Pregnancy Due Date Calculator

Find your estimated due date, how many weeks pregnant you are today, your current trimester, and the dates of key milestones. Calculate from your last menstrual period or from a known conception date.

How Is a Due Date Calculated?

Pregnancy is dated from the first day of your last menstrual period (LMP), even though conception happens roughly two weeks later. A typical pregnancy lasts 280 days — 40 weeks — from the LMP. This is Naegele’s rule, named after the German obstetrician Franz Karl Naegele, who formalised it in the 1830s. It remains the foundation that doctors’ organisations worldwide still use.

The basic formula is simple:

• From LMP: due date = LMP + 280 days, adjusted by (cycle length − 28 days)
• From conception: due date = conception date + 266 days (38 weeks)

The cycle adjustment matters. A 28-day cycle assumes ovulation on day 14, but if your cycle is 35 days, ovulation happens about a week later, so the due date moves a week later too. The calculator above applies that adjustment automatically and also shows the “LMP-equivalent” date it uses for counting weeks, so the numbers stay consistent.

Trimesters and What They Mean

Doctors also use “term” categories that matter near the end: early term is 37 weeks 0 days to 38 weeks 6 days, full term is 39 weeks 0 days to 40 weeks 6 days, late term is 41 weeks, and postterm is 42 weeks and beyond. Most babies arrive between 39 and 41 weeks — only about 5% are born exactly on their due date, so treat the date as the middle of a window, not an appointment.

The American College of Obstetricians and Gynecologists (ACOG) publishes the definitions clinicians use for pregnancy dating and term, and the World Health Organization’s maternal health pages cover recommended prenatal care throughout pregnancy.

Why Ultrasound Dating Beats LMP Dating

LMP dating assumes regular cycles and accurate recall of the last period — two things that are often shaky. A first-trimester ultrasound (especially crown-rump length measurement between 8 and 13 weeks) dates a pregnancy to within about 5–7 days, and is the most accurate method available. If ultrasound dating differs from LMP dating by more than about 5–7 days in the first trimester, your provider will usually redate the pregnancy to the ultrasound.

Irregular cycles, recent hormonal contraception, and breastfeeding can all make LMP dating unreliable. If any of those apply to you, treat the calculator’s result as a rough estimate and rely on your dating scan.

What to Do After You Know Your Due Date

• Book prenatal care early. Contact your doctor or midwife and schedule your first visit and dating scan (usually 11–14 weeks).
• Start folic acid. 400 micrograms daily is widely recommended before and during early pregnancy, unless your provider advises a different dose.
• Review medications and habits. Check every medicine, supplement, and vitamin with your provider, and avoid alcohol and smoking.
• Plan nutrition and hydration. Pregnancy raises fluid needs — check your daily water intake — and energy needs shift by trimester, which the calorie calculator can help you estimate. For a healthy weight-gain range to discuss with your provider, your pre-pregnancy BMI and ideal weight range are useful reference points.
• Know the milestones. Dating scan, anatomy scan, and glucose screening all land in specific windows; the calculator’s milestone table gives you the dates to discuss with your provider.

Recommended Weight Gain by Pre-Pregnancy BMI

How much weight gain is healthy during pregnancy depends on where you started. The Institute of Medicine guidelines below are the reference most providers use, for a single baby. They are ranges, not targets to hit exactly, and your provider may adjust them for your situation:

Gain is usually small in the first trimester — 0.5 to 2 kg is typical, and some people lose a little with nausea — with most of it coming later as the baby grows. If you are unsure of your starting point, your pre-pregnancy BMI and ideal weight range give you the reference numbers to discuss with your provider. Twin pregnancies have higher recommended gains, roughly 17 to 25 kg for a healthy starting BMI.

Counting Weeks, Months, and Trimesters

Pregnancy counting confuses almost everyone at first, because there are two clocks running. Gestational age counts from the first day of the last period, which is what doctors use and what this calculator reports. Fetal age counts from conception, about two weeks later. When a doctor says you are 10 weeks pregnant, the embryo is roughly 8 weeks old. Both numbers describe the same pregnancy; they just start at different points.

