possible health foundation before, during, and after pregnancy.

Medical Disclaimer:

The information provided by WellCal's Pregnancy Due Date Calculator is intended for general wellness and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment from a licensed healthcare provider.

Important limitations to be aware of:

Results are estimates only — only approximately 4% of babies arrive on their exact due date.
This calculator assumes a naturally conceived pregnancy. IVF pregnancies require a different calculation method based on embryo transfer date.
This tool is not appropriate for determining gestational age in high-risk pregnancies, multiple gestations (twins, triplets), or pregnancies with known foetal complications.
Women with irregular or unpredictable menstrual cycles should consult a healthcare provider for a first-trimester ultrasound to establish gestational age accurately.
Always confirm your estimated due date with a qualified midwife, obstetrician, or general practitioner.

Pregnancy Term Definitions — ACOG Classification
Term	Gestational Age
Preterm	Before 37 weeks 0 days
Early Term	37 weeks 0 days – 38 weeks 6 days
Full Term	39 weeks 0 days – 40 weeks 6 days
Late Term	41 weeks 0 days – 41 weeks 6 days
Post-Term	42 weeks 0 days and beyond

What Affects Your Due Date Accuracy — And How to Get the Best Estimate

Your estimated due date is only as accurate as the inputs behind it. Understanding the factors that influence gestational age dating helps you interpret your result with the right level of confidence — and know when to seek ultrasound confirmation.

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Last Menstrual Period (LMP) — The Starting Point of Every Calculation

The first day of your last menstrual period is the foundational input in Naegele's rule and the universal starting point used by midwives and obstetricians globally. Because conception typically occurs about two weeks after this date, gestational age technically counts from before fertilisation occurred — a convention that can initially seem counterintuitive. According to ACOG guidelines, LMP-based dating is the accepted standard when the date is reliably known. To complement your due date estimate, our Ovulation Calculator can help you identify your fertile window and cross-reference your likely conception date.

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Menstrual Cycle Length — Adjusting for Earlier or Later Ovulation

Standard Naegele's rule assumes a 28-day cycle, but the average cycle length among reproductive-age women ranges from 21 to 35 days, with ovulation shifting proportionally. A woman with a 35-day cycle ovulates approximately 7 days later than the 28-day model predicts, shifting her true EDD forward by 7 days. Conversely, a 21-day cycle pushes the EDD 7 days earlier. Entering your actual average cycle length into WellCal's calculator applies this adjustment automatically. For conception planning, our Ovulation Calculator can help you track your cycle and identify peak fertility days with greater precision.

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First-Trimester Ultrasound — The Gold Standard for Gestational Age

An ultrasound performed between 10 and 13 weeks gestation — measuring the crown-rump length (CRL) of the embryo — is the most accurate method of establishing gestational age, with a margin of error of just ±5 to 7 days. The NHS recommends a dating scan between 8 and 14 weeks for all pregnancies, and ACOG advises that ultrasound findings in the first trimester should take precedence over LMP-based calculations when they differ by more than 7 days. If your ultrasound EDD differs from this calculator's result, follow the ultrasound date — it reflects measured foetal development rather than assumed cycle timing.

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Ovulation Timing — Why the Day of Conception Matters

Conception can only occur within a narrow fertile window — typically the 12–24 hours following ovulation, though sperm can survive in the reproductive tract for up to 5 days prior. If you know your precise ovulation date from LH surge testing or basal body temperature tracking, this provides a more direct route to your EDD: simply add 266 days (38 weeks) to your confirmed ovulation date. Research from the NIH's National Institute of Child Health confirms that ovulation-date-based calculation is at least as accurate as LMP-based Naegele's rule when ovulation timing is precisely known. Pair this insight with our Ovulation Calculator for a more precise estimate.

