Last updated September 28, 2026. Methods follow published obstetric guidance; sources are listed at the end.
Your due date is the day your pregnancy reaches 40 weeks. It is an estimate, not a deadline, and it is built from a simple calendar rule that doctors refined with ultrasound research over many decades. This guide explains exactly how the number is worked out, which method suits your situation, how reliable the result is, and what to do with your dates once you have them.
How a due date is calculated
Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you conceived. That can feel odd, because you were not pregnant during those first two weeks. The reason is practical: most people can remember when their last period began, but very few know the exact day an egg was fertilized. Counting from the LMP gives everyone the same starting line.
The standard rule adds 280 days, which is 40 weeks, to the first day of the LMP. Doctors have used a shortcut for this for generations, known as Naegele's rule: take the first day of your last period, subtract three months, and add seven days. If your last period began on 1 January, for example, subtracting three months lands on 1 October, and adding seven days gives an estimated due date of 8 October.
The 280-day rule rests on two assumptions. The first is that your cycle is 28 days long. The second is that you ovulate on day 14, so conception happens about two weeks after the LMP. When both are true, the baby is about 38 weeks old at the due date, counted from conception, and 40 weeks old counted from the LMP. When your cycle is shorter or longer than average, the simple rule can drift by several days, which is why a good calculator lets you enter your cycle length.
The result is the date on which labor is expected to begin on its own. The word "expected" matters. Babies are ready to be born across a window of several weeks, and only a small share arrive on the exact predicted day.
Five ways to calculate your due date
The calculator above offers five starting points. Each one leads to the same 40-week timeline but uses a different piece of information. Choose the method that matches the most reliable date you have.
Last menstrual period (LMP)
This is the most common method and the one most people start with. You enter the first day of your last period, which is the first day of real bleeding rather than light spotting, and the calculator adds 280 days. It works best when your cycles are regular and you are confident about the date. It is less reliable if your periods are irregular, if you conceived soon after stopping hormonal contraception, or if you were breastfeeding and not having periods.
Adjusting for your cycle length
Ovulation does not always happen on day 14. Someone with a 35-day cycle typically ovulates around day 21, so conception happens a week later than the textbook assumption. The usual correction is to add or subtract the difference between your cycle length and 28 days. A 35-day cycle moves the due date seven days later, and a 24-day cycle moves it four days earlier. Cycle length is counted from the first day of one period to the first day of the next. If your cycle varies a lot from month to month, the adjustment is only a rough guide and an early ultrasound is a better anchor.
Conception date
If you tracked ovulation with test strips, basal body temperature, or fertility monitoring, or you know the date of a single act of intercourse, you can date the pregnancy from conception. The calculator adds 266 days, which is 38 weeks, because that is the average time from fertilization to a 40-week gestational age. Keep in mind that sperm can survive for up to five days in the reproductive tract while an egg survives only 12 to 24 hours, so the day of intercourse and the day of fertilization are not always the same.
IVF or embryo transfer date
After in vitro fertilization the age of the embryo is known exactly, which makes this the most precise method available. The calculator subtracts the embryo's age from the transfer date to find the equivalent conception date and then adds 266 days. In practice, a day-3 embryo transfer means the due date is 263 days after the transfer, a day-5 transfer means 261 days, and a day-6 transfer means 260 days. The same logic applies to frozen embryo transfers, using the age the embryo was when it was frozen.
Ultrasound dating
An ultrasound measures the size of the embryo or baby and converts it to a gestational age. In the first trimester the measurement used is the crown-rump length, the distance from the top of the head to the bottom of the torso, and it is the most accurate biometric method available, particularly up to 13 weeks and 6 days. The calculator takes the scan date and the gestational age on that day, works backwards to a calculated LMP, and then adds 280 days. Enter the gestational age exactly as it appears on your report, for example 8 weeks and 3 days.
A due date you already know
If your provider has already given you a due date, you can enter it to see your current gestational age, trimester, and the dates of upcoming milestones. The calculator works backwards from it: the calculated LMP is 280 days earlier and the estimated conception date is 266 days earlier.
Due date examples
These worked examples show how the same arithmetic plays out for different starting information. You can reproduce any of them in the calculator to check the result.
| Starting information | Rule applied | Estimated due date |
|---|---|---|
| LMP 10 March 2026, 28-day cycle | LMP + 280 days | 15 December 2026 |
| LMP 10 March 2026, 35-day cycle | LMP + 7 days, then + 280 days | 22 December 2026 |
| LMP 10 March 2026, 24-day cycle | LMP − 4 days, then + 280 days | 11 December 2026 |
| Conception 1 June 2026 | Conception + 266 days | 22 February 2027 |
| Day-5 embryo transfer 20 May 2026 | Transfer + 261 days | 5 February 2027 |
| Scan on 15 April 2026 showing 8 weeks 3 days | Scan date − 59 days = LMP, then + 280 days | 22 November 2026 |
Notice how much the cycle length changes the answer in the first three rows. The same last period produces due dates eleven days apart, which is one reason many providers confirm the date with an early scan.
