How does a Due Date Calculator work?
A Due Date Calculator turns one date you already know into an estimate of when your baby is likely to arrive. Doctors and midwives do not count a pregnancy from the day of conception, because that day is rarely known for sure. Instead, pregnancy is counted from the first day of your last menstrual period (LMP), which most people can remember or find in a period-tracking app. A typical pregnancy lasts about 280 days — 40 weeks — from that date, so the calculator adds 280 days to your LMP and shows the result as your estimated due date (often shortened to EDD).
If you know your conception date, or you conceived through IVF and know your embryo transfer date, the calculator works from that date instead and uses the matching number of days. Whichever method you choose, it also works out how far along you are today, which trimester you are in, how many days are left and the key dates of your pregnancy. Everything runs instantly in your browser — nothing you type is stored on our servers.
How is an estimated due date calculated?
The standard method is called Naegele’s rule, named after the German obstetrician who popularised it in the 1800s. It is still the starting point used in clinics today:
Estimated due date = first day of last period + 280 days
or, the same thing counted on a calendar: LMP + 1 year − 3 months + 7 days
Worked example: if your last period started on March 1, adding 280 days gives December 6. Counted the calendar way: March 1 + 1 year = March 1 next year, − 3 months = December 1, + 7 days = December 8 — close, but the two methods can differ by a day or two because months have different lengths. That is why this calculator always counts exact days rather than months.
Naegele’s rule assumes a 28-day cycle with ovulation on day 14. If your cycles are regularly longer or shorter, ovulation usually happens later or earlier, so the calculator shifts the due date by the difference: a 32-day cycle adds 4 days, a 25-day cycle subtracts 3. You can set your average cycle length in the calculator above.
Calculating from the conception date
Conception normally happens around two weeks after the start of the last period, so the gap between conception and the due date is 280 − 14 = 266 days (38 weeks). If you know when you conceived — for example from ovulation tests or because of a single intercourse date — the calculator adds 266 days. Remember that sperm can survive for several days, so the date of intercourse and the date of conception are not always the same; our Conception Date Calculator explains the fertile window in more detail.
Calculating an IVF due date
IVF dating is the most precise of all because the age of the embryo is known exactly. The calculator subtracts the embryo’s age from 266 days and adds the result to the transfer date:
| IVF method | Days added to the date |
|---|---|
| Egg retrieval / insemination date | 266 days |
| Day-3 embryo transfer | 263 days |
| Day-5 blastocyst transfer | 261 days |
| Day-6 blastocyst transfer | 260 days |
For frozen cycles see the FET Due Date Calculator; for fresh cycles the IVF Due Date Calculator has more detail.
Which calculation method should I use?
All three methods give the same kind of answer — an estimated due date at 40 weeks of gestational age — but they start from different information. Use the one that matches what you know most reliably:
| Method | Best when… | Keep in mind |
|---|---|---|
| Last period (LMP) | You know the first day of your last period and your cycles are fairly regular. | Ovulation may not happen exactly on day 14; enter your cycle length if it is not 28 days. |
| Conception date | You tracked ovulation, had a single possible date, or had IUI. | Conception can happen a few days after intercourse. |
| IVF transfer | You conceived through IVF or a frozen embryo transfer. | Choose the embryo’s age at transfer (or at freezing for FET). |
| Ultrasound | A scan has already measured how far along you are. | Early (first-trimester) scans are the most reliable for dating. |
If you have more than one of these dates, try each method — the results are usually within a few days of each other. When they differ by more than about a week, the date your healthcare professional confirms is the one to use. You can also enter your scan result in the Pregnancy Due Date Calculator, which accepts ultrasound dating and lists all your milestone dates.
A practical tip for the LMP method: count from the first day of real bleeding, not spotting, and not the day the period ended. If you cannot remember the exact date, a rough estimate still gives a useful starting point — just expect your due date to be refined at your first scan.
Due date chart
Want a quick answer without using the calculator? Find the month your last period started below. Each row shows the estimated due date for a period that began on the 1st or the 15th of that month, assuming a 28-day cycle. For any other day, simply count forward the same number of days from the due date shown — or use the calculator at the top of this page.
| Last period began in 2026 | If it began on the 1st | If it began on the 15th |
|---|---|---|
| January | October 8, 2026 | October 22, 2026 |
| February | November 8, 2026 | November 22, 2026 |
| March | December 6, 2026 | December 20, 2026 |
| April | January 6, 2027 | January 20, 2027 |
| May | February 5, 2027 | February 19, 2027 |
| June | March 8, 2027 | March 22, 2027 |
| July | April 7, 2027 | April 21, 2027 |
| August | May 8, 2027 | May 22, 2027 |
| September | June 8, 2027 | June 22, 2027 |
| October | July 8, 2027 | July 22, 2027 |
| November | August 8, 2027 | August 22, 2027 |
| December | September 7, 2027 | September 21, 2027 |
How accurate is a due date?
