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The due date calculator estimates when a pregnancy is likely to reach full term, based on the date of the last menstrual period (LMP). It applies Naegele's rule, the standard method clinicians use for a first estimate before an ultrasound is available. The result is a single calendar date, but it represents the middle of a several-week range during which birth is considered normal, not a fixed appointment.

Die Formel

Estimated due date = LMP + 280 days (40 weeks)

• LMP = first day of the last menstrual period
• 280 days assumes a 28-day cycle with ovulation and conception occurring around day 14
• An equivalent version of the rule adds one year and subtracts three months from the LMP date, giving the same result

Naegele's rule, attributed to German obstetrician Franz Karl Naegele in the early 19th century

How the calculation works

Naegele's rule counts forward 280 days, or 40 weeks, from the first day of the last menstrual period. This is the standard convention used to describe gestational age, even though conception itself usually happens about two weeks after the LMP, near the middle of the cycle.

The rule assumes a menstrual cycle of 28 days with ovulation on day 14. If a cycle is shorter or longer than 28 days, the actual date of conception shifts earlier or later relative to the LMP, which is one reason the estimate can be off by more than a week in either direction.

Because the calculation only needs one date, it is easy to apply, and it remains the first estimate offered at a booking appointment before any scan has been performed.

How to read your result

The result marks a single point, but pregnancy is described in trimesters and, closer to delivery, in defined term categories. The table below shows how these ranges are typically classified relative to the estimated due date.

LMP dating versus ultrasound dating

When a first-trimester ultrasound is available, the American College of Obstetricians and Gynecologists (ACOG) recommends using the measurement from that scan, typically the crown-rump length, to set or adjust the due date rather than relying on LMP alone. Early ultrasound measurements are more consistent than LMP recall, particularly when cycles are irregular, when the LMP date is uncertain, or when conception followed fertility treatment.

Once a due date is established from an accurate early ultrasound, it is generally not revised again later in pregnancy even if subsequent scans suggest a slightly different size, because growth naturally varies more as pregnancy progresses.

Only a small fraction of births happen on the estimated date itself. Data on birth timing indicate that roughly 4 percent of babies arrive on their exact due date, and most are born within about two weeks before or after it.

For pregnancies from in vitro fertilization, the transfer date and embryo age at transfer give a precise conception-based estimate that is generally treated as more reliable than either LMP or a later ultrasound, since the exact day of fertilization is already known.

How the estimate is revised across trimesters, and what term means

ACOG guidance sets a threshold for how much a scan has to disagree with the LMP-based date before the due date is actually changed, and that threshold gets tighter as pregnancy progresses. In the first trimester, a discrepancy of more than about 5 to 7 days between the ultrasound measurement and the LMP-based estimate is usually enough to prompt a change. By the early second trimester, the threshold narrows to roughly 7 to 10 days, and by the late second trimester it widens again to about 10 to 14 days, because fetal size naturally varies more between individuals as pregnancy goes on, making late measurements a worse proxy for age.

This is why a due date set from a good first-trimester scan is treated as final: it was captured at the point in pregnancy when embryos of the same true age are most uniform in size, so the measurement error is smallest. A third-trimester scan, taken alone, is considered too imprecise to redate a pregnancy, even though it remains useful for tracking growth against that already-established due date.

The word 'term' does not mean a single day; it is a set of four bands in the last few weeks of pregnancy, shown in the table above, running from early term at 37 weeks 0 days through post-term at 42 weeks 0 days and beyond. ACOG and the Society for Maternal-Fetal Medicine introduced this four-way split because outcomes for the baby differ measurably between early term and full term births, even though both were once lumped together simply as 'term.'

Grenzen

• This calculator applies a population average, not a prediction for any individual pregnancy; a range of about two weeks either side of the estimated date is considered normal.
• It assumes a 28-day cycle with ovulation on day 14. It is less accurate for anyone with irregular, long, or short cycles, or who does not know their LMP with confidence.
• It does not account for pregnancies conceived through assisted reproduction, where the conception or transfer date is known precisely and should be used instead of an LMP-based estimate.
• An early ultrasound is more accurate than LMP-based dating and should be used to confirm or adjust the estimate whenever one is available, per ACOG guidance.
• It cannot detect or predict pregnancy complications, growth concerns, or the actual timing of labor; it only estimates a reference date.
• It is not a substitute for dating performed and confirmed by a clinician during prenatal care.

Häufige Fragen

How accurate is a due date calculated from the last menstrual period?

It is a useful first estimate but carries a margin of error of a week or more in either direction, mainly because it assumes a standard 28-day cycle. An early ultrasound narrows that margin and is generally considered more reliable once one has been performed.

Why did my due date change after an ultrasound?

Early ultrasound measurements of the embryo or fetus tend to be more consistent across pregnancies than recalled LMP dates, so a due date may be adjusted if the scan and the LMP-based estimate disagree by more than a set number of days, following ACOG criteria.

Is it normal to deliver before or after the due date?

Yes. Birth occurring anywhere from about three weeks before to two weeks after the estimated date is considered within the normal range; only a small percentage of babies are born on the calculated date itself.

Does this calculator work if my cycles are irregular?

It will still produce a result, but the estimate is less reliable, because Naegele's rule assumes a consistent 28-day cycle. An ultrasound-based estimate is generally more useful in that situation.

What is the difference between early term, full term, late term, and post-term?

These are four bands covering 37 weeks 0 days through 42 weeks and beyond, replacing the older single label of 'term.' Full term, 39 weeks 0 days to 40 weeks 6 days, is associated with the lowest average rate of newborn complications, which is why elective deliveries without a medical reason are generally not scheduled before 39 weeks.

Will my due date be moved if a later ultrasound gives a different estimate?

Usually not. Once a due date has been set from an accurate first-trimester measurement, later scans are used to track growth against that date rather than to reset it, because size naturally varies more between individual babies as pregnancy progresses, making later measurements a less reliable basis for dating.

Quellen

1. American College of Obstetricians and Gynecologists. Methods for Estimating the Due Date. Committee Opinion No. 700, 2017.
2. CDC. Reproductive Health.
3. NICHD. Pregnancy.