Gestational Age from LMP Calculator
Gestational Age from LMP Calculator
Gestational age from the number of days since the first day of the last menstrual period, with Naegele’s cycle-length adjustment and the days left to the 280-day estimate. Days in, days out: this page has no calendar.
Gestational age from the LMP interval
Days since LMP to gestational age200 days since the first day of the last menstrual period, 28-day cycle
Formula
Days remaining = 280 − gestational age
- 280 days
- Naegele’s convention, which is 40 weeks 0 days. It is a convention and not a measurement: it assumes a 28-day cycle, ovulation on day 14, and a 266-day post-ovulatory pregnancy
- the cycle adjustment
- subtracted, not added. A 35-day cycle means ovulation around day 21 rather than day 14, so a pregnancy is seven days YOUNGER than the menstrual interval implies. At a 21-day cycle the adjustment runs the other way. ACOG’s dating guidance does not require the adjustment and many programmes omit it
- no dates
- this is interval arithmetic. The expression language behind every calculator on this site is numeric only — no strings, no dates, no date arithmetic — so a page here cannot take a menstrual date and print a due date. It takes a number of days and returns a number of days, which is the whole of what a dating wheel does
- the term strata
- ACOG and SMFM Committee Opinion 579 divides the old single ‘term’ block into early term (37 0/7 to 38 6/7), full term (39 0/7 to 40 6/7), late term (41 0/7 to 41 6/7) and post-term (42 0/7 and beyond). The bands above are those four plus preterm
- what this does not establish
- the menstrual interval is the WEAKEST of the dating methods. ACOG Committee Opinion 700 calls a pregnancy with no ultrasound confirming or revising the estimate before 22 weeks 0 days SUBOPTIMALLY DATED
Worked example
200 days since the first day of the last menstrual period, 28-day cycle
Cycle adjustment = 28 − 28 = 0, so the menstrual interval is used unchanged
Gestational age = 200 days, which is 200 ÷ 7 = 28 completed weeks and 4 days, written 28+4 or 28w4d
Days to the 280-day estimate = 280 − 200 = 80 days
200 days is before 259, so this sits in the preterm band
Change the cycle length to 35 days and the answer becomes 193 days (27w4d), a week younger, with 87 days remaining — the adjustment is subtracted because later ovulation means a younger pregnancy
Change it to 21 days and the answer becomes 207 days (29w4d). The two adjustments are symmetric about 28, which is the check worth making on any implementation of this rule
The term strata, in weeks and in days
| Gestational age | Days of gestation | ACOG and SMFM name |
|---|---|---|
| Before 37 weeks 0 days | under 259 | Preterm |
| 37 weeks 0 days to 38 weeks 6 days | 259 to 272 | Early term |
| 39 weeks 0 days to 40 weeks 6 days | 273 to 286 | Full term |
| 41 weeks 0 days to 41 weeks 6 days | 287 to 293 | Late term |
| 42 weeks 0 days and beyond | 294 and over | Post-term |
What the cycle-length adjustment does, at 200 days since the LMP
| Cycle length (days) | Adjustment | Gestational age (days) | Days to 280 |
|---|---|---|---|
| 21 | +7 days | 207 (29w4d) | 73 |
| 25 | +3 days | 203 (29w0d) | 77 |
| 28 | 0 days | 200 (28w4d) | 80 |
| 32 | -4 days | 196 (28w0d) | 84 |
| 35 | -7 days | 193 (27w4d) | 87 |
| 40 | -12 days | 188 (26w6d) | 92 |
Interval arithmetic, and why the menstrual date is the weakest way to date a pregnancy
This calculator works in intervals, not dates. It takes a number of days and returns a number of days: it has no calendar, so it cannot take the date of the last menstrual period and print a due date. Count the days from the first day of the last menstrual period to today on any calendar, enter that number, and the arithmetic below is the same arithmetic a dating wheel performs.