Gestational age and fetal age
Gestational age is the number of weeks and days since the first day of the LMP. It is the number your doctor, your midwife, your ultrasound report, and every pregnancy app use. Fetal age, sometimes called conceptional or embryonic age, counts from fertilization and is usually about two weeks less. When you read that a baby is "12 weeks", it almost always means 12 weeks of gestational age, which is about 10 weeks after conception.
Gestational age is written as weeks plus days. A pregnancy at 12 weeks and 3 days is often written 12w3d or 12+3, and it means the 13th week of pregnancy has begun. That is a common source of confusion: someone who is 12 weeks and 3 days is in the thirteenth week but has completed only 12 weeks. Most fetal development tables use completed weeks, so 12w3d belongs with "week 12".
This is also why pregnancy is described as lasting about ten months rather than nine. Forty weeks equals 280 days, and because a calendar month averages a little over four weeks, 40 weeks is closer to nine months and one week from conception, or ten lunar months of 28 days counted from the LMP.
The three trimesters
Clinicians divide pregnancy into three trimesters. The boundaries are set by gestational age, so your calculator result tells you exactly which one you are in.
First trimester: weeks 1 to 13 and 6 days
Implantation happens in the first weeks after conception, and by the time a period is missed the pregnancy is usually about four weeks along. Through the first trimester the embryo forms the basic structure of every major organ. Many people notice nausea, tiredness, breast tenderness, and frequent urination. A heartbeat can often be seen on a vaginal ultrasound at around six to seven weeks. This is the trimester in which the risk of miscarriage is highest, and it declines steadily once a scan shows a healthy heartbeat.
Second trimester: weeks 14 to 27 and 6 days
Many people feel better in the second trimester as early symptoms ease. The baby grows quickly, and first movements, sometimes called quickening, are usually felt between about 16 and 25 weeks, often a little later in a first pregnancy. The mid-pregnancy anatomy ultrasound is normally scheduled between 18 and 22 weeks and checks the baby's organs, growth, and the position of the placenta.
Third trimester: week 28 until birth
The baby gains most of its birth weight in the last trimester, and the lungs and brain continue to mature. Common experiences include backache, heartburn, swollen ankles, and disturbed sleep. Prenatal visits become more frequent, and this is the time to finish practical preparations, learn the signs of labor, and confirm your birth plan.
Key prenatal dates to expect
Once you know your due date, most of the pregnancy calendar falls into place. The table below shows the timing that is typical in many countries. Your own schedule may differ depending on your health, your provider, and local guidelines, so treat it as a planning aid and follow your care team's advice.
| Milestone | Typical timing | What it is for |
|---|---|---|
| First prenatal visit | 8 to 10 weeks | Health history, blood tests, and confirming the due date |
| Nuchal translucency scan and first-trimester screening | 11 weeks to 13 weeks 6 days | Screening for certain chromosomal conditions |
| Anatomy ultrasound | 18 to 22 weeks | Detailed check of the baby's organs and growth |
| Glucose screening | 24 to 28 weeks | Testing for gestational diabetes |
| Tdap vaccine | 27 to 36 weeks | Protects the newborn against whooping cough |
| Group B strep test | 36 weeks to 37 weeks 6 days | Checks for a common bacterium so treatment can be given in labor |
| Full term begins | 39 weeks | Baby is considered fully developed for birth |
After you press the calculate button, the results section lists the calendar date for each of these windows so that you can put them in your diary.
How accurate is a due date?
Only about 5 percent of babies, roughly one in twenty, are born on their predicted due date. A healthy pregnancy can end naturally at any point across several weeks, and most births happen within about two weeks either side of the estimate. It is more helpful to think of the date as the centre of a window than as an appointment.
Medical definitions reflect this window. The whole stretch from 37 to 42 weeks used to be called simply "term", but research showed that outcomes differ across it, so the labels were refined.
| Term | Gestational age at birth |
|---|---|
| Preterm | Before 37 weeks 0 days |
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Post-term | 42 weeks 0 days and beyond |
Accuracy also depends on the method. An LMP-based date is only as good as your memory and your cycle regularity. IVF dates are extremely reliable because the embryo's age is known. Early ultrasound sits in between: it is the best tool when the LMP is uncertain, but individual babies vary in size, so a scan can be off by several days.