The months question is equally slippery. Forty weeks is not nine calendar months — it is nine months plus about a week, which is why due dates often land in the tenth calendar month counted from the last period. Providers write gestational age as weeks plus days, for example 24w3d, meaning 24 weeks and 3 days. Trimesters divide the 40 weeks into three parts, but the boundaries are conventions rather than biology: the second trimester starts at 14 weeks, the third at 28 weeks, and development flows continuously across all of them.

What If You Do Not Know Your LMP?

Not knowing your last period is common, and it does not leave you stuck. There are three practical paths, in order of reliability. First, if you know your conception date — from fertility treatment, for example — use the conception-date mode on this calculator, which adds 266 days. Second, an early ultrasound is the most accurate dating method available: a first-trimester scan dates a pregnancy to within about 5 to 7 days, and if it disagrees with the period-based date by more than that, providers usually redate to the scan. Third, if you were recently on hormonal contraception, breastfeeding, or have very irregular cycles, tell your provider exactly that — these are routine situations, and they will date the pregnancy by scan rather than by cycle maths.

What matters most is getting a dating scan in the first trimester, ideally between 8 and 13 weeks, when measurements are most precise. After that window, scans become progressively less accurate for dating, because babies start growing at different rates.

Preparing for the Due Window

Since only about 5% of babies arrive on their exact due date, think of it as the middle of a window running roughly from 37 to 42 weeks. Practical preparation beats date-watching: have a hospital bag packed by about 36 weeks, know the route to your birth setting at different times of day, and agree with your provider how and when to call. Labour often starts with regular contractions that grow stronger and closer together, or with waters breaking; a bloody show and backache can precede both.

Call your provider promptly for bleeding, a noticeable reduction in the baby’s movements, severe headache or vision changes, fever, or leaking fluid before 37 weeks. These are the symptoms where waiting is the wrong choice. For everything else, the due window is a period of watchful readiness, not an appointment to be kept — most first babies arrive after the due date rather than before it.

Due Date Methods Compared

Three ways of dating a pregnancy are in common use, and they are not equally reliable. The table below compares them on the same terms, so you can see why providers trust the scan over the calendar — and why the calculator’s result is an estimate to confirm, not a date to set in stone.

The practical rule providers follow is simple: date by scan when the scan disagrees with the period-based date by more than about 5 to 7 days in the first trimester, and stick with the period date when they agree. After the first trimester, dating accuracy falls away quickly, because babies begin growing at different rates — a second-trimester scan can be off by two weeks or more for dating purposes, which is why it is used for anatomy checks rather than re-dating.

Week-by-Week Milestones

The 40 weeks of pregnancy divide into recognisable landmarks, and knowing roughly when each falls helps you understand what each appointment is for. The table below collects the milestones already mentioned on this page into one timeline, counted in gestational weeks from the last menstrual period.

Milestones are windows, not appointments with fixed days. If a scan or test falls a week earlier or later than the table suggests, that is usually normal scheduling, not a problem. Your provider’s dates override any general timeline, including this one.

Prenatal Visit Schedule

A typical low-risk pregnancy follows a rhythm of visits that starts monthly and quickens toward the end. The schedule below is the pattern many providers follow for uncomplicated pregnancies; higher-risk pregnancies, twins, and medical conditions all bring more frequent visits, and your provider may adjust the pattern for your situation.

Between visits, your job is to notice and report: the warning signs listed later on this page are the things that should trigger a call outside the schedule, not something to save for the next appointment. Write questions down as they occur to you and bring the list — providers expect it, and it stops the important questions from being forgotten in the consulting room.

• Bring to every visit: your notes on symptoms, a list of medicines and supplements, and your questions.
• Know your numbers: blood pressure readings, weight trend, and any test results from previous visits.
• Plan the logistics early: transport, childcare for other children, and who to call after hours.

Nutrition and Supplements in Pregnancy

Pregnancy raises the need for several nutrients, but the answer is rarely “eat for two”. Energy needs rise only modestly — roughly an extra snack’s worth in the second trimester and a little more in the third — while the need for specific nutrients rises much more sharply. Food first, supplements to fill gaps, and your provider’s advice over any general list.

