Due Date Calculator
Estimate your pregnancy due date from LMP, conception, ultrasound, or IVF transfer.
Due Date Calculator — your inputs
- Method
- Last menstrual period (LMP)
- Cycle length
- 28 days
- Unit system
- Metric
Key takeaways
- Your due date is 40 weeks from the first day of your last period — not 40 weeks from conception, which is about two weeks later.1
- Only about 4% of babies arrive on the due date. Treat it as the centre of a range, not a deadline.5
- Anywhere from 37 to 42 weeks is a normal length of pregnancy. Even among healthy pregnancies, the natural spread is around five weeks.2
- A first-trimester ultrasound is the most accurate dating method — within about five days. If it disagrees with your period dates by more than a week, the scan is what your care team will use.1
- The date will not move once it is set. Redating late in pregnancy is deliberately avoided, because the scan's margin of error grows as the baby grows.1
How to use this calculator
Choose an estimation method: LMP (default), conception date, ultrasound, or IVF transfer.
Enter the relevant date(s). For LMP, you can adjust cycle length.
Understanding your result
Naegele's rule sets the due date at LMP + 280 days (~40 weeks), with an adjustment for cycle lengths other than 28 days.
Only about 4% of births happen on the estimated due date — the 37–42 week window is the normal range.
Why pregnancy is counted from your last period
It is the oddest thing about pregnancy dating and the source of most of the confusion: the count starts about two weeks before you were pregnant. Week 1 is the week of your last period. Conception happens somewhere around week 3. By the time a test turns positive you are already called four weeks pregnant.
The reason is practical rather than biological. The first day of a period is a date most people can name; the day of conception is one almost nobody can. Wilcox and colleagues showed that the fertile window is about six days wide and that conception cannot be pinned to a single day even when intercourse is tracked precisely.6 Counting from a knowable date and accepting a fixed offset is more reliable than counting from an unknowable one.
Gestational age vs. how long you have been pregnant
Gestational age is counted from the last period. Fetal age — sometimes called conceptional age — is counted from conception and runs about two weeks behind. When a scan report and an app disagree by a fortnight, this is almost always why.
The four methods, and when each one is used
This calculator implements all four methods ACOG recognises.1 They do not carry equal weight, and knowing which one your own date came from tells you how much to trust it.
| Method | Typical accuracy | How it works |
|---|---|---|
| First-trimester ultrasound (success) | ±5 to 7 days | Measures crown-rump length. Early embryos grow at almost the same rate regardless of genetics, so size predicts age closely. |
| IVF transfer date (success) | ±1 day | The only method where conception is known exactly. Day-5 transfer + 261 days; day-3 transfer + 263. |
| Last menstrual period (warning) | ±1 to 2 weeks | Naegele's rule: LMP + 280 days, adjusted for cycle length. Assumes a regular cycle and a 14-day luteal phase. |
| Second/third-trimester scan (danger) | ±2 to 3 weeks | Growth diverges as pregnancy progresses, so a late scan measures size far better than it measures age. |
Naegele's rule — the LMP method — dates from the early 1800s and assumes a 28-day cycle with ovulation on day 14. Neither is true for a great many people, which is why this calculator adjusts for cycle length: a consistent 32-day cycle usually means ovulation four days later, and a due date four days later with it.
Why day-5 transfers are 261 days and day-3 transfers are 263
The count is always to the same point of development. A day-5 blastocyst is two days further along than a day-3 embryo at the moment of transfer, so it has two fewer days left to go. It is arithmetic, not a difference in how the pregnancies progress.
What 'due date' actually predicts
Very little, on its own. About 4% of babies are born on their estimated due date.5 That is barely better than the roughly 1-in-25 you would get from picking a day at random inside a month-wide window — which tells you the number is a midpoint, not a prediction.
Jukic and colleagues measured pregnancies from the day of implantation, removing the noise that ovulation timing introduces, and still found a natural spread of 37 days between the shortest and longest normal pregnancies.2 Some of that variation is real and biological: older mothers carried slightly longer, as did women who themselves had been heavier at birth. Pregnancy length varies the way adult height varies, and no formula can collapse that into a single day.
- Preterm (34–37)
- Early term (37–39)
- Full term (39–41)
- Late term (41–42)
- Postterm (42–43)
| Category | Range |
|---|---|
| Preterm | 34 to 37 weeks |
| Early term | 37 to 39 weeks |
| Full term | 39 to 41 weeks |
| Late term | 41 to 42 weeks |
| Postterm | 42 to 43 weeks |
Those bands are not folklore; ACOG defined them in 2013 precisely because "term" had been used loosely to mean anything from 37 to 42 weeks, and outcomes across that range are not equivalent.3 Babies born at 39 to 40 weeks do measurably better than those born at 37 to 38, which is why elective deliveries before 39 weeks are now discouraged.
What happens if the date passes
Most pregnancies that reach the due date are not delivered that day, and nothing is wrong. First pregnancies in particular tend to run slightly longer.
