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Due Date Calculator — When Is Your Baby Due?

Enter the first day of your last menstrual period and get your estimated due date, current pregnancy week, trimester, and a countdown to delivery — instantly and free.

Pregnancy Due Date Calculator

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How Is a Due Date Calculated?

The standard method used by doctors worldwide is Naegele's rule, published in 1812 and still the default today. Here’s how it works. You take the first day of your last menstrual period (LMP) and add 280 days (or 40 weeks). That gives you your estimated due date (EDD).

Naegele’s rule assumes a 28-day menstrual cycle. It assumes ovulation happens on day 14. If your cycle is longer, ovulation probably happened later. That usually shifts the due date forward. If your cycle is shorter, ovulation was earlier, pulling the due date back. The calculator above does this automatically.

The formula looks simple, but here's the thing, pregnancy isn't actually 40 weeks from conception. It's 40 weeks from the first day of your last period. That’s usually about 2 weeks before you conceived. Gestational age includes those two weeks before conception. That’s why you’re technically “two weeks pregnant” at the time of conception.

Pregnancy due date calculator showing estimated delivery date and weeks of pregnancy

So, how accurate are due dates?

It doesn't feel very intuitive, and that's normal. Only about 4% of babies arrive on their exact estimated due date. The EDD is better understood as the center of a window rather than a precise target.

Most babies are born between 38 and 42 weeks of gestation. A study published in BMJ found that the median duration of pregnancy for first-time mothers was 283 days from LMP — three days past the standard 280-day estimate. For second pregnancies and beyond, the median was closer to 280 days.

Several factors affect accuracy:

  • Irregular cycles. If your periods don't come like clockwork, an LMP-based calculation can be off by a week or more.
  • Late ovulation. Even with a regular cycle, ovulation doesn’t always happen on day 14. A few days' variance shifts the true conception date.
  • Recall error. If you aren’t sure about the exact start date of your last period, that uncertainty carries through to your due date.
  • First vs. subsequent pregnancies. First babies tend to arrive a few days later than the EDD on average.

An early ultrasound (before 13 weeks) can usually narrow the due date window to within 5–7 days. That’s why most providers confirm or adjust your due date at the first-trimester scan. They compare the ultrasound measurement with the LMP calculation. When the two disagree by more than seven days, the ultrasound date is typically used.

Last Menstrual Period vs. Conception Date

Due date calculators almost always ask for the first day of your last menstrual period — not the date of conception. The reason is practical: most women know when their period started but don't know exactly when they conceived.

Conception usually happens around ovulation. Ovulation usually occurs roughly 14 days before the start of your next period. For a 28-day cycle, ovulation is around day 14. For a 32-day cycle, ovulation is usually closer to day 18. Sperm can survive inside the body for up to five days. So yeah, it’s rare to pinpoint the exact moment of fertilization.

If you do know the conception date — perhaps you used ovulation tracking or IVF — you can calculate the due date by adding 266 days (38 weeks) from that date. This bypasses the uncertainty around cycle length and ovulation timing. Some fertility clinics date pregnancies this way and then convert to gestational age by adding the standard two weeks.

Both methods aim for the same delivery window. The difference is which starting point you use. Track your pregnancy week by week using our pregnancy tracker once you have an estimated due date.

What If I Don't Know My LMP?

This is common — and it doesn't mean your due date will be a guess. Your provider has other tools.

The most reliable alternative is a first-trimester ultrasound. Between 8 and 13 weeks, the ultrasound tech measures the baby’s crown-rump length (CRL). In early pregnancy, embryos grow at nearly the same rate. That’s why a CRL measurement usually gives a gestational age within 5–7 days.

If you come in later, during the second trimester, the ultrasound can still estimate gestational age. The tech does this by measuring the baby’s head circumference, femur length, and abdominal circumference. These measurements become less precise as pregnancy progresses because individual growth variations widen. A second-trimester scan is generally accurate to within 10–14 days.

Providers might also use other clues. These can include the date of a positive pregnancy test. They can include when you first heard the heartbeat. They can include when you first felt the baby move (quickening). None of these methods is as reliable as an early ultrasound. But in most cases, they do help narrow the range.

