Calculate your baby's estimated birthday using 3 medically-validated methods. Plus current pregnancy week, trimester progress, baby size, and a complete prenatal appointment timeline.
A pregnancy due date calculator estimates your baby's expected birthday by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP) — a formula called Naegele's Rule. If you know your conception date or IVF transfer date, more precise calculations are possible. Only about 4% of babies are born on their exact due date, but 70% arrive within one week of it. The most accurate dating comes from an early ultrasound between 8-13 weeks of pregnancy.
Choose your calculation method below. Most people use LMP (last menstrual period). Conception and IVF methods are more accurate if you know those dates.
This calculator gives you a complete pregnancy timeline in under a minute. Here's exactly what to do:
Your "due date" is technically called the Estimated Date of Delivery (EDD) in medical terms — and that word "estimated" is doing a lot of work. The due date is the date when you'll be 40 weeks pregnant, calculated from the first day of your last menstrual period. It's the midpoint of a normal delivery window, not a deadline.
Pregnancy is officially considered "term" anywhere between 37 and 42 weeks. According to the American College of Obstetricians and Gynecologists, this is further subdivided into early term (37-38 weeks), full term (39-40 weeks), late term (41 weeks), and post-term (42+ weeks). The optimal delivery window for baby's health outcomes is 39-40 completed weeks of pregnancy — slightly after the due date for first-time mothers.
Most pregnancy websites won't tell you this directly: due date predictions are surprisingly inaccurate. Here are the numbers backed by published research:
To put that in context: if 100 pregnancies were predicted to deliver on January 1st, only about 4 would actually arrive that day. The rest are distributed across the surrounding weeks — most before, some after. Here's the typical distribution:
| When Babies Are Born | Percentage | Cumulative |
|---|---|---|
| Before 37 weeks (preterm) | ~10% | 10% |
| 37 to 38+6 weeks (early term) | ~26% | 36% |
| 39 to 40+6 weeks (full term) | ~57% | 93% |
| 41 to 41+6 weeks (late term) | ~6% | 99% |
| 42+ weeks (post-term) | ~1% | 100% |
The accuracy of your specific due date depends heavily on how it was calculated:
A landmark 2013 study by Anne Marie Jukic published in Human Reproduction found that even among women with regular cycles and confirmed conception dates, the time from ovulation to delivery varied by up to 37 days. Some pregnancies are simply longer or shorter than others, and that's biologically normal.
This calculator offers three medically-validated methods. Each works best in specific situations:
The standard method used worldwide. Add 280 days to the first day of your last menstrual period. This is the method behind Naegele's Rule, the formula that's been used for over 200 years.
Best for: Anyone who remembers their LMP and has regular cycles.
Limitation: Assumes ovulation on day 14 of a 28-day cycle, which isn't true for everyone.
Add 266 days to the date of conception. This skips the LMP assumption entirely.
Best for: People tracking ovulation, or those who know the exact date of intercourse that led to pregnancy.
Limitation: Few people know their exact conception date with certainty.
For pregnancies achieved through in vitro fertilization. The formula varies by embryo age at transfer:
Best for: IVF pregnancies — gives the most precise due date.
Limitation: Only applies to fertility treatment pregnancies.
The formula behind nearly every due date calculator was developed by German obstetrician Franz Karl Naegele in 1812. Despite being over 200 years old, it remains the standard worldwide.
For a typical 28-day cycle: if your LMP was January 1, 2026, your due date is October 8, 2026. For a 32-day cycle, the due date shifts 4 days later to October 12, 2026.
Here's the historical context most calculator sites skip: Naegele's Rule was developed in an era when regular cycles were assumed universal. Today we know that cycle length varies significantly — from 21 to 35+ days in healthy adults — which is why the cycle adjustment matters.
This is one of the most confusing aspects of pregnancy timing. Let's clear it up with math:
So why do healthcare providers use weeks instead of months? Precision matters. A "month" can be anywhere from 28 to 31 days, but a "week" is always exactly 7 days. When a doctor says you're "20 weeks pregnant," that's a specific milestone for development checks. "Five months pregnant" is too vague — it could mean 18 to 22 weeks, which is a huge difference in fetal development.
An interesting fact you won't find on most pregnancy sites: not every country uses the same formula. While Naegele's Rule is dominant, regional variations exist:
| Country / Region | Formula | Notes |
|---|---|---|
| 🇺🇸 United States | LMP + 280 days | Standard Naegele's Rule |
| 🇬🇧 United Kingdom | LMP + 280-283 days | Sometimes adds extra days for first-time mothers |
| 🇪🇺 European Union | LMP + 280 days | Often confirmed earlier with mandatory first ultrasound |
| 🇯🇵 Japan | LMP + 280 days | Counted as "10 months" (lunar) |
| 🇨🇳 China | LMP + 280 days | Traditional count is "10 months" by lunar calendar |
| 🇦🇺 Australia | LMP + 280 days | Early ultrasound at 8-12 weeks is standard |
The bigger international difference is in when due dates get revised. In the UK and most of Europe, an early ultrasound at 11-14 weeks routinely revises the LMP-based date if there's a discrepancy of more than 5-7 days. In the United States, practices vary by provider — some defer more to LMP, others to ultrasound.
