Due Date Calculator Explained: Naegele's Rule, Cycle Adjustment, and IVF Dating

Forty weeks from a day you might not remember exactly

A pregnancy due date is arithmetic on top of a guess. The arithmetic is fixed — add 280 days to the first day of your last period and you have an estimated due date, called an EDD. The guess is which day that was, and it is the reason due dates are estimates rather than deadlines. The Calc Dragon due date calculator handles all four common ways to date a pregnancy — last menstrual period (LMP), conception or ovulation date, and IVF Day-3 or Day-5 transfer — and hands back the EDD plus the dates that mark the start of the second and third trimesters.

The 280-day figure comes from a nineteenth-century German obstetrician, Franz Karl Naegele, whose rule has stayed the clinical standard for good reasons: it is simple, it is stable across populations, and no better calendar rule has replaced it. The American College of Obstetricians and Gynecologists (ACOG) still endorses it in Committee Opinion 700, reaffirmed in 2024. This article walks through what the calculator is doing under the surface, why cycle length matters, how IVF dating differs, and the small handful of things worth knowing before you write the date on your calendar.

What a due date actually is

The estimated due date is a statistical mid-point, not a prediction. In a 2020 US cohort study covering more than nineteen million births, fewer than five per cent of babies arrived on the calculated EDD. Around sixty per cent came within seven days of it either way, and roughly ninety per cent within fourteen days. That is a wide window, and it is the reason obstetricians treat 37 to 42 weeks as term rather than treating 40 weeks as a target.

A gestational week is counted from the first day of the last menstrual period, not from conception. That convention has an odd consequence — a pregnancy is already two weeks "pregnant" on the day of conception and four weeks along by the first missed period. It exists because LMP is the one date most people can pin down without a scan, and standardising to it lets every antenatal milestone (the twelve-week scan, the anomaly scan at twenty weeks, the glucose test at twenty-eight) refer to the same clock.

The due date calculator returns four dates from that clock: the EDD, the estimated conception date, the start of the second trimester (14 weeks) and the start of the third trimester (28 weeks). The first is the one people care about; the other three are the anchors midwives use for scheduling.

How the calculation works

Naegele's rule in its arithmetic form is: EDD = LMP + 280 days. Its mental shortcut, which is what most obstetricians use in clinic, is: take the first day of the last period, add seven days, subtract three months, add one year. The two versions give the same date to within a day because 280 days is 40 weeks is nine months plus one week minus a few days of month-length noise.

Behind the shortcut is one assumption: a regular 28-day cycle with ovulation on day 14. Fertilisation happens within a day of ovulation, so on that assumption conception is 14 days after LMP and gestation from fertilisation is 280 − 14 = 266 days. That 266-day figure — sometimes called the "true" gestation length — is what the other three methods key off:

• Conception or ovulation date known: EDD = conception + 266 days.
• IVF Day-5 blastocyst transfer: The embryo is 5 days old on transfer day, so EDD = transfer + 261 days.
• IVF Day-3 embryo transfer: The embryo is 3 days old on transfer day, so EDD = transfer + 263 days.

For LMP dating, if your cycle is not 28 days the standard adjustment (from RCOG Greentop Guideline No. 41 and Mongelli's 1996 study) is to shift the EDD by (cycle length − 28) days. A 30-day cycle pushes the EDD two days later; a 26-day cycle two days earlier. The Calc Dragon due date calculator applies this automatically when you enter your cycle length, and skips it (correctly) for the IVF and conception methods where the fertilisation date is already known.

A worked example, end to end

Take a real case. Your last period started on 1 January 2026 and your cycles run roughly 28 days. Feed that into the due date calculator:

1. LMP = 1 January 2026.
2. EDD = 1 January + 280 days = 8 October 2026.
3. Conception (LMP + 14) = 15 January 2026.
4. Start of second trimester (LMP + 14 weeks) = 9 April 2026.
5. Start of third trimester (LMP + 28 weeks) = 16 July 2026.

Cross-check with Naegele's shortcut: 1 Jan + 7 = 8 Jan; subtract three months = 8 October the previous year; add one year = 8 October 2026. Same answer, no calendar arithmetic. Now vary the cycle length: with a 30-day cycle the EDD becomes 10 October, and with a 26-day cycle it becomes 6 October. Two days each way per two-day cycle shift — exactly the (cycle − 28) rule.