Energy and fluid needs shift by trimester: pregnancy raises fluid needs by about 300 ml a day, which your daily water target already accounts for, and the calorie calculator can help you estimate the modest extra energy. Your pre-pregnancy BMI and ideal weight range give you the starting point for the weight-gain discussion with your provider.

Foods to Limit or Avoid

• Alcohol: avoid entirely — no amount has been shown to be safe in pregnancy.
• High-mercury fish: shark, swordfish, king mackerel, and tilefish — choose low-mercury fish instead.
• Raw or undercooked meat, fish, and eggs: a food-poisoning risk that matters more in pregnancy.
• Unpasteurised milk and soft cheeses: a listeria risk — choose pasteurised versions.
• Excess caffeine: keep it moderate and within whatever limit your provider advises.
• Smoking: avoid, and avoid second-hand smoke where possible — discuss support for quitting with your provider.

Exercise in Pregnancy

For uncomplicated pregnancies, regular moderate exercise is recommended by doctors’ organisations worldwide, and it brings real benefits: less back pain, better sleep, better mood, and easier labour recovery. The key word is moderate — you should be able to hold a conversation while exercising — and the key rule is to listen to your body rather than chase targets.

• Good choices: walking, swimming, stationary cycling, and prenatal yoga or pilates classes.
• Keep it regular: short sessions most days beat one exhausting weekly session.
• Stay hydrated and cool: drink before, during, and after exercise, and avoid overheating.
• Avoid contact sports and fall risks: anything with a real chance of a blow to the abdomen or a hard fall.
• Skip lying flat on your back for long periods after the first trimester, and avoid heavy lifting with straining.
• Stop and call your provider for vaginal bleeding, dizziness, chest pain, painful contractions, or fluid leaking.

If you were active before pregnancy, you can usually continue at a similar level with adjustments; if you were inactive, start gently with walking and build up. Either way, clear any new exercise programme with your provider, especially if you have a medical condition, a multiple pregnancy, or any bleeding.

Warning Signs to Call Your Provider

Most pregnancy symptoms are uncomfortable but harmless. A short list is not, and those symptoms should trigger a prompt call rather than a wait until the next appointment. Do not feel foolish for calling: providers would far rather hear about a false alarm early than about a real problem late.

• Vaginal bleeding at any stage, from spotting to heavy bleeding.
• A noticeable reduction in the baby’s movements after movements have become regular.
• Severe headache, vision changes, or sudden swelling of the face and hands.
• Fever, burning on urination, or persistent vomiting with inability to keep fluids down.
• Leaking fluid before 37 weeks, or a gush of fluid at any stage.
• Severe abdominal pain or regular painful contractions before 37 weeks.
• Dizziness, fainting, or chest pain during normal activity.

When to Seek Urgent Care

Heavy bleeding, severe pain, a sudden absence of movements, seizures, or signs of a serious allergic reaction need emergency care, not a routine phone call. If you cannot reach your provider quickly, go to the nearest emergency department and tell them you are pregnant and how many weeks along you are. Bring your notes and your medication list — in an emergency, clear information saves time.

Twins and Multiples

Multiple pregnancies follow the same rules as single pregnancies, with wider margins. Due dates are calculated the same way, but twins more often arrive earlier — many twin births happen before 37 weeks — and the recommended weight gain is higher, because two babies, two placentas, and more fluid all add weight. Monitoring is also closer, with more scans and more frequent visits.

For other starting BMIs, providers set individual targets for twin pregnancies, since the single-baby table does not transfer directly. If you are carrying multiples, treat every general number on this page — including the due date itself — as a rough guide and follow your specialist’s plan instead.

After the Due Date: What Usually Happens

Going past the due date is common, especially with a first baby, and it unfolds in a predictable sequence. Knowing the sequence in advance removes much of the anxiety of those extra days.

1. 40 weeks, the due date itself: nothing special happens — only about 5% of babies arrive on this exact day, and the rest of the window is still ahead.
2. 40 to 41 weeks: monitoring continues as normal, often with extra checks on the baby’s wellbeing.
3. From 41 weeks: your provider will typically monitor you more closely and discuss options, including waiting with surveillance or planning an induction.
4. By 42 weeks: induction is usually recommended, because risks rise beyond this point and the placenta ages.
5. Throughout: keep tracking movements daily and call promptly for any reduction, bleeding, or fluid leaking.

