Pregnancy Calculator

⚙️How the due date is calculated (Naegele’s rule)

Doctors use Naegele’s rule: pregnancy lasts 40 weeks (280 days) counted from the first day of the last menstrual period (LMP), not from conception. It assumes a regular 28-day cycle with ovulation around day 14.

Example: LMP October 1, 2026 → due date July 8, 2027; approximate conception October 15, 2026.

If your cycle isn’t 28 days, the estimate adjusts: due date = LMP + 280 + (cycle length − 28). A 35-day cycle moves the due date a week later (July 15, 2027 for the example); a 21-day cycle moves it a week earlier (July 1, 2027).

📅Four ways to date a pregnancy

Use whichever starting point you actually know — the calculator converts all of them to the same 280-day timeline:

• Last period (LMP): the classic input; add the cycle-length adjustment above if you’re not on a 28-day cycle.
• Conception date: due date = conception + 266 days (38 weeks). Useful if you tracked ovulation or had fertility treatment.
• Ultrasound scan: enter the scan date and the gestational age it reported — this is the method doctors trust most (see below).
• Known due date: working backwards from a date your doctor already gave you rebuilds the full timeline and milestones.

👶Weeks vs months: why doctors count in weeks

Doctors count pregnancy in weeks, not months, because months are unequal. Roughly: month 1 is weeks 1–4, month 3 ends at week 13 (end of the first trimester), month 6 ends at week 27, and month 9 ends at week 40. When someone says “six weeks pregnant”, they mean six weeks since the first day of their last period — about four weeks since conception.

🎯Milestones worth knowing

The week-by-week schedule above is computed from your own dates. The big ones to remember: the first heartbeat is usually detectable around week 6, the first trimester ends at week 13, the anatomy scan happens around week 20, viability is reached around week 24, the third trimester begins at week 28, and early term starts at week 37 — a baby born from then on is considered full-term-adjacent.

🔬Why an ultrasound dating scan is more reliable

Naegele’s rule assumes ovulation on day 14 of a 28-day cycle, which many people don’t follow. A first-trimester ultrasound measures the embryo directly (crown-rump length) and dates the pregnancy within a few days — this is the method doctors trust most, and it can shift your due date by days or even weeks. If your cycles are longer or irregular, your true gestational age may differ from the LMP estimate.

⚖️Accuracy: treat it as a planning estimate

Adding 280 days to the LMP dates back to an 18th-century formula (Naegele’s rule: LMP + 1 year − 3 months + 7 days — mathematically the same as LMP + 280 days). Only about 5% of babies arrive on their exact due date; most arrive within two weeks on either side. Treat the result as a planning estimate, not an appointment.

🧮How to calculate due date

Everything on this page runs on one formula, and you can work it out on paper in under a minute. To calculate due date the way midwives have for two centuries, use Naegele’s rule, named after the German obstetrician Franz Naegele:

1. Start with the first day of your last menstrual period (your LMP).
2. Add 7 days.
3. Count back 3 calendar months.
4. Add 1 year. The date you land on is your due date.

Worked example. Say your last period started on January 5, 2026. Add 7 days and you get January 12, 2026. Count back 3 months and you get October 12, 2025. Add 1 year and your due date is October 12, 2026. Run the same LMP through the calculator above and you will get the same answer, because adding 280 days lands on exactly the same day — the calendar method and the day count are the same rule in two different outfits.

That date has an official name: your estimated due date, or EDD. When people search for an EDD calculator or an estimated due date calculator, this is precisely the calculation they mean — and the word estimated is doing real work in the name, because only about 5 in every 100 babies actually arrive on it.

The rule quietly assumes a 28-day cycle with ovulation on day 14. If your cycle runs longer, you ovulated later, so your due date moves later by the same amount: with a 32-day cycle, the January 5 example shifts 4 days to October 16, 2026. The calculator asks for your usual cycle length and makes that adjustment for you, which is the part most people get wrong when they count on their fingers.

Your LMP does double duty, because it also sets your gestational age — the count of weeks and days since that first day, which is the number every appointment, scan and test is scheduled by. The tool at the top of this page is therefore also a gestational age calculator: give it your LMP and it reports both your due date and exactly how far along you are. And because the whole system hangs on that one date, you will sometimes see tools like this called an LMP calculator. If the LMP itself is uncertain, every method built on it inherits that uncertainty — which is exactly the problem the next sections solve.

📆Pregnancy weeks to months

Weeks are precise; months are not. A calendar month runs anywhere from 28 to 31 days, so 40 weeks of pregnancy is not a tidy nine months — it is closer to nine months and one week. That mismatch is why providers count in weeks, and why “how many months am I?” never has a single exact answer.

The chart below uses the convention most published pregnancy weeks to months charts follow: whole weeks grouped into months of four or five weeks that add up to the full 40, with trimesters following ACOG’s definitions — the first trimester runs through week 13, the second covers weeks 14 to 27, and the third runs from week 28 until delivery. If you go past your due date, weeks 41 and 42 fall into a tenth month.