An estimated due date is exactly that — an estimate. Only a small minority of babies, often quoted at around 4 to 5 percent, arrive on the due date itself. Most are born within a few weeks either side of it, and a birth anywhere from 37 to 42 weeks is considered within the normal range. Think of your due date as the middle of a window rather than an appointment.
Several things affect how close the estimate is likely to be:
- Cycle regularity. The LMP method works best when cycles are regular and you are sure of the date. Irregular cycles, recent hormonal contraception or breastfeeding can make ovulation timing less predictable.
- Early ultrasound. A dating scan in the first trimester measures the embryo and is generally the most accurate way to estimate gestational age. If the scan and your LMP disagree by more than a set amount, your provider may change your due date to match the scan. You can enter scan results in our Accurate Due Date Calculator.
- Assisted conception. IVF dates are known precisely, so IVF due dates are usually not changed by later scans.
- Individual variation. First pregnancies, previous pregnancy lengths and many other factors mean every pregnancy has its own timing.
Can my due date change?
Yes. It is common for a due date estimated from the last period to be adjusted after the first ultrasound. Once a due date has been set from an early scan, healthcare professionals generally keep it the same for the rest of the pregnancy rather than changing it after later growth scans, because later measurements reflect the baby’s size more than its age. If you are given a new date, you can enter it in the Reverse Due Date Calculator to see your updated week of pregnancy and conception estimate.
Pregnancy timeline
A 40-week pregnancy is divided into three trimesters. Here is what each one generally involves — for a week-by-week calendar with your own dates, use the Pregnancy Timeline.
First trimester (week 1 to week 13 + 6 days)
Weeks 1 and 2 are counted from the first day of the last period, before conception has happened, which is why “4 weeks pregnant” is usually only about two weeks after conception. Implantation happens in week 3–4, and a home pregnancy test usually turns positive around the time of the missed period. The first trimester is when the major organs begin to form, and it is also when many people have their first prenatal appointment and an early ultrasound. An ultrasound in this trimester is the most precise way to confirm or adjust an estimated due date.
Second trimester (week 14 to week 27 + 6 days)
For many people the second trimester feels the most comfortable. Early nausea often eases, the bump becomes visible and the first movements (“quickening”) are commonly felt somewhere between 16 and 22 weeks. A detailed anatomy scan is typically offered between 18 and 22 weeks, and screening for gestational diabetes is commonly done between 24 and 28 weeks.
Third trimester (week 28 until birth)
The third trimester is about growth and preparation. Prenatal visits usually become more frequent, and your care team will monitor growth, position and wellbeing. A pregnancy is considered early term from 37 weeks, full term from 39 weeks, late term from 41 weeks and post-term from 42 weeks — the estimated due date sits at exactly 40 weeks.
A due date calculator and pregnancy calculator in one
This Due Date Calculator also works as a complete pregnancy calculator. Besides your estimated due date, every result shows how many weeks and days pregnant you are today, your trimester, your approximate month of pregnancy, the days remaining and a progress bar — so you can answer “when is my baby due?” and “how far along am I?” with one calculation. If you want to see what typically happens in your current week as well, open the all-in-one Pregnancy Calculator, which adds a this-week summary and a size comparison for your baby.
A due date pregnancy calculator cannot predict your baby’s sex: that is set at conception and is shown by blood screening (from about 10 weeks) or the anatomy scan (around 18–22 weeks), not by dates. For healthy weight gain targets based on your BMI, see the Pregnancy Weight Gain Calculator.
Gestational age, fetal age and “weeks pregnant”
When you read that someone is “12 weeks pregnant”, that is gestational age, counted from the last period. Fetal age (or conception age) is counted from conception and is roughly two weeks less. Ultrasound reports, prenatal apps and this calculator all use gestational age, so your results will match what your midwife or doctor tells you. If you only want to know how far along you are today, the Pregnancy Week Calculator focuses on exactly that.
Using your due date
Once you have an estimated due date, a few practical steps help:
- Book prenatal care. Contact a healthcare professional early; they will confirm the pregnancy, check your dates and plan your appointments.
- Plan around the window, not the day. Use the full-term range (39 to 40 weeks 6 days) and the wider 37–42 week window when planning leave or travel. The Date Calculator can count working days or add weeks to any date.
- Keep your dates handy. Use the copy, share or print buttons on your result so you can bring the dates to your first appointment.
Planning a different kind of due date? The Dog Due Date Calculator estimates whelping dates, and the Days Until Date counter turns any date into a countdown.