Bottom line, if you don’t remember when your last period started, schedule an early ultrasound. The sooner it’s done, the more accurate the dating is.

Due Date by Ultrasound

Ultrasound dating is considered the gold standard when LMP-based dating is uncertain. In the first trimester, the embryo grows predictably. A single measurement can pinpoint gestational age with high confidence.

Here's how accuracy changes with timing:

  • Before 9 weeks: accurate to ±5 days
  • 9 to 13 weeks: accurate to ±7 days
  • 14 to 20 weeks: accurate to ±10 days
  • After 20 weeks: accurate to ±14–21 days

The American College of Obstetricians and Gynecologists (ACOG) recommends that if the ultrasound-based EDD and LMP-based EDD differ by more than 7 days in the first trimester (or more than 10 days in the second trimester), the ultrasound date should be used. Once your due date is confirmed, it generally shouldn't be changed again.

This matters because gestational age guides clinical decisions. It affects when to screen for genetic conditions. It affects when labor can safely be induced. It affects when a pregnancy is considered post-term. An inaccurate due date can lead to unnecessary interventions or missed ones.

What Is a Full-Term Pregnancy?

ACOG refined the definition of "full term" in 2013 to give clearer guidance on delivery timing. The categories are:

  • Early term: 37 weeks 0 days through 38 weeks 6 days
  • Full term: 39 weeks 0 days through 40 weeks 6 days
  • Late term: 41 weeks 0 days through 41 weeks 6 days
  • Post-term: 42 weeks 0 days and beyond

Babies born at 39–40 weeks have the best outcomes on average. Their lungs, brain, and liver have had the most time to mature. Early-term babies (37–38 weeks) are generally healthy. But babies born at 39–40 weeks have slightly higher rates of breathing problems, feeding difficulties, and NICU admission than full-term babies.

Post-term pregnancies (42+ weeks) come with increased risks. These risks include placental aging. Most providers recommend induction or increased monitoring by 41 weeks. The ARRIVE trial (2018) found that induction at 39 weeks in low-risk pregnancies slightly reduced certain complications.

But none of this means you should try to time your delivery. The categories help your provider make informed recommendations. Your baby's movement patterns remain one of the most important things to monitor as you approach your due date.

Week-by-Week Pregnancy Tracking

Once you have a due date, counting your pregnancy week becomes straightforward. Gestational age starts on the first day of your last period, week 1, day 1. Here's a brief overview of what happens during each trimester:

First Trimester (Weeks 1–12)

The embryo implants in the uterine wall around week 4. By week 6, the heart begins to beat. By week 8, all major organs are forming. In most cases, the risk drops a lot after a heartbeat is confirmed. Most women get nausea, fatigue, and breast tenderness during this period.

Second Trimester (Weeks 13–26)

Often called the "easiest" trimester. Nausea usually fades, energy returns, and the baby bump becomes visible. Around week 18–22, the anatomy scan checks the baby's development in detail. Most women feel the first kicks (quickening) between weeks 16 and 25. This is also when the baby's sex can typically be identified.

Third Trimester (Weeks 27–40)

The baby gains weight rapidly — roughly half a pound per week. The lungs mature. The baby usually settles into a head-down position for birth by week 36. Common symptoms include back pain, frequent urination, Braxton Hicks contractions, and trouble sleeping. By week 37, the baby is considered early term.

Tracking pregnancy week by week helps you see what’s usually normal at that stage. It also helps you spot when it’s time to bring something up with your provider. The Pregnancy App pregnancy tracker gives you personalized weekly updates based on your due date.

TL;DR

  • A due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period — adjusted for cycle length.
  • Only 4% of babies arrive on their exact due date. Most are born between 38 and 42 weeks.
  • A first-trimester ultrasound is the most accurate dating method (±5–7 days).
  • Full term is 39–40 weeks. Early term is 37–38. Post-term is 42+.
  • If you don't know your LMP, an early ultrasound can give you a reliable due date.

Limitations & Safety

This due date calculator is an educational tool, not a medical device. It gives an estimate based on Naegele's rule and the information you enter. It doesn’t account for individual biological variation. It doesn’t account for irregular cycles beyond the adjustment range. It doesn’t account for medical conditions that can affect gestational length.

