Due Date Calculator
Calculate baby due date from LMP
Typical range: 21-35 days
Due Date & Milestones
Pro Tip
Only 5% of babies are born on their exact due date! The actual delivery can occur 2 weeks before or after.
Privacy & Security
Your pregnancy information is completely private and secure. All calculations are performed locally in your browser - no personal data is transmitted or stored. Your dates remain confidential.
What is a Due Date Calculator?
A due date calculator is an essential prenatal tool that estimates when your baby will be born based on your last menstrual period (LMP), conception date, or ultrasound measurements. Understanding your due date is crucial for proper prenatal care scheduling, monitoring fetal development, planning for maternity leave and birth preparations, and managing expectations throughout pregnancy. The standard pregnancy duration is 40 weeks (280 days) from the first day of your last period, though this includes the approximately two weeks before conception occurs. The calculator uses Naegele's Rule, the medical standard for due date calculation taking the first day of your last menstrual period, adding one year, subtracting three months, and adding seven days. Alternative calculation methods include using your known conception date and adding 266 days, or using ultrasound measurements from the first trimester which are the most accurate dating method. It's important to understand that your due date is an estimate, not a guarantee - only about 5% of babies arrive on their exact due date, while approximately 90% are born between 37-42 weeks. First-time mothers tend to deliver slightly later on average while women who have had babies before tend to deliver slightly earlier. Your healthcare provider will use your due date to create a prenatal care schedule with appropriately-timed tests throughout pregnancy. Whether you're newly pregnant and excited to know when baby will arrive, planning your pregnancy journey, or comparing different dating methods, this calculator provides the essential timeline information every expectant parent needs.
Key Features
LMP-Based Calculation
Calculate due date from the first day of your last menstrual period using Naegele's Rule
Conception Date Method
Determine due date from known conception date for precise tracking
Ultrasound Date Input
Calculate backwards from ultrasound-confirmed due date to determine conception
Gestational Age Display
See your current pregnancy week and days along with due date
Trimester Breakdown
View when each trimester begins and ends throughout your pregnancy
Conception Date Estimation
Estimate when conception occurred based on your due date or LMP
Full-Term Range
See the complete range of normal delivery dates from 37-42 weeks
Countdown Timer
Track days, weeks, and months remaining until your due date
How to Use the Due Date Calculator
Choose Your Input Method
Select whether you want to calculate from your last menstrual period (most common), from a known conception date, or from an ultrasound-confirmed due date.
Enter the Relevant Date
Input the first day of your last period, your conception date, or your ultrasound-determined due date. Be as accurate as possible with the date you enter.
Review Your Due Date
See your estimated due date 40 weeks from your LMP or 38 weeks from conception. Remember this is an estimate - most babies arrive within 2 weeks before or after this date.
Check Additional Information
View your estimated conception date, current gestational age if already pregnant, and which trimester you're in for tracking pregnancy progress.
Share with Healthcare Provider
Bring this information to your first prenatal appointment. Your provider may adjust the due date based on early ultrasound measurements.
Due Date Planning Tips
- Record Your LMP Date: When trying to conceive, note the first day of each menstrual period in an app or calendar to ensure you have an accurate date if you become pregnant.
- Think of It as a Range: Instead of fixating on one date, think of your due date as the center of a 4-week window (38-42 weeks) when baby might arrive.
- Prepare Early for First-Time Mothers: First-time mothers typically deliver later on average, but prepare everything by 36-37 weeks just in case of early delivery.
- Don't Over-Schedule Late Pregnancy: Avoid major commitments, travel, or important events in the final 6-8 weeks to account for potential early delivery or discomfort.
- Get Early Ultrasound for Accuracy: First-trimester ultrasounds provide the most accurate pregnancy dating, more reliable than LMP calculations alone.
- Discuss Induction Timeline: Around 39 weeks, discuss with your provider when they typically recommend induction if you haven't gone into labor spontaneously.
Frequently Asked Questions
How is my due date calculated?
Your due date is calculated using Naegele's Rule, which adds 280 days (40 weeks) to the first day of your last menstrual period. This calculation method has been the medical standard for over 150 years. To use Naegele's Rule, take the first day of your last period, add one year, subtract three months, and add seven days. For example, if your last period started on March 10, you would calculate March 10 next year, then subtract three months to December 10, then add seven days to get December 17 as your estimated due date. This method assumes a regular 28-day menstrual cycle with ovulation occurring around day 14. Alternative methods include conception-date-based calculation by adding 266 days to conception date, ultrasound dating by measuring fetal size in first trimester which is the most accurate method, or IVF transfer date plus 266 days for those who conceived through fertility treatments. Healthcare providers often rely most heavily on early ultrasound dating because fetal measurements are very consistent at this stage. If ultrasound dating differs from LMP dating by more than 5-7 days, most providers adjust to the ultrasound date. Regardless of calculation method, remember that the due date is an estimate and only about 5% of babies arrive exactly on their due date.