One thing most due date calculators miss: the appointments and tests that come with knowing your due date. Below is a standard prenatal schedule based on current ACOG recommendations:
| Week | Visit / Test | What Happens |
|---|---|---|
| 8-10 weeks | First prenatal visit | Confirm pregnancy, medical history, blood work, dating ultrasound |
| 10-13 weeks | First trimester screening | NIPT/cell-free DNA, nuchal translucency (NT) scan |
| 16-20 weeks | Quad screen | Optional blood test for chromosomal conditions |
| 18-22 weeks | Anatomy ultrasound | Detailed scan; gender reveal possible; checks for ~80% of structural concerns |
| 24-28 weeks | Glucose challenge test | Screens for gestational diabetes |
| 27-36 weeks | Tdap vaccine | Recommended for whooping cough antibody transfer |
| 28+ weeks | Rh immunoglobulin | If you're Rh-negative |
| 35-37 weeks | GBS (Group B Strep) test | Bacterial screening before labor |
| 36-41 weeks | Weekly visits | Cervical checks, fetal monitoring |
The calculator above auto-generates these dates based on your due date. Save or screenshot the timeline so you don't miss the windows for time-sensitive tests like NIPT and gestational diabetes screening.
The dates generated by this calculator are based on standard recommendations. Your specific schedule may differ based on your health history, age, multiple pregnancies (twins, etc.), or prior complications. Always follow your healthcare provider's specific schedule.
Let's walk through an actual calculation using the LMP method.
Step 1: Apply Naegele's Rule
Step 2: Count Forward 280 Days
Step 3: Adjust for Longer Cycle (if applicable)
For a 28-day cycle with LMP January 1, 2026, the estimated due date is October 8, 2026 — exactly 280 days later.
Only about 4% of babies are born on their exact predicted due date. Roughly 70% are born within one week of the estimated due date (37-42 weeks). The most accurate dating comes from an early ultrasound at 8-13 weeks, which can predict delivery within 5-7 days. LMP-based calculations are less precise because they assume a 28-day cycle and ovulation on day 14, which doesn't apply to everyone.
The standard formula is called Naegele's Rule: add 280 days (40 weeks) to the first day of your last menstrual period. For a 28-day cycle, this assumes ovulation on day 14 and conception shortly after. If your cycle is longer or shorter, the formula adjusts: due date = LMP + 280 days + (cycle length − 28 days). When conception date is known, the formula simplifies to conception date + 266 days.
According to the American College of Obstetricians and Gynecologists (ACOG), pregnancy is divided into: Early term (37 0/7 weeks through 38 6/7 weeks), Full term (39 0/7 weeks through 40 6/7 weeks), Late term (41 0/7 weeks through 41 6/7 weeks), and Post-term (42 0/7 weeks and beyond). The optimal delivery window is 39-40 weeks for the best newborn outcomes.
Pregnancy lasts about 280 days from the last menstrual period. If you count by calendar months (averaging 30.4 days each), 280 days equals approximately 9.2 months. If you count by 4-week "lunar months", 280 days equals exactly 10 months. Healthcare providers use weeks for precision because each week represents significant fetal development that monthly counting would obscure.
Going past your due date is very common — about 30% of first-time pregnancies extend past 40 weeks. Doctors generally allow pregnancy to continue safely up to 41-42 weeks with monitoring. Beyond 42 weeks (post-term), induction is typically recommended due to increased risks. If you reach 41 weeks, your provider will likely schedule additional monitoring including non-stress tests and fluid level assessments.
The due date calculation itself is the same for twins and singletons — both are based on the LMP or conception date. However, twin pregnancies are typically delivered earlier on average (around 36-37 weeks for fraternal twins, sometimes earlier for identical), so the actual delivery is often weeks before the calculated due date. Twin pregnancies require closer monitoring throughout.
Yes — switch to the "IVF Transfer" mode at the top of the calculator. IVF pregnancies actually have the most accurate due dates because the conception date is known precisely. The formula uses the embryo's age at transfer: add 263 days for Day 3 embryos, or 261 days for Day 5 blastocysts.
No. This calculator provides educational estimates using standard medical formulas, but it does not replace professional prenatal care. Your healthcare provider will confirm your due date using ultrasound and clinical assessment, and provide personalized guidance based on your specific health situation. Always follow your provider's recommendations.
No. All calculations happen entirely in your browser. Your dates, cycle length, and other inputs stay on your device — nothing is sent to our servers, no cookies are set for the calculator, and no personal data is stored after you close the page.
This calculator provides educational estimates based on standard medical formulas. It does not replace professional medical advice, diagnosis, or treatment. Always consult your healthcare provider for personalized prenatal care.