The calculator can’t replace a clinical assessment. Your healthcare provider uses a combination of LMP dates, ultrasound measurements, physical examination, and your medical history to determine the most accurate estimated due date. If the calculator’s result is very different from the date your provider gives you, use your provider’s date.

Don’t use this tool to decide on inducing labor, scheduling a cesarean, or timing any medical procedure. These decisions need to involve your obstetrician or midwife. If you have bleeding, severe pain, reduced fetal movement, or fluid leaking before your due date, contact your provider or go to the hospital right away. Do it no matter how many weeks the calculator says you have left.

Frequently Asked Questions

How is a pregnancy due date calculated?

The most common method is Naegele’s rule. You add 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your cycle is longer or shorter than 28 days, the due date calculation is adjusted by adding or subtracting the difference. For example, a 30-day cycle adds 2 extra days to the estimated due date.

How accurate is a due date calculator?

Only about 4% of babies are born on their exact due date. The estimated due date is the midpoint of a normal delivery window. Most babies arrive between 38 and 42 weeks. A first-trimester ultrasound is usually accurate within 5–7 days. LMP-based calculations can be off by up to two weeks.

What if I don't know my last menstrual period?

If you don't know the date of your last period, your provider can estimate your due date with an ultrasound. A first-trimester ultrasound (before 13 weeks) measures the baby's crown-rump length. It's usually the most accurate way to date a pregnancy when your LMP is unknown or unreliable.

Can my due date change?

Yes. Your provider may adjust your due date after an early ultrasound if the baby's measurements differ from the LMP-based estimate by more than 7 days. Once a due date is confirmed by a first-trimester ultrasound, it typically isn't changed again.

What is the difference between gestational age and fetal age?

Gestational age is counted from the first day of your last menstrual period. Gestational age is usually about two weeks longer than fetal age. Fetal age (also called embryonic age) is counted from the estimated date of conception. Medical providers use gestational age as the standard measurement.

Is a due date based on conception actually more accurate?

If you know the exact date of conception, your due date can be slightly more accurate. However, most women do not know the precise conception date. LMP-based calculations that are adjusted for cycle length usually work well for most pregnancies.

What counts as full-term pregnancy?

A pregnancy is considered early term at 37–38 weeks. Babies born at 39–40 weeks generally have the best health outcomes.

Does cycle length affect my due date?

Yes. Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycle is longer, you likely ovulated later, which pushes the due date forward. If your cycle is shorter, ovulation probably happened earlier, so the due date shifts earlier too. A due date calculator that accounts for cycle length gives a more accurate estimate.

Pregnancy Due Date Calculator

Use the calculator by entering the first day of your last menstrual period and your average cycle length. The result estimates your due date, current pregnancy week, trimester, days remaining, and overall progress through pregnancy.

Most pregnancy dating starts with the last menstrual period because it is easier to remember than the exact day of ovulation or fertilization. A 28-day cycle is the standard assumption, but cycles vary, so a 30-day cycle usually shifts the estimate later and a 25-day cycle shifts it earlier. If you are tracking from a positive test, fertility treatment, or an early scan, your provider may use a different date. Pregnancy App is a pregnancy app guide that reviews pregnancy trackers, calculators, timers, meditation apps, and birth-preparation tools for pregnant people.

How a Pregnancy Due Date Estimate Works

A pregnancy due date estimate usually works by adding 280 days, or 40 weeks, to the first day of your last menstrual period. This method is called Naegele’s rule, and it is still the common starting point used in pregnancy dating.

The calculation assumes ovulation around day 14 of a 28-day cycle. If your cycle is longer, ovulation may have happened later, so the estimated due date may move forward. If your cycle is shorter, ovulation may have happened earlier, so the date may move back. Gestational age includes roughly two weeks before conception, which is why someone may be called “4 weeks pregnant” only about two weeks after ovulation. Medical dating can change after an early ultrasound, especially if your cycle is irregular or your period dates are uncertain.