What if I have irregular periods?
If you have irregular periods, LMP-based due date calculation is less reliable because the standard calculation assumes a 28-day cycle with ovulation on day 14, which doesn't apply to irregular cycles. With irregular cycles, you might ovulate earlier or later than day 14, and cycle lengths varying month to month mean your LMP doesn't accurately predict when conception occurred. For women with irregular periods, alternative dating methods are more accurate. First, if you were tracking ovulation using ovulation predictor kits, basal body temperature charting, or fertility apps, you can use your known ovulation or conception date to calculate your due date by adding 266 days. Second, early ultrasound dating becomes especially important, with your provider ordering a dating ultrasound ideally between 8-13 weeks that measures crown-rump length to determine gestational age with accuracy of 3-5 days regardless of your cycle regularity. Third, if you have polycystic ovary syndrome or other conditions affecting cycle regularity, discuss this with your healthcare provider at your first appointment so they can rely primarily on ultrasound for dating rather than LMP. Don't worry if you can't provide an accurate LMP date, as healthcare providers are experienced in determining accurate due dates through examination and ultrasound even without knowing exact LMP.
Can my due date change during pregnancy?
Yes, your due date can be adjusted during pregnancy, and this is normal and common, especially if early ultrasound measurements differ significantly from LMP-based calculations. Due dates are most commonly adjusted based on first-trimester ultrasound measurements at 8-13 weeks, which are the most accurate way to date pregnancy. If the ultrasound measurement differs from LMP-based dating by more than 5-7 days, most healthcare providers will adjust the due date to match the ultrasound. For example, if your LMP suggested you were 11 weeks pregnant but ultrasound measurements show development consistent with 9 weeks, your due date would be moved forward by 2 weeks. This doesn't mean anything is wrong, it likely means you ovulated later than assumed so conception occurred more recently than LMP calculations indicated. First-trimester ultrasounds are used for adjustments because fetal growth is remarkably consistent at this early stage, making size-based dating very accurate. Later ultrasounds in second and third trimester are NOT used to adjust due dates because individual growth variations increase as pregnancy progresses. Once your due date is established by early ultrasound, it typically remains the same for the rest of your pregnancy unless there is clear evidence of a significant dating error. Having your due date adjusted is not cause for concern, it is simply a recalibration to ensure accurate pregnancy dating.
When should I tell my healthcare provider I'm pregnant?
You should contact your healthcare provider as soon as you have a positive pregnancy test or suspect you might be pregnant, typically within a few days to a week of getting a positive test. Early prenatal care is important for confirming the pregnancy, establishing accurate dating, screening for potential risks, addressing any health concerns or medications, and beginning prenatal vitamins with folic acid if you haven't already started. Your provider will schedule your first comprehensive prenatal appointment typically around 8-10 weeks of pregnancy. This timing allows for a good first ultrasound to confirm viable pregnancy, see heartbeat, and establish accurate dating while being early enough to begin important prenatal care. However, you should contact your provider immediately if you experience any concerning symptoms including vaginal bleeding or spotting, severe abdominal pain or cramping, severe nausea and vomiting preventing you from keeping down food or water, fever over 100.4 degrees F, dizziness or fainting, or any symptoms that worry you. Additionally, contact your provider promptly if you have pre-existing medical conditions like diabetes or thyroid disorders that may need medication adjustments during pregnancy, take medications that might not be safe during pregnancy, have a history of pregnancy complications, or are over 35 years old. Even if you're early in pregnancy at 4-6 weeks, making that initial contact ensures you're on their radar and can receive guidance about early pregnancy.
What percentage of babies are born on their due date?