Now imagine the same person had IVF instead, with a Day-5 blastocyst transfer on 15 January 2026. EDD = 15 January + 261 days = 3 October 2026. Notice this is five days earlier than the LMP-derived EDD. That is not a contradiction; it is a better estimate. IVF fixes the fertilisation date exactly, so it does not need to assume ovulation on day 14. Whenever IVF dating and LMP dating disagree by more than a week, ACOG says the IVF date wins.

Factors that shift the estimate

Your cycle length

A woman with 32-day cycles ovulates around day 18 rather than day 14. If dating is done by LMP with the standard 280-day rule, the EDD lands four days too early, because conception happened four days later than the rule assumes. This is the single most common source of LMP misdating and is why the calculator asks for cycle length up front. If your cycles vary by more than five days between periods, the calendar estimate becomes unreliable and a dating ultrasound at 8 to 13 weeks supersedes it.

Which method you picked

LMP has the most measurement error because it depends on recall of a single day. Conception or ovulation dating is tighter if you were tracking, because ovulation windows are narrow. IVF dating is the tightest of all — fertilisation happens in a dish on a known day. The pecking order for precision, from loosest to tightest, is LMP < conception < IVF. Any dating scan in the first trimester tightens LMP by about half.

Individual gestation length varies

A 2013 study in Human Reproduction by Jukic and colleagues measured ovulation biochemically and found that natural gestation length from ovulation varied by up to 37 days between women — with a median of 268 days from ovulation rather than the assumed 266. The rule is a population average, and about a two-day systematic bias exists at that median. No calendar method can correct for how long your particular pregnancy will actually run.

Multiple pregnancies

Twins deliver on average around 36 weeks; triplets around 32. The calculator returns the singleton EDD in every case because that is what antenatal guidelines are built around (scans, screening, testing all use the LMP clock even for twins). If you know you are carrying multiples, treat the EDD as a scheduling anchor, not a birth prediction, and expect to deliver a few weeks earlier.

First birth versus subsequent births

First-time mothers deliver, on average, three to five days later than their EDD in large cohort data. Second and later pregnancies cluster more tightly around the EDD itself. Neither pattern is strong enough to build into the rule — the variance between individuals dwarfs the between-parity signal — but it explains why first-birth stories so often involve waiting past the due date.

Practical tips

• Use LMP if you know it, otherwise conception or IVF. More precise inputs give more precise EDDs. If you have a confirmed IVF transfer date, that is always the best input.
• Enter your actual cycle length. Leaving it at 28 when your cycles are 32 will put the EDD four days early. The calculator's adjustment is automatic once you tell it the truth.
• Get a dating scan in the first trimester. A crown-rump length measurement between 8 and 13 weeks is accurate to about ± 5 days and, when it disagrees with LMP by more than a week, replaces it as the official EDD.
• Do not update the EDD after 20 weeks. Third-trimester scans are much less accurate for dating than first-trimester scans. Once the EDD is set by a first-trimester scan, ACOG guidance is to keep it fixed for the rest of the pregnancy.
• Treat "40 weeks" as a midpoint, not a target. Roughly half of babies come after the EDD, half before. If you are booking work leave or family visits, plan for a window between 38 and 42 weeks.
• Use the day counter for milestones. If you want to know exactly how many days from today until 20 weeks or how many weeks pregnant you are on a specific date, the day counter is the fastest tool.

Common mistakes

Counting from conception instead of LMP

Every antenatal system counts from the first day of the last period, not from conception. If you enter the conception date but pick the LMP method, the calculator will treat conception as LMP and give you a due date two weeks too early. Match the method to the date you have.

Confusing spotting with the start of the period

LMP is the first day of full menstrual bleeding, not spotting. Counting the first spot of pink discharge as day one can shift the EDD by two or three days. If you spotted before your period started, use the day the flow properly began.

Ignoring cycle length after coming off hormonal contraception

Cycles are often irregular for the first three to six months after stopping the pill, the implant or the coil. LMP dating in that window is unreliable, and a dating scan is more or less mandatory. Do not lean on the calendar alone if your last three cycles varied by more than a week.