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IVF Embryo Transfer Date — A Separate Calculation for Assisted Conception

For pregnancies achieved through in vitro fertilisation (IVF), due date calculation bypasses LMP entirely and uses the embryo transfer date as the reference point, since conception timing is precisely documented. For a day-5 blastocyst transfer, the EDD is 261 days from the transfer date; for a day-3 embryo transfer, it is 263 days. According to the American Society for Reproductive Medicine (ASRM), IVF pregnancies carry a higher rate of multiples and should always be monitored by a reproductive endocrinologist. If you are tracking related health goals after IVF, our BMI Calculator can help you establish a pre-pregnancy or inter-pregnancy health baseline.

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Irregular Cycles — When LMP-Based Dating Is Less Reliable

Women with polycystic ovary syndrome (PCOS), thyroid disorders, or other hormonal conditions often have menstrual cycles that vary by more than 7 days from month to month, making LMP-based due date calculation significantly less reliable. Approximately 14–25% of reproductive-age women experience irregular cycles, according to data from the CDC. For these women, an early dating ultrasound is the recommended method for establishing gestational age — the LMP result should be treated as a rough estimate only. Monitoring hormonal health is also important; our Diabetes Risk Calculator can be a useful tool, as insulin resistance is strongly associated with cycle irregularity and PCOS.

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Prenatal Nutrition — How Maternal Health Influences Pregnancy Duration

While your EDD is a fixed calculation, maternal nutritional status and overall health can influence the likelihood of preterm or post-term delivery. Deficiencies in folate, vitamin D, and omega-3 fatty acids have been associated with elevated preterm birth risk, according to research published in The Lancet. Meeting pregnancy's increased caloric demands — typically an additional 300–500 kcal/day in the second and third trimesters — supports healthy foetal growth and full-term delivery. Use our Calorie Calculator to estimate your pregnancy energy requirements, and our Protein Calculator to ensure adequate intake for foetal tissue development.

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Hydration — An Underestimated Factor in Pregnancy Health

Adequate fluid intake is essential throughout pregnancy to support amniotic fluid levels, foetal circulation, and the prevention of urinary tract infections — which are significantly more common during pregnancy and can trigger preterm labour if untreated. The National Academies of Sciences, Engineering, and Medicine recommend approximately 3 litres (10 cups) of total daily water intake for pregnant women, compared to 2.7 litres for non-pregnant adults. Blood volume increases by up to 50% during pregnancy, further elevating fluid needs. Our Water Intake Calculator can help you set a personalised daily hydration target to support both your health and your baby's development.

What a Pregnancy Due Date Calculator Doesn't Tell You — 6 Important Limitations

Your estimated due date is a powerful planning tool — but it has real clinical limitations. Understanding what this calculator cannot reveal helps you use it effectively and know when professional assessment is essential.

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It Is an Estimate, Not a Date

Only around 4% of babies are born on their exact due date. Research in BJOG found that spontaneous labour occurs anywhere between 37 and 42 weeks in healthy pregnancies — a 5-week window centred on your EDD. Treat your due date as the midpoint of a probable delivery range, not a fixed event. Your care provider will monitor foetal wellbeing in the weeks surrounding your EDD to guide timing decisions.

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Doesn't Reflect Foetal Growth

A due date calculator tells you how far along your pregnancy is by dates — but it cannot measure how well your baby is actually growing. Foetal growth restriction (FGR), macrosomia (large-for-gestational-age babies), and other growth anomalies are only detectable through serial ultrasound measurements. These conditions can significantly influence delivery timing and are invisible to any date-based calculation.

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Not Designed for Multiple Pregnancies

Twin, triplet, and higher-order pregnancies follow different gestational norms. The average delivery for uncomplicated twin pregnancies is approximately 37–38 weeks — not 40. For triplets, it is closer to 33–34 weeks. Using a standard due date calculator for a multiple pregnancy will produce an EDD that is 2–7 weeks later than what your clinical team will actually plan around. Always rely on specialist obstetric guidance for multiple gestations.

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Cannot Assess Pregnancy Risk Level

A due date calculator provides no information about your individual risk profile. Factors such as pre-existing diabetes, hypertension, thyroid disorders, previous preterm birth, and advanced maternal age all influence whether a pregnancy will be managed as low- or high-risk — and whether delivery will be induced before the EDD. Use our Diabetes Risk Calculator to check your metabolic health baseline alongside pregnancy planning.