Naegele’s rule is three assumptions stacked on each other: that the cycle was 28 days, that ovulation happened on day 14, and that pregnancy from ovulation runs 266 days. Add the last two and you get the 280-day convention, which is where the 40-week figure comes from. None of the three is measured in an individual pregnancy, and the first two are often wrong. The cycle-length field above corrects the second assumption using the first, and it is worth knowing that many screening programmes do not apply that correction at all — so a gestational age from your unit may differ from this one by exactly the adjustment, with neither being an error.
The practical consequence is that menstrual dating is the least accurate method available and is routinely superseded. ACOG Committee Opinion 700 makes a first-trimester crown-rump length the measurement of choice through 13 weeks 6 days, and calls a pregnancy with no ultrasound confirming or revising the estimate before 22 weeks 0 days suboptimally dated. The same opinion is explicit that a pregnancy is dated ONCE: once an estimate has been set from the earliest accurate measurement, later scans do not reset it except in rare circumstances. That is not administrative tidiness. A fetus that is small because growth has slowed looks, on a late scan, exactly like a fetus that is small because the dates were wrong — and re-dating it at every scan makes growth restriction disappear by definition. The redating criteria interpreter carries the thresholds and that warning.
What the number is used for downstream is worth saying plainly, because it is why dating is the foundation of everything else. Every screening marker is read as a multiple of the median for a gestational DAY — the PAPP-A and free beta-hCG medians change daily in the first trimester — and every fetal measurement is read against a centile for gestational age. A dating error of a week moves both. The screening biochemistry of pregnancy lives in this site’s reproductive, fertility and pregnancy laboratory category: that category owns the assay, this one owns the pregnancy. Those pages need a gestational age and none establishes one, which is what the dating pages here are for. An obstetric measurement is read alongside the whole clinical picture — history, serial trend, the other biometry — and never instead of it. It supports a clinician’s judgement rather than replacing it.
Frequently asked questions
Why does this page not take a date?
Because the expression language behind every calculator here is numeric only: no strings, no date type, no date arithmetic. Rather than pretend otherwise, the page asks for the one number a reader can produce from any calendar — the days since the first day of the last menstrual period — and returns intervals: the gestational age, the completed weeks and days, and the days left to the 280-day estimate.
Should I adjust for cycle length?
It depends on your unit. Naegele’s rule assumes a 28-day cycle, and a longer cycle means later ovulation and a younger pregnancy, so the surplus is subtracted. Many screening programmes do not apply the correction, and ACOG’s dating guidance does not require it. Leave the field at 28 for the unadjusted figure. The difference can be two weeks between a 21-day and a 40-day cycle, which is wider than most of the redating thresholds.
Is 280 days a measurement?
No. It is a convention: 266 days from ovulation plus 14 days from the first day of the cycle to ovulation. Observed pregnancy length varies around it by considerably more than the spread of any single dating method, which is why the term strata in ACOG Committee Opinion 579 cover five weeks rather than naming one day.
What is a suboptimally dated pregnancy?
ACOG Committee Opinion 700’s term for a pregnancy that has had no ultrasound confirming or revising the estimated due date before 22 weeks 0 days. It matters because the dating method decides whether a pregnancy looks preterm or post-term, and both of those change what is offered.
Can a gestational age be re-set at a later scan?
ACOG Committee Opinion 700 says subsequent changes to the estimated due date should be reserved for rare circumstances, and the estimate should come from the earliest accurate measurement. Serial re-dating is the specific hazard: a growth-restricted fetus measures small, and re-dating it to match its size removes the finding. This page states the criteria and does not make that call.
Related calculators
References
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150-4.
- American College of Obstetricians and Gynecologists; Society for Maternal-Fetal Medicine. Committee Opinion No. 579: Definition of Term Pregnancy. Obstet Gynecol. 2013;122(5):1139-40.
- International Society of Ultrasound in Obstetrics and Gynecology. Fetal Biometry: Dating, Assessing Size and Estimating Fetal Weight (Basic Training lecture). London: ISUOG.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/