When an ultrasound date replaces the LMP date
At your first scan the sonographer measures the baby and compares the result with the date you calculated from your period. If the two agree closely, your LMP date stands. If they differ by more than a set threshold, guidance from obstetric bodies such as the American College of Obstetricians and Gynecologists recommends using the ultrasound date instead. The threshold widens as pregnancy advances because babies naturally vary more in size the later you measure them.
| Gestational age at first scan | Change the due date if the difference is more than |
|---|---|
| Before 9 weeks 0 days | 5 days |
| 9 weeks 0 days to 15 weeks 6 days | 7 days |
| 16 weeks 0 days to 21 weeks 6 days | 10 days |
| 22 weeks 0 days to 27 weeks 6 days | 14 days |
| 28 weeks 0 days and later | 21 days |
An important exception is pregnancy after IVF, where the embryo's age is known. In that case the transfer date is normally the reference, and scan measurements are not used to override it in the same way.
Once a due date is set on the strength of an early scan, it is normally not changed again, even if later scans suggest the baby is bigger or smaller than expected. A baby measuring small or large later in pregnancy may reflect genuine growth patterns and is followed up on its own merits rather than by moving the date.
Can your due date change?
Yes, and it is perfectly normal. The most common reason is a first-trimester ultrasound that disagrees with the LMP date by more than the threshold above. It also happens when a person's cycle is longer or shorter than average, or when the last period was misremembered. A change of a few days does not mean anything is wrong with the pregnancy. It means the estimate has become more accurate.
If your date changes, your gestational age changes with it. Your baby has not grown faster or slower than expected. Only the calendar reference has moved, and all your milestone dates move along with it. It is worth updating any apps, work plans, and childbirth class bookings when this happens.
Twins and other multiples
A twin or triplet pregnancy is dated in exactly the same way as a single baby, using the LMP, conception, IVF, or ultrasound date. The difference is what happens next. Multiples tend to arrive earlier, so the due date is used more as a reference for monitoring than as a realistic delivery date. Delivery for a healthy twin pregnancy is often planned before 40 weeks, and the exact timing depends on whether the babies share a placenta and on your own health.
| Pregnancy | Average gestational age at delivery |
|---|---|
| Single baby | About 39 weeks |
| Twins | About 35 weeks |
| Triplets | About 32 weeks |
| Quadruplets | About 30 weeks |
These are averages seen in large studies and describe how often labor begins or is induced early. They are not a target. If you are expecting more than one baby, expect a closer schedule of scans and visits, and ask your provider which milestones apply to you.
Irregular cycles and uncertain dates
Many people cannot give a confident LMP. Irregular cycles, polycystic ovary syndrome, recent hormonal contraception, breastfeeding, and simply not tracking periods are all common reasons. In these situations an early ultrasound is the recommended way to establish the due date, because it does not depend on memory or on the timing of ovulation.
If you can only estimate your last period, enter your best guess in the calculator and treat the result as provisional. If you conceived while breastfeeding or shortly after stopping birth control, say so at your first appointment so that the scan can be interpreted with that in mind. If you have irregular cycles and used ovulation tracking, the conception method is often more accurate than the LMP method.
Whatever the situation, try to book your first prenatal visit and dating scan early. A confirmed date makes every later decision about screening, growth checks, and timing of delivery more reliable.
What to do once you know your due date
A due date is a starting point for planning. These steps are worth taking in the first weeks.
- Book prenatal care. Contact a doctor or midwife as soon as you have a positive test. Many practices see new patients around 8 weeks, and earlier if you have a medical condition or a past pregnancy complication.
- Start or continue folic acid. A daily supplement containing 400 micrograms of folic acid is widely recommended before conception and through early pregnancy to reduce the risk of neural tube defects. Ask your provider if you need a higher dose.
- Review your medicines. Check every prescription, over-the-counter product, and herbal supplement with a pharmacist or your provider. Do not stop a prescribed medicine without advice.
- Avoid alcohol, tobacco, and recreational drugs. No amount of alcohol has been shown to be safe in pregnancy. Ask for support if you find it hard to stop.
- Adjust your diet. Eat a varied diet, cook meat and eggs thoroughly, avoid unpasteurized dairy, limit caffeine to about 200 milligrams a day, and follow local guidance on fish and mercury.
- Stay active. Unless you have been told otherwise, regular moderate exercise such as walking or swimming is generally encouraged.
- Ask about vaccines. Vaccines such as influenza and Tdap are commonly recommended during pregnancy, and other vaccines may be offered depending on the season and where you live.
- Plan work and money. Count back from your due date to plan leave, and remember that babies often arrive a little early or late.
