About Pregnancy Due Date Calculator
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Pregnancy Due Date Calculator: Estimate Your Baby's Delivery Date With Naegele's Rule
TL;DR: Enter the first day of your last menstrual period (LMP). The calculator adds 280 days to find your estimated due date (EDD), then shows your current gestational age and trimester. Only about 4% of babies arrive on the exact EDD, but 80% deliver within two weeks of it.
Table of Contents
- How Naegele's Rule Predicts Your Due Date
- Six Scenarios Where This Calculator Saves You Time
- The Formula Behind Estimated Due Date Calculation
- Calculating Your EDD Step by Step
- Putting the Formula to Work: Two Real-World Examples
- Where People Go Wrong With Due Date Estimates
- FAQ
- Assumptions and Notes
- Your Next Step
- Further Reading
How Naegele's Rule Predicts Your Due Date
Most people hear "you're due on March 15th" and treat it as a fixed appointment. It is a statistical midpoint. A pregnancy due date calculator uses Naegele's Rule, the same method the American College of Obstetricians and Gynecologists (ACOG) recommends as the clinical starting point for estimating delivery. The rule assumes a 28-day menstrual cycle with ovulation on day 14, then projects 280 days (40 weeks) from the first day of your last menstrual period.
The biological basis is straightforward: human gestation averages 266 days from conception. Because most people know their LMP date but not their exact ovulation date, the formula adds 14 days to cover the follicular phase. Genetic variation affects gestational length, too. Studies show that first-time mothers tend to deliver about 1.3 days later on average than women who have given birth before, and maternal age, ethnicity, and cycle length all shift the window by several days.
Plug in your LMP above and get your due date, gestational age, and trimester in about ten seconds.
Six Scenarios Where This Calculator Saves You Time
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You just got a positive pregnancy test and want to know how far along you are. Home tests typically turn positive around 4 weeks gestational age (about 14 days after ovulation). Entering your LMP date tells you whether you are 4, 5, or 6 weeks along, which determines when to schedule your first prenatal visit, usually between weeks 8 and 10.
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Your OB appointment is weeks away and you need a quick reference date. The average wait for a first obstetric appointment in the US is 24 days. Having your EDD calculated before that visit lets you start planning around a concrete timeline rather than guessing.
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You want to figure out which trimester you are in. Each trimester carries different prenatal screening windows. The nuchal translucency scan, for instance, must happen between weeks 11 and 14. Knowing your gestational age down to the week prevents missed screening deadlines.
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You are comparing a dating ultrasound result to Naegele's Rule. First-trimester ultrasound is accurate to plus or minus 5 days. If the ultrasound EDD and Naegele's EDD differ by more than 7 days, ACOG guidelines recommend using the ultrasound date. This calculator gives you the Naegele's baseline for that comparison.
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You need to plan maternity or paternity leave around a delivery window. About 80% of births occur between weeks 38 and 42. If your EDD is September 12th, your realistic delivery window runs from August 29th to September 26th, a 4-week span your employer needs to account for.
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Your menstrual cycle is regular and you want the fastest possible estimate. Naegele's Rule is most accurate for cycles between 26 and 30 days. If your cycle falls in that range, the formula's margin of error is roughly plus or minus 10 days, comparable to a first-trimester ultrasound.
The Formula Behind Estimated Due Date Calculation
Naegele's Rule converts your last menstrual period into an estimated delivery date using simple date arithmetic.
EDD = LMP + 280 days
Alternative form (Naegele's original method):
EDD = LMP + 7 days − 3 months + 1 year
Gestational Age = Today − LMP (expressed in weeks + days)
Trimester:
1st trimester: 0–13 weeks
2nd trimester: 14–27 weeks
3rd trimester: 28–40 weeks
Both forms produce the same result. The "add 7 days, subtract 3 months" version is what obstetricians use for mental math during appointments.
| Trimester | Week Range | Key Milestones |
|---|---|---|
| 1st | 0–13 | Organ formation, first heartbeat (week 6), nuchal translucency scan (weeks 11–14) |
| 2nd | 14–27 | Anatomy scan (week 20), fetal movement felt (weeks 16–22), viability threshold (week 24) |
| 3rd | 28–40 | Lung maturity, growth scans, delivery preparation, term at week 37 |
| Cycle Length | Adjustment to Standard EDD |
|---|---|
| 21 days | Subtract 7 days |
| 25 days | Subtract 3 days |
| 28 days | No adjustment (standard) |
| 32 days | Add 4 days |
| 35 days | Add 7 days |
The adjustment accounts for a shifted ovulation day. A 35-day cycle typically means ovulation around day 21 instead of day 14, pushing conception (and therefore delivery) 7 days later.
Limitations: Naegele's Rule assumes a regular cycle and a single fetus. Irregular cycles, polycystic ovary syndrome (PCOS), recent hormonal contraceptive use, and multiple pregnancies all reduce accuracy. In those cases, first-trimester ultrasound becomes the primary dating method.
Calculating Your EDD Step by Step
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Identify your LMP date. This is the first day of your last menstrual period, not the last day. Many people confuse the two, which shifts the result by 3 to 7 days.
