How accurate is a pregnancy due date calculator?

A due date estimate is a statistical estimate, not a guarantee. Only about 5% of babies are born on their exact calculated due date, most arrive within two weeks of it.

What is Naegele's rule?

Naegele's rule is the standard formula for estimating a due date: take the first day of the last menstrual period and add 280 days, named after Franz Naegele.

Why is pregnancy counted as 40 weeks if it lasts 9 months?

Pregnancy is measured in weeks from the last menstrual period, not conception, which adds about two weeks before conception actually occurs, making 40 weeks equal roughly 9.2 months.

What is the difference between gestational age and fetal age?

Gestational age counts from the last menstrual period, fetal age counts from actual conception, roughly two weeks later, so fetal age is always about two weeks behind gestational age.

How does IVF change the due date calculation?

IVF provides an exact fertilisation date, so clinics count 266 days from fertilisation minus the embryo age at transfer, giving 263 days for day 3 transfers and 261 days for day 5 transfers.

What counts as full term pregnancy?

Full term is 39 weeks 0 days to 40 weeks 6 days per ACOG guidelines, with early term, late term, and postterm covering the ranges before and after.

Can I use this calculator with an irregular cycle?

Yes, entering your typical cycle length adjusts the calculation, though results are less precise than a regular cycle. Using a known conception date instead often improves accuracy.

Does this tool store or share my pregnancy information?

No. All calculations run locally in your browser, nothing entered is ever sent to a server, logged, or stored.

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Pregnancy Due Date Calculator, Estimate Your Baby’s Arrival

This pregnancy due date calculator estimates your delivery date from your last menstrual period, conception date, or IVF transfer date. See your current gestational age, trimester, and key pregnancy milestones instantly, with no sign up and nothing sent to a server.

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Calculation Method

First Day of Last Period

Average Cycle Length (days)

Cycles longer or shorter than 28 days shift ovulation, so the calculator adjusts your due date accordingly.

Choose a method above, enter your date, and click Calculate Due Date to see your estimated delivery date, current week, and trimester.

Not medical advice. This is an estimate from average cycle data. Irregular cycles, PCOS, thyroid conditions, hormonal contraception and recent pregnancy all shift the result, sometimes by weeks. Do not use it to plan or prevent a pregnancy. See a doctor or midwife for anything that matters.

About This Tool

How this pregnancy due date calculator works, and why it offers four different starting points

A due date is a probability estimate dressed up as a single number. Only about 5 percent of babies are actually born on their calculated due date, most arrive within roughly two weeks of it in either direction. This tool computes that estimate using the method appropriate to what you actually know, the first day of your last period, a known conception date, or an IVF embryo transfer date, since each starting point requires a different offset to land on the same target: roughly 40 weeks of gestation.

All four calculations run locally using plain date arithmetic in your browser. Nothing you enter is uploaded or retained once you leave the page.

Naegele’s rule, and why it isn’t just adding nine months

The default method uses the first day of your last menstrual period. It’s built on Naegele’s rule, named for the 19th-century German obstetrician Franz Karl Naegele, who formalized a method still in near universal clinical use today: add 280 days, or 40 weeks, to the first day of the last period.

Step 1 Anchor on LMP Naegele’s rule assumes a standard 28-day cycle with ovulation on day 14. The tool asks for your actual average cycle length and adjusts the due date by the difference from 28 days, a refinement clinics also apply when cycle length is known.

Step 2 Estimate conception Ovulation, and therefore likely conception, is estimated as cycle length minus 14 days after the LMP, matching the same ovulation-timing logic used in the ovulation calculator.

Step 3 Track gestational age Gestational age counts from the LMP, not from conception, which is the clinical convention. That means you’re already considered roughly two weeks pregnant on the day you actually conceive, a source of frequent confusion worth knowing up front.

// adapted from computeDueDate(), LMP method var ovulationOffset = cycleLen – 14; conceptionEstimate = addDays(lmp, ovulationOffset); dueDate = addDays(lmp, 280 + (cycleLen – 28));

Conception date and IVF transfer methods

When you know the actual conception date rather than just a period date, the offset shortens, since you’re skipping the two week gap between LMP and ovulation entirely. IVF transfers shift the anchor again, because a lab transferred embryo already has a known age in days.

Method	Days Added	Reasoning
Last menstrual period	280 days from LMP	Naegele’s rule, cycle-length adjusted
Known conception date	266 days from conception	280 minus the ~14-day LMP-to-ovulation gap
Day 3 embryo transfer	263 days from transfer	266 minus the embryo’s 3-day age at transfer
Day 5 embryo transfer	261 days from transfer	266 minus the embryo’s 5-day age at transfer

Those IVF offsets are the standard convention fertility clinics use, since an embryo transferred on day 5 is already five days past fertilization, the due date math has to account for the head start.