- Look into childbirth education. Classes fill up quickly, and most are best taken in the late second or early third trimester.
When to call your doctor or seek urgent care
Most pregnancies progress without serious problems, but some symptoms need prompt attention whatever week you are in. Contact your provider or local emergency service straight away if you have:
- Vaginal bleeding, or fluid leaking from the vagina
- Severe or persistent abdominal pain, or regular painful contractions before 37 weeks
- A severe headache, changes in vision, or sudden swelling of the face and hands, which can be signs of high blood pressure conditions such as preeclampsia
- Fever, or pain or burning when you pass urine
- Vomiting so severe that you cannot keep food or fluids down
- A clear drop in your baby's movements once you are used to feeling them
- Chest pain, difficulty breathing, fainting, or a seizure, which need emergency care at once
Trust your instincts. If something feels wrong, calling is always appropriate, and you should never worry about being a nuisance.
Common due date myths
"My baby will arrive on the due date"
Very few do. Being a few days early or a week late is completely ordinary, so avoid telling everyone your exact date and building up pressure for that one day.
"First babies are always late"
Some studies suggest that first pregnancies last slightly longer on average, but the difference is measured in days and is far smaller than the natural variation between individuals. There is no rule that a first baby will be late.
"Pregnancy lasts nine months"
Forty weeks is a little more than nine calendar months from conception and about ten lunar months from the LMP. Counting in weeks avoids the confusion.
"A later scan can correct my date"
Dating is most accurate early. Later scans measure the baby's size, which varies from one healthy baby to another, so they are used to monitor growth rather than to redate the pregnancy.
"The date tells me when I must deliver"
It does not. If labor has not begun naturally, your provider will discuss options as the pregnancy passes 41 weeks, and many recommend delivery by 42 weeks, but the plan is based on your health and your baby's wellbeing at the time.
About this calculator
This tool applies standard obstetric arithmetic in your browser and does not send your dates to a server. It uses the 280-day rule for the LMP and known due date methods, 266 days for conception, and embryo-age adjustments for IVF. The LMP method includes the cycle length correction described above. Every result is an estimate, and it cannot take account of your medical history, your ultrasound findings, or anything else your care team knows about you. For decisions about testing, timing of delivery, and your health, rely on your doctor or midwife.
Sources and further reading
- American College of Obstetricians and Gynecologists, Committee Opinion No. 700: Methods for Estimating the Due Date, 2017. PubMed record
- Butt K, Lim K. Guideline No. 388: Determination of Gestational Age by Ultrasound. Journal of Obstetrics and Gynaecology Canada, 2019. PubMed record
- Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates. American Journal of Obstetrics and Gynecology, 1996. PubMed record
- American College of Obstetricians and Gynecologists, Committee Opinion No. 579: Definition of Term Pregnancy, 2013.
Medical disclaimer. This page is for general education and is not medical advice, diagnosis, or treatment. Always speak to a qualified healthcare professional about your pregnancy.
Frequently asked questions
How is my due date calculated?
Doctors add 280 days, which is 40 weeks, to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the date is adjusted by the difference from 28 days, and an early ultrasound can confirm it.
How do I calculate my due date from the conception date?
Add 266 days, or 38 weeks, to the date of conception. Because fertilization can happen up to five days after intercourse, ovulation tracking gives a more reliable conception date than the day of intercourse.
How is the due date worked out after IVF?
The embryo age is subtracted from the transfer date to find the equivalent conception date, then 266 days are added. That means 263 days after a day-3 transfer, 261 days after a day-5 transfer, and 260 days after a day-6 transfer.
How accurate is an estimated due date?
It is a useful estimate, but only about 5 percent of babies are born on the exact day. Most arrive between 37 and 42 weeks. Early ultrasound and IVF dates are generally more precise than dates based on memory of the last period.
Can my due date change?
Yes. If your first-trimester ultrasound differs from the LMP date by more than the accepted threshold, your provider will usually switch to the ultrasound date. This is routine and does not signal a problem.
What if I do not know the date of my last period?
Use the ultrasound option and enter the scan date with the gestational age from your report. You can also use a known conception date, an IVF transfer date, or a due date your provider has already given you.
How many weeks pregnant am I today?
Choose your method, enter your dates, and the calculator shows your gestational age for today. You can also pick any other date to see how far along you will be, or were, on that day.
When is my baby considered full term?
Full term is 39 weeks 0 days to 40 weeks 6 days. Early term is 37 weeks 0 days to 38 weeks 6 days, late term is 41 weeks, and post-term starts at 42 weeks.
Does this calculator replace medical advice?
No. It gives a general estimate using standard formulas. Your doctor or midwife can confirm your date with an examination or scan and advise you on care that suits your health.