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Pregnancy Due Date Calculator

Enter your last period or conception date to get your due date, current week and trimester.

Enter a valid date that is not in the future.

What a due date really is, and why the count starts before conception

An estimated due date, or EDD, is the midpoint of a window, not an appointment. The standard way to work it out is Naegele’s rule: take the first day of your last menstrual period, add 280 days and you have 40 weeks of pregnancy. Every clinic in the world uses some version of this count, which is why your notes say something like 24w3d rather than a number of months.

The part that confuses almost everyone is that the clock starts about two weeks before there is anything to count. Weeks one and two of a pregnancy are the weeks of the period itself and the run up to ovulation. Conception lands around week two, implantation around week four, and a positive home test usually arrives somewhere in week four or five. Counting from the day you think you conceived gives a number roughly two weeks lower than the one your provider will use, and that gap is the single most common reason people think an online calculator is broken.

The reason for the convention is practical rather than biological. Most people can name the first day of their last period. Almost nobody can name the hour they ovulated. Dating from the period gives every clinic the same starting line, and an early ultrasound then corrects it when the two disagree.

Which start date to enter

  • Last period, regular cycles: the default. Enter the first day of bleeding, not the last. If your cycles run close to 28 days, this is as good as arithmetic gets.
  • Last period, long or irregular cycles: still usable, but read the result as provisional. A 35 day cycle usually means ovulation about a week later than the rule assumes, so the true date is about a week later than the calculator shows.
  • Known conception date: choose the conception option if you tracked ovulation with LH strips or basal body temperature, or if there was a single possible date. The calculator works backward to a notional last period and then forward again.
  • IVF or a timed transfer: use the conception option with the fertilization or retrieval date, then compare it with the date your clinic gives you. The clinic date wins, because it accounts for the age of the embryo at transfer.
  • You have already had a dating scan: use the date your provider recorded. A first trimester scan beats every calculation on this page, including this one.

How to find out how many weeks pregnant you are

  1. Leave the start option on last menstrual period, which is what clinicians use.
  2. Enter the first day of that period. If you are unsure by a day or two, enter your best guess and treat the result as approximate.
  3. Read the three cards: how far along you are in weeks and days, which trimester that falls in, and how many days are left to the estimated date.
  4. Check the timeline below them. It shows where you are between week 0 and week 40, with the usual points of interest marked.
  5. Open the weekly size panel for the average length and weight at your current week, and expand the chart if you want to see the whole run from week 4 to week 40.

How to date from conception or an IVF transfer

  1. Switch the start option to conception date and enter the day of ovulation, fertilization or egg retrieval.
  2. The calculator adds 266 days, which is the same 280 day estimate minus the two weeks that precede conception in the standard count.
  3. For IVF, compare the answer with your clinic’s date. Clinics count from the transfer and subtract the age of the embryo: a day 5 blastocyst transfer is usually 261 days from the transfer date, a day 3 transfer 263 days. A day or two of difference between methods is normal.
  4. Whichever date you end up with, keep using the same one. Switching between a period based date and a scan based date halfway through makes every later measurement harder to interpret.

Getting the date out of this page and into your week

  • Put it in a calendar: add the due date as an all day event, then add the week 20 scan window and the week 37 mark from the milestone list as separate events. Seeing them on the same calendar as everything else is more useful than a countdown.
  • Share it: nothing you type is stored in the link, so sending this page to someone will not carry your dates with it. Send the due date itself, or the week you are in, which is the number a midwife or doctor will ask for anyway.
  • Paperwork: employers and insurers usually ask for the EDD as a plain date, sometimes with a letter from your provider confirming it. The number from a calculator is a starting point for that conversation, not a document.
  • On a phone: the date field opens your device’s own date picker on iPhone and Android, so you can scroll to the month rather than typing it.

Where the numbers differ between apps, clinics and countries

  • Period dating against ultrasound dating: these are the two systems that actually compete. Guidance from obstetric bodies treats a first trimester crown to rump measurement as the more accurate of the two and sets thresholds for when the scan should replace the calculated date, on the order of five days in the earliest weeks and widening as pregnancy progresses. If your provider moved your date, this is why.
  • IVF pregnancies: dating comes from the laboratory, not from a period, and it is the most precise dating there is. Clinics still differ in whether they quote the date from retrieval, from fertilization or from transfer.
  • Apps and clinic portals: some recalculate the whole pregnancy when you enter a scan result, some keep the original date and show a discrepancy, and some silently round the week boundary differently. If two apps disagree by a day, this is usually the cause.
  • Countries and languages: French speaking practice distinguishes weeks of amenorrhea, counted from the last period, from weeks of pregnancy counted from conception, a difference of exactly two weeks. Other traditions describe pregnancy in months, including lunar months of 28 days, which is where the old idea of ten months comes from. The week number itself is the only figure that travels safely.
  • Trimester boundaries: this page splits at 13 and 27 weeks. Plenty of reputable sources split at 14 and 28 instead. Both are conventions for talking about the pregnancy rather than anything that happens on the day.

