Due Date Calculator (2026)
Whether you just found out you're expecting, you're mid-way through an IVF cycle, or you're counting down the days until you finally meet your baby - one question comes before almost everything else: when is this baby actually due? This calculator gives you a real answer using whichever piece of information you already have, whether that's your last period, a known conception date, or an embryo transfer date from a fertility clinic.
| Milestone | Estimate |
|---|
What a Due Date Actually Represents
A due date - officially called the "estimated date of delivery" on medical charts - isn't a prediction of the exact day your baby will arrive. It's the midpoint of a normal, healthy range of delivery timing, calculated from population averages of how long pregnancies typically last. Think of it less as a deadline and more as the center of a several-week window where the vast majority of healthy babies show up.
- A due date is an estimate, not a prediction - only about 4-5% of babies arrive exactly on it
- Full-term is a range: 39 weeks 0 days to 40 weeks 6 days is now the official medical definition
- There are four common ways to calculate it: last period, conception date, or IVF transfer (Day 3 or Day 5)
- A first-trimester ultrasound is considered the most accurate dating method of all
- Doctors will often update your due date if the ultrasound and period-based estimate disagree by more than about a week
- IVF due dates are usually the most precise, since the exact embryo age and transfer date are both known
Four Ways to Calculate a Due Date
This calculator supports all four common methods, so you can use whichever date you actually know:
- Last Menstrual Period (LMP) - Naegele's rule. Add 280 days to the first day of your last period. This is the most widely used method and the one most pregnancy apps default to, since almost everyone can identify this date.
- Known Conception Date. Add 266 days to your conception date. This skips the ovulation-estimation step entirely, since you already know when fertilization happened.
- IVF Transfer Date (Day 3 embryo). Add 263 days to the transfer date. A Day 3 embryo is already 3 days past fertilization at transfer, so 3 fewer days are needed compared to a conception-date calculation (266 − 3 = 263).
- IVF Transfer Date (Day 5 blastocyst). Add 261 days to the transfer date. A Day 5 blastocyst is 5 days past fertilization, so 261 days (266 − 5) completes the standard 266-day post-conception timeline.
LMP: Due Date = Last Period Start + 280 days
Conception: Due Date = Conception Date + 266 days
IVF (Day 3): Due Date = Transfer Date + 263 days
IVF (Day 5): Due Date = Transfer Date + 261 days
Your Pregnancy by Trimester
| Trimester | Weeks | What's Generally Happening |
|---|---|---|
| First Trimester | Weeks 1 – 13 | Major organ development; morning sickness and fatigue are common |
| Second Trimester | Weeks 14 – 27 | Often called the "golden period" - energy tends to return, bump becomes visible |
| Third Trimester | Weeks 28 – 40+ | Rapid weight gain for baby; increasing discomfort as delivery approaches |
Worked Examples
Last period started April 10. Due date = April 10 + 280 days = January 14 (of the following year).
Conception occurred April 24. Due date = April 24 + 266 days = January 15 - just one day different from Example 1, since April 24 is roughly 14 days after April 10, the expected conception timing for a standard 28-day cycle.
Blastocyst transferred on May 1. Due date = May 1 + 261 days = January 16. IVF due dates are usually trusted more than period-based ones, since the embryo's exact age at transfer removes the ovulation-timing guesswork entirely.
Why Only 5% of Babies Arrive On Their Due Date
Research consistently shows that only about 4% to 5% of babies are born on their exact calculated due date. Most first-time mothers deliver a few days after their due date, while it's completely normal to deliver anywhere from about two weeks before to two weeks after. A due date is genuinely just the statistical middle of a wide, normal range - not a deadline your body is expected to hit precisely.
Early, Full, Late, and Post-Term Explained
| Category | Gestational Age |
|---|---|
| Early Term | 37 weeks 0 days – 38 weeks 6 days |
| Full Term | 39 weeks 0 days – 40 weeks 6 days |
| Late Term | 41 weeks 0 days – 41 weeks 6 days |
| Post-Term | 42 weeks 0 days and beyond |
This four-category system, adopted by the American College of Obstetricians and Gynecologists, replaced the older blanket "term" label (37-42 weeks) because research showed meaningful differences in newborn outcomes even within that range - which is exactly why your due date matters less than which of these windows you deliver in.
When Your Doctor Changes Your Due Date
A first-trimester ultrasound - typically done between weeks 8 and 13 - measures the crown-rump length of the fetus, which correlates very reliably with gestational age at that early stage. If this measurement puts you more than about 5 to 7 days away from your period-based due date, most providers will officially move your due date to match the ultrasound, since it's considered the single most accurate dating tool available, especially useful when your last period date is uncertain, your cycles are irregular, or you weren't tracking ovulation.
Gestational Age vs. Fetal Age
This trips up a lot of people early on: when your doctor says you're "8 weeks pregnant," that's counted from your last period - not from conception. Since conception typically happens around day 14 of a cycle, your baby (fetal age) is actually closer to 6 weeks of development at that point. Gestational age is the medical standard used everywhere - due date calculators, pregnancy apps, ultrasound reports - so it's worth knowing the two numbers aren't the same thing, even though they're often talked about interchangeably.