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Less Reliable with Irregular Cycles

Even with cycle-length adjustment, LMP-based calculation loses accuracy when cycles vary by more than 7 days from month to month. Women with PCOS, thyroid imbalances, or perimenopause may have anovulatory cycles or cycles with unpredictable ovulation timing, making the formula unreliable without additional data. In these cases, a first-trimester dating ultrasound — measuring crown-rump length — is the only method that directly reflects foetal size and developmental age.

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Doesn't Guide Your Health Habits

Knowing your EDD is just the beginning. Pregnancy significantly changes your caloric, protein, hydration, and sleep requirements — all of which affect both maternal wellbeing and foetal development. This calculator provides your date, but your wider health habits need dedicated attention. Explore WellCal's Calorie Calculator, Water Intake Calculator, and Sleep Calculator to build a comprehensive wellness plan around your pregnancy.

Frequently Asked Questions About Pregnancy Due Date Calculators

What is a pregnancy due date and how is it calculated?

A pregnancy due date — formally called the estimated due date (EDD) — is the projected date on which labour is most likely to begin, typically 40 weeks (280 days) from the first day of the last menstrual period (LMP). The method used is Naegele's rule, which adds 280 days to the LMP date, or equivalently subtracts 3 months and adds 7 days and 1 year. This standard is endorsed by ACOG and the NHS as the clinical foundation for antenatal care planning. Because gestational age is counted from the LMP rather than conception, a pregnancy is considered to have begun approximately two weeks before fertilisation actually occurred — which can feel counterintuitive but is a universal clinical convention. WellCal's Pregnancy Due Date Calculator applies Naegele's rule with an optional cycle-length adjustment for greater personalisation.

How accurate is an online pregnancy due date calculator?

An LMP-based due date calculator provides a clinically useful estimate, but only about 4% of babies are born on their exact due date. Research published in BJOG: An International Journal of Obstetrics and Gynaecology found that spontaneous labour most commonly occurs between 39 and 41 weeks. The accuracy of an LMP-based calculation depends heavily on how reliably you recall your LMP date and how predictable your cycle is. For maximum accuracy, a first-trimester ultrasound between 10 and 13 weeks — measuring the embryo's crown-rump length — has a margin of error of just ±5–7 days and is the gold standard recommended by ACOG. If your ultrasound date and calculator date differ by more than a week, your care provider will typically adopt the ultrasound date. Use our Ovulation Calculator to cross-check your likely conception date if you're uncertain about your LMP.

What formula does the pregnancy due date calculator use?

WellCal's pregnancy due date calculator uses Naegele's rule as its base formula: EDD = First Day of LMP + 280 days. When a custom cycle length is entered, the calculator applies a cycle-length adjustment by subtracting 14 from your cycle length and adding the result to the standard 280-day estimate. For example, a woman with a 35-day cycle has an EDD adjusted 7 days later (35 − 14 = 21; 21 − 14 = +7 days), because ovulation occurs approximately 7 days later in her cycle than assumed by the standard formula. A woman with a 21-day cycle receives an EDD 7 days earlier. This adjustment brings the calculation closer to actual conception timing. For known ovulation dates, an alternative formula — EDD = confirmed ovulation date + 266 days — can be even more precise, as described by the NIH.

Does my menstrual cycle length affect my estimated due date?

Yes — significantly. Standard Naegele's rule assumes a 28-day cycle with ovulation on day 14, but cycle length varies widely among healthy women. Research from the National Institutes of Health shows that cycle lengths ranging from 21 to 35 days are within normal variation, each shifting ovulation — and therefore the likely conception date — accordingly. Every 1 day that your average cycle differs from 28 days shifts your EDD by approximately 1 day. Entering your actual cycle length into this calculator applies that adjustment automatically. If your cycles are highly irregular — varying by more than 7 days from month to month — an LMP-based due date should be treated as a rough guide only, and a dating ultrasound is strongly recommended. You can also track your ovulation patterns more closely with our Ovulation Calculator.