Applying the cycle adjustment to IVF dates

Cycle length only matters for LMP dating, because it is used to infer the ovulation day. In IVF the fertilisation day is known exactly, so cycle length is irrelevant. The Calc Dragon calculator ignores the cycle field automatically when you pick the IVF or conception methods. Manual spreadsheets tend not to.

When to seek professional advice

A calculator is a scheduling tool, not a clinical assessment. Book an early appointment with a midwife or obstetrician as soon as you have a positive test — most health services aim for a booking appointment by 8 to 10 weeks and a dating scan between 11 and 14 weeks. If you have very irregular cycles, are on fertility treatment outside of IVF, have had recent pregnancy loss, or are unsure of your LMP, get the scan sooner rather than later. The calendar-based EDD is a starting point; the scan confirms or corrects it.

Frequently asked questions

How accurate is the due date the calculator gives me?

Naegele's rule is a calendar estimate, not a measurement. The 2013 Human Reproduction study by Jukic et al. found that natural pregnancies, measured from ovulation, varied by up to 37 days between women. About four per cent of babies arrive on their due date; roughly ninety per cent within fourteen days either side. A first trimester dating ultrasound is typically accurate to ± 5 days and supersedes the LMP estimate if the two disagree by more than a week.

Why does the IVF method give a different answer to LMP?

Because IVF pins the fertilisation date exactly. Egg retrieval, insemination and embryo age at transfer are all known. LMP-based dating has to infer ovulation from cycle length, which is where most of the error comes from. ACOG recommends IVF dating take precedence whenever the transfer date is known: EDD = transfer + 266 − embryo age, giving + 261 days for a Day-5 blastocyst and + 263 for a Day-3 embryo.

My cycle is not 28 days — does that matter?

Yes, for LMP dating it does. A longer cycle means ovulation happened later, so conception happened later relative to LMP, so the due date is later. The RCOG-endorsed adjustment is to shift the EDD by (cycle length − 28) days. A 30-day cycle pushes it two days later; a 26-day cycle two days earlier. The calculator applies this automatically. For highly irregular cycles (variation greater than ± 5 days), a dating scan is more reliable than any calendar rule.

What counts as the first day of my last period?

The first day you saw full menstrual bleeding from your last period before pregnancy — not the day it ended, not the period before that, and not the spotting that sometimes precedes it. If you are unsure, enter your best estimate and confirm with a dating ultrasound at the booking appointment.

When do the trimesters start and end?

The most common convention, used by NHS, ACOG and the WHO, is trimester 1 from 0 to 13 weeks 6 days, trimester 2 from 14 weeks 0 days to 27 weeks 6 days, and trimester 3 from 28 weeks 0 days to birth. Some textbooks use 13 and 27 as boundaries instead; the difference is a few days and does not affect clinical care.

Is the calculated due date the same as the date my baby will be born?

No. In a 2020 cohort study of nineteen million US births, fewer than five per cent delivered on the calculated EDD, about sixty per cent within a week either side, and roughly ninety per cent within two weeks. Doctors treat 37 to 42 weeks as term. The EDD is most useful as an anchor for scheduling scans, vaccinations and screening tests — not as a prediction of the actual birthday.

Can twins or triplets use the same calculator?

Yes, but with a caveat. Antenatal guidelines all use the singleton EDD for scheduling, so the calculator returns it whether you are carrying one baby or three. Statistically, twins deliver at around 36 weeks and triplets at around 32, so treat the EDD as a scheduling anchor rather than a birth prediction if you already know you are carrying multiples.

What about pregnancies conceived with fertility drugs but not IVF?

For ovulation-induction cycles (Clomid, letrozole, gonadotropins) with a monitored ovulation date, use the conception method and enter the ovulation date. For intrauterine insemination timed to a natural cycle without precise ovulation confirmation, LMP-based dating with the actual cycle length is a reasonable start; expect a dating scan to firm it up.

Related calculators

• Due Date Calculator — the parent tool, with LMP, conception and IVF variants.
• Period Calculator — track cycles and predict upcoming period dates.
• Day Counter — days, weeks and months between any two dates.
• Age Calculator — exact age in years, months and days.
• Age in Weeks Calculator — infant age in weeks, handy in the first year.
• BMI Calculator — body mass index with WHO categories.