Only about 5% of babies arrive exactly on their estimated due date, while approximately 90% are born somewhere between 37-42 weeks in the full-term range. This wide variation exists because pregnancy length naturally varies among individuals and many factors influence when labor begins. The due date represents the midpoint of normal delivery timing, not a precise prediction. Understanding the distribution helps set realistic expectations with about 10% of babies arriving before 37 weeks (preterm), about 20% arriving between 37-38 weeks (early term), about 35% arriving between 39-40 weeks (full term), about 25% arriving between 40-41 weeks, and about 10% arriving between 41-42 weeks (late term). First-time mothers typically deliver slightly later than average with median delivery around 40 weeks 3 days, while women who have had previous babies tend to deliver slightly earlier with median delivery around 39 weeks 5 days. Other factors affecting delivery timing include genetics, age, weight and BMI, stress levels and physical activity, and whether it's a singleton pregnancy or multiples. Instead of fixating on your exact due date, think of it as the center of a roughly 4-week window when baby might arrive. This mindset reduces stress if your due date comes and goes without labor starting, which happens to about 50% of first-time mothers.
How do fertility treatments affect due date calculation?
Fertility treatments provide exceptionally accurate due date calculations because the exact conception date is known, eliminating the estimation inherent in LMP-based dating. For IVF (in vitro fertilization), the embryo transfer date is precisely known and due date is calculated by adding 266 days to a Day 5 blastocyst transfer or adding 268 days to a Day 3 embryo transfer. This dating is highly accurate because you know exactly when conception occurred in the lab and when the embryo was transferred. For IUI (intrauterine insemination), you know the exact insemination date typically timed with ovulation, so due date is calculated by adding 266 days to the insemination date. For timed intercourse with ovulation induction medications like Clomid or Letrozole, you typically know your ovulation date from monitoring allowing conception-date-based calculation. Despite knowing exact conception dates, healthcare providers still perform first-trimester ultrasounds to confirm that fetal growth matches the expected timeline. The advantage of IVF and other assisted reproduction is that guesswork is removed from due date calculation. You have precise information from the start, and your due date is less likely to be adjusted later based on ultrasound as it's already based on the most accurate information possible.
What is the difference between gestational age and fetal age?
Gestational age and fetal age are different ways of measuring pregnancy progress, with gestational age being the medical standard used by healthcare providers. Gestational age is calculated from the first day of your last menstrual period making a full-term pregnancy 40 weeks. This calculation includes the approximately two weeks before conception occurs during your period and the follicular phase leading to ovulation. When your doctor says you're 10 weeks pregnant, they mean 10 weeks from your LMP even though conception likely occurred around week 2. Fetal age (also called embryonic age or conception age) measures time since actual conception occurred, which is typically about 14 days after the LMP for women with regular 28-day cycles. This makes fetal age approximately two weeks less than gestational age. If your gestational age is 10 weeks, the actual fetus is only about 8 weeks old. Healthcare providers use gestational age rather than fetal age because most women know when their period started but don't know the exact conception date, it provides a consistent standardized reference point for all pregnancies, and ultrasound growth charts and medical guidelines are all based on gestational age. Understanding this distinction helps prevent confusion when reading pregnancy information or discussing your pregnancy with healthcare providers.
Should I plan important events around my due date?
When planning important events like weddings, major travel, work commitments, or other significant occasions during pregnancy, it's crucial to account for the uncertainty around your due date and potential early delivery. Since only 5% of babies arrive on the exact due date and delivery can happen anywhere from 37-42 weeks in a normal pregnancy, planning requires flexibility and buffer time. For events you're hosting or central to, avoid scheduling them in the final 6-8 weeks of pregnancy. Even if you reach your due date, you'll likely feel uncomfortable and exhausted. For important work commitments or travel, create clear backup plans for any commitments after 36 weeks. Discuss coverage plans with your employer and avoid scheduling critical deadlines in the weeks surrounding your due date. For travel, most airlines prohibit flying after 36 weeks of pregnancy, and healthcare providers generally advise against traveling far from your planned delivery location after 36-37 weeks. For attending others' important events in your final trimester, be honest about the possibility you might not make it. RSVP with the caveat that your attendance depends on not being in labor. Practical planning strategies include completing major projects and preparations by 36 weeks, having backup plans for all commitments after 37 weeks, and being honest with others about the uncertainty of your availability near your due date.
Why Use Our Due Date Calculator?
Knowing your baby's due date is one of the first and most important pieces of information in your pregnancy journey. Our due date calculator provides accurate estimates based on your last menstrual period, conception date, or ultrasound results using medically-standard calculation methods. Whether you just discovered you're pregnant and want to know when baby will arrive, you're planning your pregnancy timeline, or you need to calculate from different date types, this calculator gives you instant, reliable due date information to guide discussions with your healthcare provider.