Health · Pregnancy
Due Date Calculator
Estimate your pregnancy due date with Naegele's rule. Enter the first day of your last period and your average cycle length to see your estimated due date, current gestational week and trimester — free and no sign-up.
How the due date calculator works
This tool uses Naegele's rule, the standard method used by clinicians worldwide. Your estimated due date (EDD) is calculated from the first day of your last menstrual period (LMP):
- EDD = LMP + 280 days (40 weeks), the typical length of pregnancy measured from your last period.
- Cycle adjustment: if your average cycle is not 28 days, the EDD shifts by (cycle length − 28) days. A longer cycle means later ovulation and a slightly later due date.
- Estimated conception: ovulation usually happens about (cycle length − 14) days after your LMP, which is the approximate conception date.
- Gestational age: the number of weeks since your LMP — this is how pregnancy is counted, even though conception happens about two weeks later.
Trimesters by gestational week
| Trimester | Gestational weeks |
|---|---|
| First trimester | Weeks 1 – 13 |
| Second trimester | Weeks 14 – 27 |
| Third trimester | Week 28 – birth |
Things to keep in mind
Your due date is an estimate, not a deadline
Only about 1 in 20 babies arrive exactly on their estimated due date. Most births happen within two weeks before or after. A full-term pregnancy is considered anywhere from 37 to 42 weeks.
Ultrasound dating is more precise
If your cycles are irregular or you are unsure of your LMP, an early ultrasound (dating scan) is generally the most accurate way to estimate your due date. Your provider may adjust the date based on measurements.
Cycle length really does shift the date
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycles run long or short, entering your real average gives a more personalized estimate than the textbook 28-day default.
Frequently asked questions
How is my due date calculated?
It uses Naegele's rule: 280 days (40 weeks) added to the first day of your last period, then adjusted by your cycle length minus 28 days.
Why does my cycle length matter?
A 28-day cycle assumes ovulation on day 14. A longer cycle means later ovulation, so the due date shifts later by roughly the same number of days your cycle differs from 28.
How accurate is the estimated due date?
Only about 5% of babies are born on their exact due date. It's a reference point — most births land within two weeks either side. An early ultrasound confirms it best.
What are the trimesters?
Measured from your LMP: first is weeks 1–13, second is weeks 14–27, third is week 28 through birth.
Is this a substitute for medical advice?
No. It's an estimate only and not medical advice. Always confirm your due date and any concerns with your OB/GYN or healthcare provider.
Where Naegele's rule comes from
The 280-day method this calculator uses is named after Franz Karl Naegele, the German obstetrician who popularized it in the early nineteenth century. The rule estimates the due date by taking the first day of the last menstrual period (LMP), counting back three months, and adding seven days and one year. Counting forward, that is mathematically the same as adding 280 days — exactly 40 weeks. It remains the standard starting point used by clinicians worldwide because it needs only one easily remembered piece of information: the date your last period began.
The convention has one quirk worth understanding. Pregnancy is dated from the first day of your last period, not from conception. Yet conception (fertilization) does not happen until roughly two weeks later, around ovulation. That means at the moment you are "4 weeks pregnant" by this calendar, the embryo has only existed for about two weeks. This is why a pregnancy is described as 40 weeks even though the developing baby is actually present for closer to 38 weeks. Dating from the LMP is used because almost everyone knows the date their period started, while the exact moment of conception is rarely known.
LMP dating versus conception dating
There are two ways to anchor a due date, and they answer slightly different questions.
- LMP method (used here): EDD = first day of last period + 280 days, adjusted for cycle length. Best when you know your period date but not your conception date, which is the usual situation.
- Conception method: EDD = date of conception + 266 days (38 weeks). Useful if you know the date of ovulation or fertilization — for example after IVF or carefully tracked ovulation. The 14-day gap between the two methods is the same gap as the difference between 280 and 266 days.
This tool also estimates a conception date for you, calculated as roughly (cycle length − 14) days after your LMP. For a textbook 28-day cycle that places ovulation near day 14; for a 32-day cycle it shifts to about day 18, which is exactly why entering your real average cycle length produces a more personalized result than leaving it at the default 28.
