How Is Your Pregnancy Due Date Calculated? (Methods and Accuracy)
Your due date is normally the first day of your last menstrual period (LMP) plus 280 days, which is 40 weeks. That is Naegele's rule. If you know your conception date, your provider can count 266 days from it instead, and an ultrasound before 14 weeks can correct the date when it disagrees with your last period by more than 5 to 7 days.
The result is an estimate, not a deadline. Only about 4% of babies are born on the estimated date, and healthy pregnancies vary by as much as 37 days even when the day of ovulation is known. To run your own numbers from a last period, conception date or IVF transfer, use our Pregnancy Due Date Calculator.
Due Date Formulas at a Glance
| Starting point | Formula | Best when |
|---|---|---|
| First day of last period (LMP) | LMP + 280 days | Your cycles are regular and you know the date |
| Conception or ovulation date | Conception + 266 days | You tracked ovulation or know the day you conceived |
| IVF day-5 (blastocyst) transfer | Transfer date + 261 days | You had a blastocyst transfer |
| IVF day-3 transfer | Transfer date + 263 days | You had a day-3 embryo transfer |
| Ultrasound before 14 weeks | Crown-rump length matched to a growth chart | Your cycles are irregular, or the last-period date is unknown or disputed |
Method 1: Naegele's Rule (Last Menstrual Period)
The most common method is Naegele's rule, named after the German obstetrician Franz Naegele, whose 1812 work described it. Count 280 days from the first day of your last period. The calendar shortcut gets you to the same place: add one year, subtract three months, and add seven days.
Due date = first day of last period + 280 days (40 weeks)
Example: last period began January 15, 2026
January 15, 2026 + 280 days = October 22, 2026
Shortcut: January 15, 2027 minus 3 months = October 15, 2026
October 15, 2026 plus 7 days = October 22, 2026The two versions usually agree, but months have different lengths, so the shortcut can land up to 3 days away from the exact 280-day count. We checked every possible LMP date in 2026: the two results matched on 214 of 365 dates and differed by 1 to 3 days on the rest. For example, an LMP of March 31, 2026 gives January 5, 2027 by day count and January 7, 2027 by shortcut. The day count is the exact 40 weeks.
Why Count From Your Period, Not Conception?
Most people can name the first day of their last period, but very few know the day they conceived. Conception is not a single moment you can point to, either: in a study of 221 women who were trying to conceive, pregnancy happened only when intercourse fell within the six days ending on the day of ovulation (Wilcox and colleagues, 1995). Starting the count at your period gives providers a consistent starting point that is about two weeks before conception.
That means weeks 1 and 2 of a pregnancy, as counted from your LMP, come before conception. Implantation usually happens around weeks 3 to 4, and by the time you miss a period and get a positive test you are typically counted as about 4 weeks pregnant.
Limits of Naegele's Rule
- It assumes a 28-day cycle with ovulation on day 14. Cycles of 21 to 35 days are considered normal. With a 35-day cycle you likely ovulated near day 21, so add about 7 days. Our calculator has an Average Cycle Length field for this reason.
- It depends on remembering the date. If your period is irregular, or you conceived soon after stopping hormonal birth control, the starting point can be unreliable.
- It ignores natural variation. In a study of 125 naturally conceived single births with a known ovulation day, the median time from ovulation to birth was 268 days (38 weeks 2 days), and even after excluding six preterm births the range spanned 37 days (Jukic and colleagues, 2013). If you ovulated on day 14, 268 days after ovulation is about 282 days after your LMP.
Method 2: Conception Date Calculation
If you know the conception date, for example through ovulation tracking or insemination, add 266 days (38 weeks). The 14-day gap from the LMP method is the roughly two weeks between the start of a period and ovulation.
Due date = conception date + 266 days
Example: conception on January 29, 2026 (day 14 of a cycle that began January 15)
January 29, 2026 + 266 days = October 22, 2026This matches the LMP result because the example uses a textbook 28-day cycle. When your cycle is longer or shorter and you know the conception date, it is usually the better starting point, because it does not depend on the day-14 assumption. A guessed conception date is less reliable than a last-period date, so use this method only when you tracked ovulation or know the insemination date.
Method 3: IVF Transfer Date
With IVF, the embryo's age and the transfer date are known exactly, so the count starts there. The Cleveland Clinic describes it this way: for a day-5 transfer, count 261 days from the transfer date, and for a day-3 transfer, count 263 days.
| Transfer | Add to transfer date | Example transfer date | Due date |
|---|---|---|---|
| Day-5 (blastocyst) | 261 days | March 10, 2026 | November 26, 2026 |
| Day-3 | 263 days | March 10, 2026 | November 28, 2026 |
The numbers are the 266-day conception count minus the embryo's age at transfer (266 minus 5 is 261, and 266 minus 3 is 263). Our Pregnancy Due Date Calculator has both IVF options.
