Due Date Calculator - Naegele Estimate from LMP

A due date calculator answers a classroom question: if the first day of the last menstrual period (LMP) is known, what calendar day falls 280 days later, and how does an optional cycle length other than 28 days shift that estimated due date (EDD)? Obstetric gestational age is counted from LMP, not from fertilization, so “40 weeks” is a 280-day convention, not a claim that the embryo existed for 40 weeks. The default inputs on this page are LMP 2026-01-01 and cycle 28 days, which land on EDD 2026-10-08. Conception for a 28-day cycle is shown as LMP + 14 days (2026-01-15). None of those dates is a diagnosis, a guarantee of live birth timing, or a substitute for clinical dating.

Franz Karl Naegele popularized adding one year, subtracting three months, and adding seven days to LMP. For many calendar dates that identity matches adding 280 days. This implementation uses JavaScript UTC date arithmetic (setUTCDate / setUTCMonth), not string concatenation or “month 13” hacks. When LMP falls near the end of a short month, the +1 year −3 months form can land on a different civil day than +280; the page reports both and treats LMP + 280 + (cycle − 28) as primary.

Scope is educational. A first-trimester ultrasound, a known ovulation (LH kit, basal temperature interpreted by a clinician), or an assisted-reproduction transfer date supersedes LMP math. Irregular cycles, uncertain LMP, recent combined hormonal contraception, lactation amenorrhea, and spotting mistaken for a period all degrade the input. This tool is not a fertility treatment planner, not an IVF calendar, not a miscarriage risk score, and not medical advice. If you are pregnant or trying to conceive, talk to a qualified clinician about dating and prenatal care-not a browser form.

Related health tools on this site answer different questions: adult BMI, healthy weight range, and target heart rate. None of them date a pregnancy.

1. Enter the first day of the last menstrual period. Use the bleeding that you and a clinician would call day 1, not implantation spotting.
2. Set cycle length in whole days. Leave 28 for the classic 280-day EDD. If typical cycles are longer or shorter, EDD and the conception estimate shift by (cycle − 28).
3. Optionally enter an as-of date (today or any day after LMP) to see obstetric gestational age in weeks plus leftover days.
4. Copy the result if you need a record. Inputs stay in this browser for up to 30 days and are never uploaded. Treat the EDD as a planning estimate until imaging or clinical dating says otherwise.

2026 is not a leap year. 1 January is day 1 of the year; 8 October is day 281. The difference is 280 days, so LMP 2026-01-01 plus 280 days is 2026-10-08. Adding one year yields 2027-01-01; subtracting three months yields 2026-10-01; adding seven days yields 2026-10-08-the same civil date. Conception at cycle 28 is 2026-01-15. Gestational age on 2026-10-08 would be 280 days = 40 weeks + 0 days if you filled that as-of date.

If cycle length is 32, the optional classroom adjustment adds 4 days: EDD 2026-10-12 and conception 2026-01-19. If cycle length is 24, subtract 4 days: EDD 2026-10-04 and conception 2026-01-11. The biological story behind the shift is that luteal phase length is often near 14 days while the follicular phase stretches or shrinks, so ovulation (and therefore fertilization) moves relative to LMP. That story is a population average. Some people ovulate on day 12 of a 32-day cycle; some ovulate on day 20. A home ovulation test interpreted in clinic still beats this slider.

Gestational age on this page is (as-of − LMP) in whole days, then weeks = floor(days / 7) and remainder days. Embryonic or “fetal” age is typically about two weeks less because fertilization is assumed around LMP + 14 on a 28-day cycle. Trimester boundaries used in teaching (through 13 weeks 6 days, 14–27 weeks 6 days, 28 weeks onward) are administrative cuts, not sudden physiological switches. Post-term surveillance conversations belong with obstetric care, not with a red banner on a website.

Crown–rump length in the first trimester is a more repeatable dating method than recalled LMP in most guidelines for uncertain periods. If ultrasound and LMP disagree by more than the clinic’s threshold, the scan usually wins. This calculator will not ingest biparietal diameter or a Hadlock table; it will not “reconcile” two EDDs. If your clinic already assigned a due date, use that date for appointments.

In vitro fertilization and frozen embryo transfer use the age of the embryo and the transfer day, not LMP. A day-5 blastocyst transferred on a known calendar day has a dating convention that this form does not implement. Entering a made-up LMP to force an IVF pregnancy onto Naegele’s rule is the wrong model. We deliberately did not add a “transfer date” mode so the page cannot be mistaken for a fertility-clinic tool.

Polycystic ovary syndrome, thyroid disease, perimenopause, and recent depot medroxyprogesterone can produce cycles that are not 28 ± a few days. The 20–45 day input guard is only to catch typos, not to certify that 45-day cycles are “normal.” Bleeding after a positive pregnancy test may not be a period. When LMP is unknown, do not invent a date to satisfy the field.