A due date is a statistical midpoint, not a deadline. Naegele’s rule was calibrated on a 28-day cycle assumption that doesn’t match everyone, and full-term birth itself spans a real window, 37 to 42 weeks, not a single day. This tool’s estimate is a planning reference. A first-trimester dating ultrasound, which measures the fetus directly, is more accurate than any calendar-based method and is what your provider will actually use to confirm or adjust your due date.

Trimesters, milestones, and the size comparison

Trimester boundaries

First trimester runs through week 13, second through week 27, third from week 28 onward, the standard clinical convention used throughout the tool’s milestone timeline, including the viability milestone around 24 weeks and full term beginning at 37 weeks.

Size comparisons

The fruit and vegetable size references for each week are a widely used, informal convention across pregnancy tracking tools and apps, approximate and meant for a relatable sense of scale, not a clinical growth measurement.

Naegele’s rule, 280 days Only ~5% born on exact due date Dating ultrasound is more accurate

ACOG Committee Opinion, Methods for Estimating the Due Date, 2017, the clinical guidance behind the LMP and ultrasound dating methods used here.
CDC, gestational age conventions for how LMP-based dating is standardized in the United States.
NICHD, Pregnancy Overview, background on trimester structure and fetal development milestones.

This calculator is an educational planning estimate, not a medical diagnosis or a substitute for prenatal care. It cannot replace a dating ultrasound or your provider’s clinical judgment. Talk to a doctor or midwife to confirm your due date and for any decision related to your pregnancy.

Where a due date matters

Getting a quick estimate before a first prenatal appointment, cross checking a clinic’s quoted due date against your own tracked dates, planning parental leave or travel around key milestones, and building a personal calendar of trimester transitions and the estimated due date using the tool’s calendar export.

Common Questions

FAQ: Pregnancy Due Date Calculator

A due date from any calculator, including this one, is a statistical estimate, not a guarantee. Naegele’s rule assumes a textbook 28 day cycle with ovulation on day 14, but real cycles vary. Research shows only about 5% of babies are born on their exact calculated due date, most arrive somewhere within two weeks before or after it. A first trimester ultrasound dating scan is generally considered more accurate than date based calculation alone, since it measures the embryo directly.

Naegele’s rule is the standard formula doctors and midwives use to estimate a due date: take the first day of your last menstrual period, add one year, subtract three months, and add seven days, which is mathematically the same as adding 280 days. It is named after Franz Naegele, the German obstetrician who popularised the method in the 1800s, and it remains the default calculation used in prenatal care worldwide, alongside dating ultrasounds.

Pregnancy is measured in weeks from the first day of your last menstrual period, not from conception, even though conception typically happens about two weeks later. That means the first two weeks of a pregnancy count toward the 40 week total before conception has actually occurred. 40 weeks works out to about 9.2 calendar months, which is where the common “9 months” description comes from, it is a rounded approximation rather than an exact match.

Gestational age, the number this calculator and most doctors use, counts from the first day of the last menstrual period. Fetal age (also called conceptional age) counts from the actual day of conception, roughly two weeks later. So at “10 weeks gestational age,” the embryo itself is closer to 8 weeks old developmentally. Medical appointments, due dates, and trimester boundaries are almost always expressed in gestational age.

With IVF, the exact fertilisation date is known, which makes due date estimation more precise than LMP based methods. Clinics count 266 days from fertilisation, then subtract however many days old the embryo was at transfer, 3 days for a cleavage stage transfer or 5 days for a blastocyst transfer. That gives 263 days from a day 3 transfer and 261 days from a day 5 transfer, both of which this calculator supports directly.

The American College of Obstetricians and Gynecologists defines early term as 37 weeks 0 days to 38 weeks 6 days, full term as 39 weeks 0 days to 40 weeks 6 days, late term as 41 weeks 0 days to 41 weeks 6 days, and postterm as 42 weeks and beyond. This calculator’s milestone table marks 37 weeks as the start of the full term range so you can see how close you are to it.

You can enter your typical cycle length and the calculator will adjust the standard 280 day count accordingly, but the result will be less precise than for a regular cycle. If you know your conception date more precisely, for example from ovulation test strips or fertility tracking, switching to the Conception Date method will generally produce a more reliable estimate than the Last Period method for irregular cycles.

No. Every calculation happens locally in your browser with JavaScript date arithmetic. Nothing you type in, including your last period date, conception date, or transfer date, is ever sent to a server, logged, or stored. The calendar file it generates downloads directly to your device without touching any backend.

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