Method 4: Ultrasound Dating
ACOG calls ultrasound in the first trimester, up to and including 13 weeks 6 days, the most accurate way to establish or confirm a due date. The sonographer measures the crown-rump length (CRL), the distance from the top of the embryo's head to its bottom, and matches it to a growth chart.
Early embryos grow at nearly the same rate, so size tells you age. Later, healthy babies grow at different speeds, so a scan measures size more than age. That is why ACOG lets the gap between the scan and the LMP date grow as pregnancy advances before it changes the due date:
| Gestational age at ultrasound | Change the due date if the scan and LMP dates differ by more than |
|---|---|
| 8 weeks 6 days or earlier | 5 days |
| 9 weeks 0 days to 13 weeks 6 days | 7 days |
| 14 weeks 0 days to 15 weeks 6 days | 7 days |
| 16 weeks 0 days to 21 weeks 6 days | 10 days |
| 22 weeks 0 days to 27 weeks 6 days | 14 days |
| 28 weeks 0 days and later | 21 days |
If the gap is smaller than the threshold and your last-period date is certain, the LMP date normally stays.
Gestational Age vs. Fetal Age
- Gestational age counts from the first day of your last period. A full-term pregnancy is about 40 weeks by this count, and doctors, ultrasound reports and pregnancy apps all use it.
- Fetal age (conceptional age) counts from conception, so it runs about 2 weeks behind. At 40 weeks gestational age, the fetal age is about 38 weeks.
When your doctor says you are 12 weeks pregnant, that is gestational age, and your baby has been developing for about 10 weeks. Milestone charts use gestational age unless they say otherwise. For example, MedlinePlus says the embryo becomes a fetus at the end of the 10th week of pregnancy, which is 8 weeks after conception.
Why a Due Date Is an Estimate, Not a Deadline
Only about 4% of babies are born on their due date, according to the Cleveland Clinic. The national birth data shows how spread out the rest are. In 2024, based on the obstetric estimate of gestational age on US birth certificates:
| Gestational age at birth (US, 2024) | Share of births |
|---|---|
| Before 37 weeks (preterm) | 10.41% |
| 37 to 38 weeks (early term) | 30.26% |
| 39 to 40 weeks (full term) | 54.61% |
| 41 weeks (late term) | 4.47% |
| 42 weeks or later (post term) | 0.25% |
The preterm rate was the same 10.41% in 2023, 2024 and, in provisional data, 2025. Medical decisions shape those numbers. NCHS reports that labor was induced in 34.5% of singleton births in 2024, up from 24.9% in 2016, so many babies are born on a date a provider chose rather than when labor started on its own.
ACOG uses these labels for the weeks around your date:
| Label | Gestational age |
|---|---|
| Early term | 37 weeks 0 days to 38 weeks 6 days |
| Full term | 39 weeks 0 days to 40 weeks 6 days |
| Late term | 41 weeks 0 days to 41 weeks 6 days |
| Post-term | 42 weeks 0 days and beyond |
A baby born before 37 weeks is preterm. ACOG's guidance on medically indicated early deliveries notes that the complications of elective delivery before 39 weeks are well described, which is why elective delivery is discouraged before then, while delivery earlier than 39 weeks is still recommended when there is a medical reason for it.
What Shifts When You Actually Deliver
- Your own timing. In the 2013 ovulation study, an individual woman's deliveries tended to occur at similar gestational ages, and pregnancies with later implantation also took longer from implantation to delivery. The study did not find a link with BMI, alcohol intake, parity or the baby's sex.
- Multiples. March of Dimes reports that more than half of twins and nearly all triplets and other higher-order multiples are born earlier than they should be.
- Health conditions. Problems such as preeclampsia or gestational diabetes can lead to a medically indicated delivery before the due date.
- Induction. A planned induction moves the birth date. In the ARRIVE trial of 6,106 low-risk first-time mothers, induction at 39 weeks lowered the cesarean rate (18.6% versus 22.2% with expectant management), which is one reason some providers now offer it. ACOG's January 2025 clinical practice update integrates ARRIVE and later studies into its guidance for first-time mothers without a medical reason for delivery at 39 to 41 weeks.
The Trimesters at a Glance
ACOG's dating guidance defines the first trimester as up to 13 weeks 6 days, the second as 14 weeks 0 days to 27 weeks 6 days, and the third as 28 weeks 0 days until birth. A few milestones from MedlinePlus, all in gestational weeks:
- Weeks 6 to 7: the heartbeat can be seen on a vaginal ultrasound.