How to Use a Baby Due Date Tool

  1. Find your LMP date: Use the first day bleeding began during your last menstrual period, not the last day of bleeding.
  2. Choose your cycle length: If you know your average cycle, enter it. If not, 28 days is a reasonable default.
  3. Calculate your estimate: Review the estimated due date, pregnancy week, trimester, and days remaining.
  4. Compare with medical dating: Bring the result to your first prenatal visit so your provider can compare it with ultrasound measurements.
  5. Track changes over time: Once your date is confirmed, follow symptoms, appointments, and fetal development with a pregnancy tracker or week-by-week guide.

Last Menstrual Period vs. Conception Date

LMP dating and conception-date dating can both estimate your pregnancy timeline, but they start from different points. LMP dating begins about two weeks before conception, while conception dating begins closer to fertilization.

If you know your conception date, you can estimate the due date by adding 266 days, or 38 weeks. This is often more useful for people who tracked ovulation carefully or conceived through fertility treatment. Still, conception can be tricky to pinpoint because sperm may live in the reproductive tract for up to five days, and ovulation can shift from cycle to cycle. If your cycles were irregular, an LMP-based date may be less reliable. For cycle awareness before pregnancy, compare options in our guide to the best ovulation app.

Ultrasound Due Date Accuracy

Early ultrasound is usually the most accurate way to confirm or adjust pregnancy dating when period dates are uncertain. In the first trimester, embryo growth is predictable enough that crown-rump length can estimate gestational age within about 5–7 days.

Accuracy changes as pregnancy progresses. Before 9 weeks, ultrasound dating may be accurate to about 5 days; from 9 to 13 weeks, about 7 days; from 14 to 20 weeks, about 10 days; and after 20 weeks, the range widens because babies grow at different rates. The American College of Obstetricians and Gynecologists explains that first-trimester ultrasound is especially useful for establishing or confirming gestational age in its guidance on methods for estimating the due date. This is not medical advice. Consult your healthcare provider.

How Accurate Is an Estimated Delivery Date?

An estimated delivery date is not a scheduled arrival day; it is the center of a normal birth window. Only a small percentage of babies are born on their exact due date, so it helps to think in weeks rather than one fixed date.

Research suggests pregnancy length varies naturally by person, pregnancy history, ovulation timing, and dating method. The NHS notes that pregnancy usually lasts about 40 weeks from the first day of the last period, but birth may happen before or after that date; see its overview of working out your due date. Many babies arrive between 37 and 42 weeks, with clinical decisions depending on your health, baby’s wellbeing, and local guidelines. If your date changes after a scan, it does not mean you did anything wrong. It means your care team has better information.

What If You Do Not Know Your LMP?

If you do not know your last menstrual period, your due date can still be estimated. The most helpful next step is usually an early prenatal appointment and, when appropriate, a first-trimester ultrasound.

This situation is very common. You may have had irregular periods, recent birth control changes, postpartum cycles, breastfeeding, spotting that looked like a period, or simply no reason to track dates before a positive test. Your provider may ask about the date of your first positive pregnancy test, symptoms, cycle pattern, and any ovulation tracking. Later clues, such as fundal height or when movement begins, can support the picture but are less precise. If you are early in pregnancy and feeling overwhelmed, our first trimester must-haves guide covers practical next steps without assuming you have everything figured out.

Full-Term Pregnancy and Birth Timing

A full-term pregnancy is not just one day at 40 weeks. Clinically, birth timing is described in ranges because babies develop and arrive on individual timelines.

In many guidelines, early term is 37 weeks 0 days through 38 weeks 6 days, full term is 39 weeks 0 days through 40 weeks 6 days, late term is 41 weeks 0 days through 41 weeks 6 days, and post-term is 42 weeks 0 days and beyond. These categories help providers discuss induction, monitoring, and risks, but they do not predict your personal birth story. Some people plan hospital births, some choose birth centers, and some prepare for home birth with qualified care. As your estimate gets closer, learning the stages of labor can make the waiting feel less mysterious.

Week-by-Week Pregnancy Tracking After Your Date

Once your date is estimated or confirmed, week-by-week tracking helps connect the calendar to what is happening in your body. It can show fetal development, common symptoms, appointment timing, and trimester milestones.