- 41 weeks (late term). Most care teams begin discussing induction here, and will usually offer additional monitoring.
- 42 weeks (postterm). Reached by a small minority of pregnancies. The concern is the placenta, which is not built to function indefinitely while the baby keeps growing and needing more from it.
- Induction. A Cochrane review of induction at or beyond 37 weeks found that a policy of induction at 41 weeks or later reduced perinatal deaths compared with waiting, which is why most guidelines now recommend not going far past 41 weeks.4
This calculator does not replace monitoring
Any bleeding, reduced fetal movement, severe headache, or contractions before 37 weeks is a reason to contact your maternity team immediately, whatever this page says about your dates.
Reading the week-by-week table
Two things in the breakdown above are worth knowing, because both routinely confuse people reading a growth table for the first time.
- Why the length jumps at week 21
- Up to week 20 the measurement is crown-rump — head to bottom — because the legs are curled up and cannot be measured reliably. From week 21 it becomes crown-heel, the full length. The baby did not grow ten centimetres in a week; the ruler changed.
- Why the week number is one higher than your gestational age
- At 8 weeks and 4 days you have completed 8 weeks and are in week 9 — the same way a one-year-old is in their second year. Both numbers are correct and both appear on this page, which is why they are labelled separately.
- Why the sizes should not worry you
- They are population averages. Healthy babies at any given week span a wide range, and a single measurement means far less than a growth trend across scans.
What to do next
- Book a first-trimester scan if you have not. It is the only thing that will meaningfully improve on the date this calculator gives you.
- Write down the range, not the day. 37 to 42 weeks is where the answer actually lives, and the dated table above shows exactly when those fall.
- Check the conception window with the Conception Calculator if you want to work backwards from the date.
- Plan weight gain from your pre-pregnancy BMI, not your current weight — the Pregnancy Weight Gain Calculator applies the Institute of Medicine ranges.
- Read the full explanation in How Your Due Date Is Calculated if you want the reasoning behind each method.
Terms used on this page
- Gestational age
- Time since the first day of your last menstrual period, written as weeks and days — for example 8w 4d. The standard unit of pregnancy dating everywhere in medicine.
- Naegele's rule
- The LMP formula: due date = first day of last period + 280 days. Named for Franz Naegele, who popularised it in the 1810s. Still the default when no early scan is available.
- Crown-rump length
- The measurement from the top of the head to the bottom of the torso, used on ultrasound in the first trimester. The most accurate single measurement for dating a pregnancy.
- Luteal phase
- The stretch between ovulation and the next period, usually close to 14 days and much less variable between people than the first half of the cycle. This stability is what lets a cycle-length adjustment work.
The formula
- LMP = Jan 1, 28-day cycle
- Due = Jan 1 + 280 days
Source: Naegele's Rule. Published clinical/scientific formula.
Limitations
- Only ~4% of births occur on the estimated due date.
- Cycle-length adjustment assumes a regular cycle and a 14-day luteal phase.
- First-trimester ultrasound is the most accurate dating method.
References
- 1.Committee Opinion No. 700: Methods for Estimating the Due Date. American College of Obstetricians and Gynecologists, Obstetrics & Gynecology 129(5), 2017.
- 2.Length of human pregnancy and contributors to its natural variation. Jukic et al., Human Reproduction 28(10):2848–2855, 2013.
- 3.Committee Opinion No. 579: Definition of Term Pregnancy. American College of Obstetricians and Gynecologists, 2013 (reaffirmed 2021).
- 4.Induction of labour at or beyond 37 weeks' gestation (Cochrane review). Middleton et al., Cochrane Database of Systematic Reviews, 2020.
- 5.How accurate are 'due dates'?. Moore, K., BBC News Magazine, 2015.
- 6.Timing of sexual intercourse in relation to ovulation. Wilcox et al., New England Journal of Medicine 333(23):1517–1521, 1995.
Frequently asked questions
Related calculators
Pregnancy Calculator
Find out how many weeks pregnant you are today, from whichever date you know.
Open calculatorConception Calculator
Estimate the conception date and fertile window from a due date or birth date.
Open calculatorOvulation Calculator
Predict your ovulation date and fertile window for the next three cycles.
Open calculatorRelated articles
How Your Due Date Is Calculated (and Why It's an Estimate)
Four methods, one number, and a wide range of normal. What Naegele's rule assumes, why ultrasound usually wins, and what the date actually means.
Read articleOvulation and the Fertile Window: How to Time Conception
Why the fertile window is six days rather than one, where the day-14 assumption goes wrong, and what tracking methods actually tell you.
Read articlePregnancy Weight Gain: What's Recommended and Why It's a Range
The Institute of Medicine ranges by pre-pregnancy BMI, where the weight actually goes, and why the trajectory matters more than the total.
Read articleNot medical advice. This calculator is for general informational purposes only and is not a substitute for professional medical guidance. Always consult a qualified healthcare provider before making decisions about your health. Read the full medical disclaimer.
Printed from mystatura.com — for general information only, not medical advice.