- End of week 10: the embryo becomes a fetus.
- Weeks 19 to 21: many people first feel movement (quickening).
- Weeks 27 to 30: the lungs begin producing surfactant, which helps the air sacs fill with air after birth.
- Weeks 35 to 37: a baby weighs about 5.5 pounds (2.5 kilograms).
Related tools: use the Ovulation Calculator to estimate your fertile window and conception date, and the Period Tracker to keep a record of the cycle dates your provider will ask for. More health tools are in our Health Calculators hub.
Frequently Asked Questions
How accurate is a due date based on my last period?
It is accurate only if your cycle is close to 28 days and you remember the first day of your last period. If you have a 35-day cycle, you probably ovulated around day 21 instead of day 14, so your real due date is about a week later than the standard calculation. ACOG calls a first-trimester ultrasound (up to 13 weeks 6 days) the most accurate way to date a pregnancy, and providers switch to the ultrasound date when it differs from the last-period date by more than 5 days (scan at 8 weeks 6 days or earlier) or more than 7 days (scan at 9 weeks 0 days to 13 weeks 6 days).
Can my due date change during pregnancy?
Yes, mostly in the first trimester. If an early ultrasound and your last-period date differ by more than ACOG's cutoff for that week (5 days at 8 weeks 6 days or earlier, 7 days from 9 weeks 0 days to 15 weeks 6 days), your provider will usually move the due date to the ultrasound date. Later scans need a much bigger gap before the date is changed (10 days at 16 to 21 weeks, 14 days at 22 to 27 weeks, and 21 days from 28 weeks). Once the date is confirmed, it normally stays fixed, because later scans mostly show how big your baby is rather than how old.
What happens if I go past my due date?
Going past the date is common, but it is not open-ended. ACOG Practice Bulletin 146 says antenatal fetal monitoring may be indicated from 41 weeks, induction can be considered from 41 weeks 0 days, and induction is recommended once you reach 42 weeks 0 days, by 42 weeks 6 days, because the risk of complications rises. In practice many births happen sooner: about 34.5% of singleton births in the US were induced in 2024, and only about 4.7% of 2024 births happened at 41 weeks or later. Your own plan depends on your health, your baby, and your provider's protocol.
Is the due date different for an IVF pregnancy?
Yes. With IVF the embryo's age and the transfer date are known, so the due date is counted from the transfer date instead of your last period. For a day-5 (blastocyst) transfer, add 261 days to the transfer date. For a day-3 transfer, add 263 days. For example, a day-5 transfer on March 10, 2026 gives a due date of November 26, 2026. Ask your clinic to confirm the date it gives you, because it is based on the embryo's age and not on your last period.
How can I get the most accurate due date?
Combine an ultrasound before 14 weeks with what you know about your cycle. Tell your provider your usual cycle length, and if you tracked ovulation with temperature charting or ovulation tests, share the date, because a known ovulation date is more reliable than a last-period date when your cycle is not 28 days. You can compare the different starting points side by side with our Pregnancy Due Date Calculator and check the gap between any two dates with the Date Difference Calculator.
Sources
- ACOG Committee Opinion 700: Methods for Estimating the Due Date
- ACOG Committee Opinion 579: Definition of Term Pregnancy
- ACOG Practice Bulletin 146: Management of Late-Term and Postterm Pregnancies
- ACOG Clinical Practice Update: Management of Full-Term Nulliparous Individuals Without a Medical Indication for Delivery (January 2025), which updates Practice Bulletin 146
- ACOG Committee Opinion 831: Medically Indicated Late-Preterm and Early-Term Deliveries
- Jukic AM, et al. Length of human pregnancy and contributors to its natural variation. Human Reproduction, 2013
- Wilcox AJ, et al. Timing of sexual intercourse in relation to ovulation. New England Journal of Medicine, 1995
- Grobman WA, et al. Labor induction versus expectant management in low-risk nulliparous women (ARRIVE). New England Journal of Medicine, 2018
- NCHS National Vital Statistics Reports 75(2): Births: Final Data for 2024 (June 2026)
- NCHS: Births: Provisional Data for 2025 (April 2026), preterm rate unchanged at 10.41%
- NCHS Data Brief 554: Induction of Labor Increases in the United States, 2016 to 2024
- Cleveland Clinic: Due Date Calculator (Naegele's rule, IVF dating, and the 4% figure)
- MedlinePlus: Fetal Development
- March of Dimes: Being Pregnant With Twins, Triplets and Other Multiples