The conception line is LMP + 14 + (cycle − 28), a single day for display. Fertile windows in real couples span several days because sperm can survive in the reproductive tract and because ovulation timing jitters. Intercourse on that estimated day is neither required nor sufficient. We will not compute “peak fertility hours,” cervical mucus scores, or intercourse schedules.

Home pregnancy tests detect hCG after implantation, which itself occurs days after fertilization. A negative test on the conception estimate date is expected. Serial quantitative hCG and ultrasound viability checks are clinical, not something this page will trend. If you are using assisted reproduction, follow the clinic’s calendar instead of Naegele.

Multiple gestation, molar pregnancy, and ectopic pregnancy are not detectable from LMP arithmetic. Pain, bleeding, dizziness, or a positive test with severe symptoms need urgent care. A due date widget cannot triage those presentations.

Counting forty weeks from LMP predates ultrasound and became a shared language for midwives and later obstetricians. Mean interval from LMP to spontaneous birth in some modern cohorts is a few days different from 280, and first births tend to run later than subsequent births on average. Public-facing calculators still teach 280 because it is the convention encoded in “40 weeks,” not because nature signed a contract. Only about 5% of spontaneous births occur on the EDD itself; a window of weeks is the honest planning language.

Search queries cluster on “due date from LMP,” “Naegele’s rule,” “how many weeks pregnant am I,” and “conception date calculator.” How many weeks is (today − LMP) / 7 using obstetric counting if you fill the as-of box. Conception date is an estimate around ovulation, not a photograph of fertilization. Anything beyond that-induction timing, genetic screening windows, vaccine schedules-belongs in prenatal care.

Worked extras: LMP 2026-02-14 cycle 28 → EDD 2026-11-21 (2026-02-14 + 280; 2026 is not leap so February has 28 days). LMP 2025-11-01 cycle 28 → EDD 2026-08-08. LMP 2026-01-01 cycle 35 → EDD 2026-10-15 (280 + 7). LMP 2026-03-31 cycle 28: adding 280 days is the reliable path; the −3 month Naegele form can collide with month-length. Those examples show why this page prefers day counts.

After you have an educational EDD, other calculators still do not replace clinic visits. Energy needs in pregnancy are not the same as the adult TDEE calorie calculator. Strength training loads are not the one-rep max calculator. Running math lives on the pace calculator. Use the tool that matches the actual question.

Teaching charts often split pregnancy into first trimester through 13 weeks 6 days, second through 27 weeks 6 days, and third from 28 weeks. Those cuts help schedule education and some screening conversations; they are not biological on/off switches. Aneuploidy screening, anatomy ultrasound, and anti-D immunoglobulin timing (when indicated) use clinic protocols keyed to gestational age that this page does not encode. Filling an as-of date so you can say “I am 12 weeks 3 days on this LMP clock” is still only LMP math.

Mean gestational length in spontaneous singleton births is not exactly 280 days in every dataset. First births skew later; subsequent births a little earlier on average. Stillbirth risk and post-term surveillance are obstetric topics. A website will not color your calendar red after day 287. If your clinic already set an EDD from a scan, ignore this form for appointment planning.

Twins and higher-order multiples are dated with different growth curves and often delivered earlier by plan. Entering an LMP for a twin pregnancy into Naegele’s singleton 280-day rule is the wrong model. IVF dating, as already noted, uses embryo age plus transfer day. Home “contraception calendars” that try to invert this EDD into a fertile-day planner are also out of scope; inversion is not how ovulation works day-to-day.

Leap years: 29 February exists in 2024 and 2028, not 2026. Adding 280 days across 29 February in a leap year still works if you use date objects rather than “always 28 days in February” string math. That is another reason this implementation uses UTC calendar arithmetic. If you need weekday of EDD, the result panel already formats the civil date in your locale with a UTC timezone so 2026-10-08 does not slip to the 7th in US evening timezones.

Privacy: LMP is sensitive reproductive data. It stays in localStorage on this device for up to 30 days with a unique key, then the timestamp expires. Clearing the form removes the fields from the UI; you can also clear site data in the browser. We do not email an EDD, do not build a baby registry, and do not upsell prenatal vitamins. If that is the product you wanted, this is the wrong site-and that is intentional.

Miscarriage, ectopic pregnancy, and molar pregnancy cannot be ruled in or out with Naegele arithmetic. Cramping plus bleeding plus a positive test is a clinical pathway, not a “recalculate EDD” pathway. If a prior ultrasound already assigned a due date, writing a new LMP here does not overwrite the medical record. Medications such as misoprostol or mifepristone, and procedures such as D&C, also sit outside this form. We will not estimate “how far along” after pregnancy loss; that question belongs with the clinician who is actually caring for you.

Travel across the International Date Line does not change gestational age; only the civil timestamp of “today” in the as-of box does. If you are unsure which midnight to use, pick the date your clinic uses on the chart. The default LMP 2026-01-01 remains the grading check: 280 days later is 2026-10-08 at cycle 28, conception estimate 2026-01-15, and Naegele +1 year −3 months +7 days agrees on that particular calendar. That agreement is why it is the default, not because January 1 is a typical LMP.