The decision at 41 to 42 weeks is a shared one between you and your provider, weighing your preferences, the baby’s condition, and the monitoring results. There is no prize for waiting longest and no failure in accepting induction — the goal is a healthy birth, by whichever route gets there safely.

Preparing for Labour: A Practical Checklist

Preparation is mostly logistics, and logistics done early leave the final weeks calmer. Work through this list by about 36 weeks, so the due window itself is about waiting rather than rushing.

• Hospital bag packed: documents, clothes for you and the baby, toiletries, phone charger, snacks, and any birth plan.
• Route and backup route known: drive both at different times of day, and know the after-hours entrance.
• Support person briefed: who to call, when to call, and what your preferences are for labour and pain relief.
• Home ready: cot or bassinet set up, nappies and clothes washed, freezer stocked with simple meals.
• Paperwork ready: insurance or registration details, identification, and your provider’s number in every phone.

Frequently Asked Questions

How accurate is a due date calculator?

It is an estimate. LMP-based dates assume a regular cycle and accurate recall; only about 5% of babies arrive on their exact due date. An early ultrasound is the most accurate dating method.

Why is pregnancy counted from my last period, before I conceived?

Because the LMP is a date you can usually identify, while conception day is rarely known exactly. Gestational age therefore runs about two weeks ahead of the baby’s actual age — when a doctor says 10 weeks pregnant, the embryo is roughly 8 weeks old.

My cycle is longer than 28 days. Does that change my due date?

Yes. Longer cycles mean later ovulation, so the due date shifts later by the difference. A 35-day cycle pushes the due date about 7 days later than the standard 280-day calculation.

What if I know my conception date but not my LMP?

Use the conception-date mode: the calculator adds 266 days (38 weeks) to the conception date, which is equivalent to the LMP method when ovulation happened on day 14.

What does it mean if I go past my due date?

Going a few days past is common and usually not a concern. Your provider will typically monitor you more closely from 41 weeks and discuss options if you reach 42 weeks. Always follow your provider’s guidance rather than a calculator.

Can my due date change later in pregnancy?

Yes, but usually only after an early scan. If a first-trimester ultrasound disagrees with the period-based date by more than about 5 to 7 days, your provider will usually redate the pregnancy to the scan. Later scans are less accurate for dating, because babies grow at different rates, so dates rarely change after the first trimester.

Does a 35-day cycle really shift the date a full week?

Yes, if ovulation happens later. A 35-day cycle usually means ovulation around day 21 instead of day 14, so conception happens about a week later than the standard calculation assumes, and the due date moves about a week later too. This calculator applies that adjustment automatically when you enter your cycle length.

Does this calculator work for IVF pregnancies?

Partly. If you know the conception or embryo transfer date, the conception-date mode gives a reasonable estimate, but IVF dating follows its own conventions — transfer day plus the embryo’s age — and providers date IVF pregnancies from the transfer, not from a last period. Confirm the date with your fertility clinic rather than relying on this calculator.

What if my periods are very irregular?

Then period-based dating is unreliable, and the calculator’s result should be treated as a rough estimate only. An early ultrasound is the way to date the pregnancy accurately — ideally between 8 and 13 weeks — and your provider will usually redate to the scan when the two disagree by more than about 5 to 7 days.

Is spotting in early pregnancy normal?

Light spotting is common in early pregnancy and often harmless, but it is also a symptom that should always be reported. Contact your provider the same day for any bleeding, and seek urgent care for heavy bleeding, severe pain, dizziness, or fainting.

Can I travel while pregnant?

Most providers consider travel safe during uncomplicated pregnancies, with the second trimester usually the most comfortable time. Check your provider’s advice, your travel insurance terms, and any airline or carrier rules before booking — many airlines restrict travel in the final weeks — and keep moving, hydrated, and near medical care on long journeys.

Disclaimer: This calculator provides estimates for general information only and is not medical advice. Always confirm your due date and pregnancy care with your doctor or midwife, and seek prompt care for any concerning symptoms.