Why IVF Due Dates Are Usually More Precise
IVF pregnancies have one real advantage over natural conception when it comes to dating: there's no guesswork involved. The embryo's exact age (Day 3 or Day 5) and the exact transfer date are both known with certainty, which removes the ovulation-timing estimation that period-based due dates rely on. This is part of why IVF due dates tend to align very closely with actual delivery timing compared to LMP-based estimates, particularly for people who had irregular cycles before starting treatment.
Common Mistakes
- Treating the due date as a fixed deadline. It's a statistical estimate - normal delivery spans a window of several weeks around it.
- Adding 280 days to a known conception date instead of 266. The 280-day count starts from the last period, not conception - using the wrong number shifts the estimate by about two weeks.
- Forgetting to adjust for embryo age in IVF calculations. A Day 5 transfer needs 261 days added, not 280 or 266, since the embryo is already several days into development at transfer.
- Ignoring an ultrasound-based adjustment. If your doctor moved your due date after an early scan, that revised date is more reliable than your original period-based calculation.
- Panicking about going "overdue." Anything through 40 weeks 6 days is still considered full term - it's not overdue until past 42 weeks by medical definition.
Glossary
- Naegele's rule
- The standard formula for estimating a due date: 280 days after the first day of the last menstrual period.
- Gestational age
- Pregnancy duration counted in weeks from the first day of the last menstrual period, the medical standard.
- Fetal (conceptional) age
- Pregnancy duration counted from the actual date of conception, roughly two weeks less than gestational age.
- Crown-rump length (CRL)
- An early ultrasound measurement of the fetus from head to bottom, used to date pregnancy accurately in the first trimester.
- Full term
- The medically defined range of 39 weeks 0 days to 40 weeks 6 days, considered optimal timing for delivery.
- Post-term pregnancy
- A pregnancy that continues past 42 weeks 0 days, sometimes prompting closer monitoring or induction discussions.
- Blastocyst
- A Day 5 embryo, more developed than a Day 3 embryo, commonly used in IVF transfers.
Frequently Asked Questions
Q: How is a due date calculated?
A: The most common method, called Naegele's rule, calculates a due date by adding 280 days (40 weeks) to the first day of your last menstrual period. If you know your conception date instead, add 266 days (38 weeks), since conception typically occurs about two weeks after a period begins. For IVF pregnancies, the due date is calculated from the embryo transfer date instead, using a slightly different day count depending on whether a 3-day or 5-day embryo was transferred.
Q: How many babies are actually born on their due date?
A: Only about 4% to 5% of babies are born on their exact calculated due date. The large majority arrive within a two-week window on either side, and full-term birth is now medically defined as anywhere from 39 weeks 0 days to 40 weeks 6 days, with early term (37-38 weeks), late term (41 weeks), and post-term (42+ weeks) as separate categories - all considered within a normal range of variation.
Q: Can my due date change during pregnancy?
A: Yes, and it's fairly common. If a first-trimester ultrasound measurement (usually the crown-rump length) differs from the period-based due date by more than about 5 to 7 days, most doctors will adjust the due date to match the ultrasound, since early ultrasound dating is considered more accurate than menstrual history alone, especially for people with irregular cycles or uncertain last period dates.
Q: How is an IVF due date calculated differently?
A: For IVF pregnancies, the due date is calculated from the exact, known embryo transfer date rather than an estimated ovulation date, which makes IVF due dates generally more precise than period-based calculations. For a Day 3 embryo transfer, add 263 days to the transfer date; for a Day 5 blastocyst transfer, add 261 days, since the embryo's age at transfer is subtracted from the standard 266-day post-conception timeline.
Q: What is the difference between gestational age and fetal age?
A: Gestational age is counted from the first day of the last menstrual period and is the standard measurement used by doctors, pregnancy apps, and due date calculators. Fetal age (also called conceptional or embryonic age) is counted from the actual date of conception, which is roughly two weeks less than gestational age at any given point - so a pregnancy described as "10 weeks" by a doctor is about 8 weeks along by fetal age.
Q: Is a due date calculator accurate for irregular cycles?
A: Not as accurate as it is for regular cycles. Due date calculations based on the last menstrual period assume ovulation happens around day 14 of a roughly 28-day cycle, which doesn't hold precisely for people with longer, shorter, or irregular cycles. In these cases, a first-trimester dating ultrasound is significantly more reliable than a period-based calculation and is what most doctors will use to set the official due date.
This calculator provides estimates for informational purposes only and is not a substitute for medical advice, diagnosis, or a clinical dating ultrasound. It uses standard obstetric dating conventions (Naegele's rule and standard IVF transfer-date methodology), which are population averages and may not reflect your individual pregnancy exactly. Please speak with your doctor or midwife for guidance specific to your care. Sources: Naegele's rule and gestational dating methodology as used across obstetric practice; American College of Obstetricians and Gynecologists (ACOG) full-term/early-term/late-term/post-term classification; standard IVF embryo transfer dating conventions used across fertility medicine.