Two honest warnings about any chart like this. First, the boundaries are conventions, not biology: week 27, for example, groups into month 7 here, yet it is still the final week of the second trimester — so trust the trimester column for anything medical, and treat the month column as the friendly translation you give family. Second, pair this chart with the milestone table in your results above: together they form your personal pregnancy calendar, translating every appointment, scan and test into the week, month and trimester it falls in. Your results also include the full 40-week calendar with start dates — print it, or save it as a calendar file (.ics) that imports into Google Calendar, Apple Calendar and Outlook.

🔄When the ultrasound date and your period date disagree

It happens constantly: your period dates say one due date, the first scan says another, and the two can be a week or more apart. Period dating assumes you ovulated on day 14, and plenty of people don’t. A first-trimester ultrasound instead measures the embryo directly (its crown–rump length) and dates the pregnancy to within about 5 to 7 days — the most accurate dating there is, which is why the scan usually wins. If your report gives the crown–rump length itself rather than an age in weeks, choose Crown-rump length, CRL (mm) in the ultrasound tab above: the calculator converts the length to an age with the Robinson & Fleming formula (1975), the standard crown-rump dating equation — the same measurement, entered the other way round.

ACOG’s Committee Opinion No. 700, Methods for Estimating the Due Date (2017), gives providers a clear rule: keep the period-based date unless the ultrasound disagrees by more than a set margin. The margin grows as pregnancy progresses, because babies naturally vary more in size the older they get:

Worked example. With an LMP of January 5, 2026 you would be exactly 10 weeks pregnant at a scan on March 16. If that scan instead measures 8 weeks 6 days — eight days behind — the gap is more than the 7-day margin for that stage, so under ACOG’s rule your provider would move your due date eight days later, from October 12 to October 20, 2026, and the ultrasound date becomes the official one.

Once set, a due date is rarely changed again: later scans measure growth, not age, and a big or small measurement late in pregnancy says something about growth rather than dates. If you reach 22 weeks with no ultrasound that has confirmed or revised your dates, ACOG considers the pregnancy “suboptimally dated” — one more reason that early dating scan matters. Whichever date your provider settled on, enter it above (your LMP, or the scan’s own date and measurement) and the calculator rebuilds the same timeline either way.

📍How far along am I? Reading your result

It is the first question everyone asks — “how many weeks am I?” — so the results panel answers it first. Think of that panel as your pregnancy week calculator: it converts your dates into the four numbers you will actually use at appointments.

• Gestational age, in weeks and days — the number your provider quotes, counted from your last period.
• Your due date — the 280-day mark everything else is planned around.
• Days remaining — the countdown to that due date.
• Your trimester and progress — where you sit on the 280-day journey bar.

Worked example. If your last period started on August 3, 2026, then on October 20 you are 78 days in: 11 weeks and 1 day pregnant, in your first trimester and in month 3 of the chart above. Your due date is May 10, 2027, there are 202 days to go, and you are about 28% of the way through the 280 days. Change any input and every one of those numbers updates instantly.

Two things about the result surprise people. First, the count starts at your period, so you are “two weeks pregnant” before conception has even happened — gestational age runs about two weeks ahead of the baby’s actual age since fertilisation, and every medical chart uses the gestational figure. Second, weeks completed and the week you are in differ: at 11 weeks and 1 day you have completed 11 weeks and are living your 12th week. Providers quote the completed figure, and so does this calculator.

🧬Twins, IVF and other special cases

Twins are dated exactly the same way at first: the due date is set 40 weeks from the LMP and confirmed on the early ultrasound using the larger baby’s crown–rump length. What changes is the expectation at the end — twin pregnancies finish earlier on average, with most twins arriving around weeks 36 to 37, so your provider plans around an earlier window even though the official due date stays at 40 weeks. Select Twins under “How many babies?” in the calculator above and your results will show that earlier window as a 37-week planning date, alongside the official 40-week due date.

After IVF, nobody has to guess from a period: the clinic counts from fertilisation itself. Your care team converts the egg-retrieval and transfer dates, together with the embryo’s age at transfer (day 3, day 5 or day 6), into an estimated due date. If you have that date, enter it under “Known due date” above and the calculator rebuilds your full week-by-week timeline from it.

However your pregnancy started, the note below this guide applies: these dates are planning estimates, and the dating your own provider set early on — and wrote into your record — is the one your care will follow.

📚Sources

American College of Obstetricians and Gynecologists (ACOG), Committee Opinion No. 700, Methods for Estimating the Due Date (2017) — the source for Naegele’s rule, ultrasound dating accuracy and the redating thresholds above; ACOG patient guidance on pregnancy dating and trimester definitions. Robinson, H.P. & Fleming, J.E.E., A critical evaluation of sonar “crown-rump length” measurements, British Journal of Obstetrics and Gynaecology (1975) — the source of the crown-rump length dating formula used by the CRL entry in the ultrasound tab. The weeks-to-months groupings follow the convention stated in that section. This page provides general information only and is not medical advice.