What is considered a full-term pregnancy?

According to ACOG's official definition of term pregnancy, a full-term pregnancy spans 39 weeks 0 days to 40 weeks 6 days. The full classification system is: preterm (before 37 weeks), early term (37 weeks 0 days to 38 weeks 6 days), full term (39–40 weeks 6 days), late term (41 weeks 0 days to 41 weeks 6 days), and post-term (42 weeks or beyond). Babies born at full term generally have the best outcomes across measures of lung maturity, feeding ability, and neurological development. The risk of certain complications — including respiratory distress syndrome — declines sharply after 39 weeks. It's worth noting that post-term pregnancies (beyond 42 weeks) carry increased risks and are typically managed with monitoring and induction decisions by your care team.

How does due date calculation differ for first-time versus subsequent pregnancies?

The mathematical formula is identical for all pregnancies, regardless of how many times a woman has previously given birth. However, statistically, first-time mothers (nulliparous women) tend to deliver slightly later than women who have already given birth (multiparous women). A large-scale study published in BJOG found a mean spontaneous delivery at approximately 40 weeks and 5 days for nulliparous women versus 40 weeks and 3 days for multiparous women. This difference of about 2 days is clinically modest but routinely noted by care providers when counselling patients about delivery expectations. Your obstetric history — including previous preterm births, caesarean sections, or postdates pregnancies — will significantly influence how your care provider monitors and manages your approach to your EDD. Share your full history at your first antenatal appointment for personalised guidance.

What should I do after getting my estimated due date?

Your estimated due date is the starting point for your entire antenatal care plan. The first priority is to book a first prenatal appointment with a midwife or obstetrician as soon as possible — ideally before 10 weeks gestation. Your care provider will confirm your EDD via ultrasound, conduct initial blood work and screening, and establish your schedule of antenatal check-ups. In the meantime, begin or continue taking a prenatal supplement containing 400–800 mcg of folic acid daily (recommended by both the NHS and CDC to reduce neural tube defect risk), and avoid alcohol, smoking, and raw or undercooked foods. Supporting your nutrition is also important: use our Calorie Calculator to estimate your increased energy needs, and our Water Intake Calculator to set a daily hydration target. Register with your local antenatal services promptly, as some areas have waiting lists for midwifery care.

Can I use this due date calculator if I conceived through IVF?

Yes, but the LMP-based method should be replaced with a transfer-date method for IVF pregnancies, since the exact timing of fertilisation is documented. For a day-5 blastocyst transfer, the EDD is calculated as 261 days (37 weeks) from the transfer date. For a day-3 embryo transfer, it is 263 days. This distinction matters because using LMP in an IVF pregnancy — where natural ovulation may have been suppressed — introduces meaningful inaccuracy. Additionally, IVF pregnancies carry a higher risk of multiple gestations, gestational diabetes, and hypertensive disorders, meaning they should always be managed under specialist obstetric or reproductive endocrinology care rather than standard community midwifery alone. The American Society for Reproductive Medicine (ASRM) recommends that IVF pregnancies receive early obstetric involvement for monitoring. Our Diabetes Risk Calculator can help you assess your gestational diabetes risk profile before your first antenatal appointment.

How often should I update or recalculate my estimated due date?

In most cases, you will only need to calculate your due date once — and it will be reviewed and confirmed (or adjusted) by your care provider at your first-trimester ultrasound, typically at 10–13 weeks. After a first-trimester ultrasound confirms gestational age, that date becomes your official EDD and is unlikely to change. A second-trimester anomaly scan (around 18–22 weeks) may occasionally prompt a minor adjustment if growth measurements significantly diverge from the established EDD, but this is uncommon in otherwise uncomplicated pregnancies. You may wish to recalculate if you experience a pregnancy loss and conceive again, if you realise your LMP date was incorrect, or if you are unsure whether a new bleed represented a period or implantation bleeding. Our Pregnancy Due Date Calculator is always free to use with no limit on calculations, and you can revisit it whenever your inputs change.