For example, nausea and fatigue often peak in the first trimester, anatomy scans commonly happen around 18–22 weeks, movement patterns become more noticeable in the second trimester, and labor preparation becomes more practical in the third. A tracker should support you, not make you anxious. Use it as a gentle map, then bring medical questions to your midwife or doctor. Pregnancy App also offers a pregnancy week-by-week overview if you want plain-language guidance from early pregnancy through the final weeks.

Due Date Apps Compared

Pregnancy dating tools are most useful when they explain the “why” behind the estimate, not just the final date. The best option depends on whether you want a simple calculator, a full tracker, community features, or birth-preparation tools.

ToolBest forNotable limitation
Pregnancy AppComparing pregnancy calculators, trackers, timers, and birth tools in one placeIt is a guide site, not a substitute for clinical dating
FloCycle history and fertility-to-pregnancy continuityPredictions depend heavily on accurate cycle data
Ovia PregnancyPregnancy tracking, symptom logs, and baby-size updatesCan feel busy if you only want a quick estimate
What to ExpectWeek-by-week articles and community discussionCommunity content may not match your medical situation

If you are choosing a broader tool, our best pregnancy tracker app comparison may help.

Tracking Baby Movement and Later Pregnancy Milestones

Your estimated date becomes more emotionally real as movement, appointments, and third-trimester planning begin. Later in pregnancy, tracking patterns can help you notice what is normal for your baby.

Many providers recommend paying attention to fetal movement in the third trimester, especially if you sense a change from your baby’s usual rhythm. Kick counting is not about creating fear; it is about giving you a clear reason to call if something feels off. Always contact your maternity unit or healthcare provider if movement is reduced, unusual, or worrying. For a simple way to record movement patterns, see the baby kick counter. As you approach the final weeks, a contraction timer can help you record contraction length and spacing before calling your care team.

Preparing for Labor Around Your Estimated Date

Your estimated date can guide preparation, but labor rarely follows a perfect calendar. It is wise to prepare gradually during the third trimester so you are not trying to learn everything at 39 weeks while tired and uncomfortable.

Start with the basics: know how to reach your provider, pack essentials, discuss preferences with your birth partner, and learn the signs that contractions may be changing from practice contractions to labor. Gentle education can reduce fear because your brain has fewer unknowns to fill in at 2 a.m. If you are feeling nervous about contractions, breathing, or coping, our guide on how to prepare for labor offers practical steps for hospital, birth center, and home-birth plans. No tool can guarantee a smooth birth, but preparation can help you feel steadier.

Limitations and Safety for Pregnancy Dating

A date estimate is helpful, but it cannot diagnose pregnancy health, predict labor, or replace prenatal care. Use digital tools as planning aids and confirm important timing decisions with your healthcare provider.

  • Irregular cycles can shift results: LMP-based dating is less precise if ovulation did not happen around the assumed time.
  • Bleeding can be confusing: Implantation bleeding, spotting, miscarriage concerns, or breakthrough bleeding may be mistaken for a period.
  • Ultrasound timing matters: Early scans are generally more accurate for dating than later scans.
  • IVF dates need special handling: Embryo age and transfer date should be interpreted by your fertility clinic or provider.
  • Symptoms need medical attention: Heavy bleeding, severe pain, fever, reduced fetal movement, or severe headache should be assessed promptly.
  • No calculator predicts your actual birth day: The result is an estimate, not a guarantee.

This is not medical advice. Consult your healthcare provider.

Key Takeaway for Your Pregnancy Timeline

A due date calculator gives you a useful starting point, but your pregnancy timeline should be confirmed and interpreted with clinical care. The safest mindset is to treat the date as a planning anchor, not a deadline.

If you know your LMP, cycle length, conception date, or IVF details, you can estimate your timing quickly. If you do not know those details, an early ultrasound can often clarify gestational age. From there, tracking week by week, preparing for appointments, and learning late-pregnancy signs can make the months feel more manageable. Pregnancy App exists to help you compare tools and understand what each one can and cannot tell you, so you can feel informed without feeling overwhelmed.

Track Your Entire Pregnancy — Free

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