Important limits

This calculator is not medical advice and cannot replace care from a doctor or midwife. It applies a formula to a date you typed. It does not know your cycle, your history, whether you are carrying more than one baby, or anything a scan would show. Confirm your date with your own provider, and contact them rather than a web page about any symptom that worries you.

The milestone list on this page is deliberately generic. Prenatal visit schedules are set nationally and differ a great deal: the number of funded visits, when screening is offered and which vaccines are given all depend on where you are. Use the list as a rough map of the pregnancy, and use your own clinic’s schedule for actual appointments.

The weekly length and weight figures are population averages gathered from general reference sources. They exist to give you a picture, and a baby well above or below them can be perfectly healthy. Note that the length measurement changes around week 20, from crown to rump to crown to heel, which is why the number jumps.

Frequently asked questions

How is a due date calculated?

The standard method is Naegele's rule: take the first day of your last menstrual period and add 280 days, which is 40 weeks. That is what clinicians mean by an estimated due date, or EDD. If you know the date of conception or an IVF transfer instead, the same estimate is conception plus 266 days, because conception normally happens about two weeks after the period starts. This calculator does both.

How many weeks pregnant am I?

Count from the first day of your last period, not from the day you conceived. Divide the days elapsed by seven and you get the gestational age that clinicians use, written as weeks plus days, such as 24w3d. This is why a positive test a few days after a missed period usually already reads as four or five weeks.

Why does pregnancy counting start before conception?

Because the first day of a period is something most people can name, while the day of ovulation usually is not. Dating from the last period is a convention that gives every clinic the same starting line. It adds roughly two weeks of calendar time at the front, so weeks one and two of a pregnancy happen before there is an embryo at all.

My cycle is not 28 days. Does that change the due date?

It can. Naegele's rule assumes ovulation on day 14 of a 28 day cycle. If your cycles run longer, ovulation is usually later, so a date based on your last period tends to run early and the real due date is later than the rule suggests. A rough adjustment is to add the difference between your usual cycle length and 28 days. A first trimester ultrasound settles it far more reliably than any arithmetic.

How do I work out a due date after IVF?

IVF gives an exact date, so dating is more precise than for a spontaneous pregnancy. A common approach is to count from the retrieval or fertilization date and add 266 days, adjusting for the age of the embryo at transfer: a day 5 blastocyst transfer is 261 days from transfer, a day 3 transfer is 263 days. Your clinic will give you the date it uses, and that date is the one to trust.

Why did my doctor change my due date after the scan?

An early ultrasound measures crown to rump length, and in the first trimester that measurement predicts gestational age more reliably than a remembered period date. Professional guidance says the scan date should replace the period based date when the two differ by more than a set margin, around five days in the earliest weeks and a wider margin later in pregnancy. Once a date is set it is normally kept for the rest of the pregnancy.

When does each trimester start and end?

This calculator uses weeks 0 to 12 for the first trimester, 13 to 26 for the second and 27 onward for the third. Other sources split at 14 and 28 weeks instead. The boundaries are conventions rather than biological events, so a week either side changes nothing about the pregnancy. What matters is the week number, which is the same everywhere.

How likely is my baby to arrive on the due date itself?

Few babies do. The figure usually quoted is around 4 percent, and most births happen within a couple of weeks on either side of the estimate. Treat the due date as the center of a window rather than an appointment, which is also why many clinicians now prefer the phrase estimated due date.

What counts as full term?

Obstetric practice now splits the end of pregnancy into bands rather than one line. Early term runs from 37 weeks, full term from 39 weeks, late term from 41 weeks and post term from 42 weeks. Birth before 37 weeks is preterm. This calculator marks the 37 week point so you can see when the term window opens.

Are the weekly baby size comparisons accurate?

They are averages meant to give you a mental picture, not a measurement of your baby. Healthy babies vary widely at every week, and the switch around week 20 from crown to rump length to crown to heel length makes the numbers jump. Only a scan can tell you how your baby is growing.

Sources

  • Naegele’s rule, the 280 day count from the first day of the last menstrual period, is the standard method described in obstetric textbooks and used by clinics worldwide.
  • The American College of Obstetricians and Gynecologists publishes a committee opinion on methods for estimating the due date. It recommends that the earliest accurate ultrasound be used to establish or revise the date, gives week by week thresholds for when a scan should override a period based date, and advises that once the date is set it should not be changed again. acog.org
  • The term definitions used here, early term from 37 weeks, full term from 39 weeks, late term from 41 weeks and post term from 42 weeks, come from the same body of obstetric guidance.
  • Weekly length and weight averages are compiled from general pregnancy reference sources and are approximate. Individual growth varies widely.

Related calculators

Once the baby arrives, the age calculator gives an exact age in years, months and days, which is handy for the first year of appointments. For tracking your own numbers during and after pregnancy, the BMI calculator shows where a weight sits on the standard scale, though BMI is not a useful measure during pregnancy itself.

Related tools

Privacy: every calculation runs in your browser. The date you enter is never sent anywhere, never stored and never logged. Nothing about your pregnancy leaves this page.