How pregnancy weeks and trimesters are counted
Gestational age is expressed as completed weeks plus days — for example, "27 weeks and 4 days" is written 27+4. The week count is measured from the first day of your LMP, which is why a person can be told they are "6 weeks pregnant" only about four weeks after conception. Pregnancy is then grouped into three trimesters, each covering a distinct stage of development:
| Stage | Gestational weeks | What is happening |
|---|---|---|
| First trimester | Weeks 1 – 13 | Major organs and structures form |
| Second trimester | Weeks 14 – 27 | Growth, movement, often the most comfortable phase |
| Third trimester | Weeks 28 – 40+ | Rapid growth and preparation for birth |
Clinicians also use specific labels for when a baby arrives relative to the due date. A delivery before 37 weeks is preterm; 37 to under 39 weeks is early term; 39 to under 41 weeks is full term; 41 to under 42 weeks is late term; and 42 weeks or beyond is post term. These categories, defined by the American College of Obstetricians and Gynecologists, matter because outcomes are generally best in the full-term window.
How accurate is a due date?
It is helpful to think of the due date as the center of a range rather than a fixed appointment. Only about 1 in 20 babies — roughly 5% — actually arrives on the calculated date. The large majority are born within the two weeks on either side, which is why "full term" spans several weeks rather than a single day. A first pregnancy tends to run a little longer than later ones, and individual factors can shift the date in either direction.
The single biggest source of inaccuracy in the LMP method is uncertainty about the period date itself, or having irregular cycles where ovulation does not reliably occur on day 14. For this reason an early ultrasound dating scan, usually done in the first trimester, is generally considered the most reliable way to confirm a due date, and your provider may revise the estimate based on the baby's measurements. Treat the number from any calculator, including this one, as a well-reasoned starting estimate to be confirmed clinically.
Common misconceptions
"My due date is when the baby will be born." It is a statistical midpoint, not a deadline. Most babies arrive in the surrounding weeks, not on the exact date.
"I'm 8 weeks pregnant, so conception was 8 weeks ago." Because weeks are counted from your last period, conception was about two weeks more recent than the week number suggests.
"Cycle length doesn't change anything." It does. A cycle longer than 28 days pushes ovulation — and therefore the due date — later, while a shorter cycle pulls it earlier.
Tips for using the estimate well
- Enter your real average cycle length rather than the default 28 if you know it; it can move the estimate by several days.
- If you are unsure of your LMP or your cycles are irregular, rely on an ultrasound dating scan and treat this figure as approximate.
- Use the gestational week and trimester output to follow week-by-week pregnancy guides, but confirm milestones with your provider.
- The whole calculation runs in your browser, so nothing about your pregnancy is stored or transmitted.
More questions
Can I use this if I know my conception date instead of my period?
This tool is built around the last menstrual period. If you know your conception or ovulation date, a rough due date is that date plus about 266 days (38 weeks). Confirm with your provider, especially after IVF where dating is precise.
What does "40 weeks" really mean if conception was later?
Pregnancy is dated from the first day of your last period by convention, about two weeks before conception. So the 40-week count includes roughly two weeks before the baby was actually conceived.
Why does an ultrasound sometimes give a different date?
An early ultrasound measures the baby directly and does not depend on knowing your exact period date or assuming a 28-day cycle, so it is generally more accurate. Providers often update the due date based on it.
What is the difference between early term, full term, and post term?
Early term is 37 to under 39 weeks, full term is 39 to under 41 weeks, and post term is 42 weeks or beyond. These labels, defined by ACOG, describe when delivery happens relative to 40 weeks.
Does a longer cycle change my due date?
Yes. The calculator adds (cycle length − 28) days to the estimate, because a longer cycle usually means later ovulation and conception, pushing the due date back.
Is my information saved anywhere?
No. The calculation happens entirely in your browser. Your dates are never sent to a server or stored.