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Add 280 days (40 weeks). Or use the shortcut: add 7 days to the LMP date, then subtract 3 months, then add 1 year if the result crosses into the next calendar year.
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Check the result against your cycle length. If your cycle is shorter or longer than 28 days, adjust using the cycle-length table above. A 32-day cycle adds 4 days to the EDD.
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Calculate your current gestational age. Subtract your LMP from today's date. Express the result in weeks and days (e.g., 22 weeks + 4 days).
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Determine your trimester. Match your gestational age to the trimester table: weeks 0–13 (first), 14–27 (second), 28–40 (third).
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Compare with ultrasound dating if available. If your first-trimester ultrasound EDD differs by more than 7 days from the Naegele's result, use the ultrasound date. A second-trimester ultrasound uses a wider threshold of 10–14 days before overriding LMP-based dating.
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Record your EDD and share it with your provider. One non-obvious point: the EDD should rarely change after it is established in the first trimester. Later ultrasounds measure fetal size, not gestational age, and using them to "update" the due date leads to unnecessary inductions.
Putting the Formula to Work: Two Real-World Examples
Example 1: A 27-Year-Old With a Regular 28-Day Cycle
Priya, a graduate student, gets a positive pregnancy test on January 20th. Her LMP was December 29th.
Step 1: LMP = December 29th. Step 2: Add 7 days = January 5th. Subtract 3 months = October 5th. Add 1 year = October 5th of the following year. EDD = October 5th. Step 3: Her cycle is 28 days. No adjustment needed. Step 4: From December 29th to January 20th = 3 weeks + 1 day gestational age at the time of her positive test.
| Output | Value | Clinical Meaning |
|---|---|---|
| Estimated Due Date | October 5th | Target date for delivery planning |
| Gestational Age (at test) | 3 weeks, 1 day | Too early for ultrasound; recheck at 7–8 weeks |
| Trimester | 1st | Schedule first prenatal visit for week 8–10 |
Priya should book her first prenatal appointment for around February 23rd, when she will be approximately 8 weeks along. Her anatomy scan window opens around mid-May.
Example 2: A 34-Year-Old With a 33-Day Cycle
Marcus and his partner are expecting their second child. Her LMP was March 3rd and her average cycle length is 33 days.
Step 1: LMP = March 3rd. Step 2: Add 7 days = March 10th. Subtract 3 months = December 10th. Same year. EDD (unadjusted) = December 10th. Step 3: Cycle is 33 days, so ovulation likely occurred on day 19 instead of day 14. Add 5 days. Adjusted EDD = December 15th. Step 4: On April 14th (when they confirm with an ultrasound), gestational age from LMP = 6 weeks + 0 days.
| Output | Value | Clinical Meaning |
|---|---|---|
| Estimated Due Date | December 15th | Adjusted for longer cycle |
| Gestational Age (at ultrasound) | 6 weeks, 0 days | Heartbeat typically visible on ultrasound |
| Trimester | 1st | Nuchal translucency scan due around mid-June |
Because this is their second pregnancy, the average gestational length is statistically 1–2 days shorter. The practical difference is negligible, but their provider may note it.
Where People Go Wrong With Due Date Estimates
Using the wrong LMP date. The LMP is the first day of bleeding, not the last. Picking the last day of a 5-day period shifts the EDD forward by 5 days, which can trigger unnecessary early-delivery concerns later. Fix: mark the start date of each period in a tracking app.
Ignoring irregular cycle length. A person with a 35-day cycle who uses the standard 280-day formula will get an EDD that is 7 days too early. Roughly 30% of women have cycles outside the 26–30 day range. Fix: adjust by the difference between your cycle length and 28 days.
Treating the due date as a deadline. Only 4–5% of babies are born on the exact EDD. Full term begins at 37 weeks and extends to 42 weeks. Expecting delivery on the precise date creates unnecessary anxiety after week 39. Fix: think in terms of a 5-week delivery window (weeks 37–42).
Confusing gestational age with fetal age. Gestational age counts from the LMP, which is approximately 2 weeks before conception. A "6-week" pregnancy contains an embryo that is about 4 weeks old. Mixing up these two measurements when reading medical literature leads to confusion about developmental milestones. Fix: always ask your provider whether they mean gestational or fetal age.
Recalculating the EDD after every ultrasound. A third-trimester ultrasound might estimate fetal weight at the 90th percentile, prompting someone to think the baby is "ahead of schedule." Fetal size in the third trimester reflects growth rate, not gestational age. Changing the EDD based on late ultrasounds increases the rate of unnecessary inductions by up to 20%. Fix: lock in the EDD from your first-trimester dating and do not change it.
Not accounting for recent contraceptive use. Coming off hormonal birth control (especially the pill or hormonal IUD) can delay the first natural ovulation by 2–6 weeks. The first post-pill "period" may be a withdrawal bleed, not a true menstrual period. Using that date as the LMP produces an EDD that is too early. Fix: if your pregnancy occurred within 3 months of stopping hormonal contraception, rely on ultrasound dating instead.
Assumptions and Notes
- Margin of error: Naegele's Rule carries a margin of plus or minus 10–14 days. About 80% of deliveries occur within 2 weeks of the EDD. First-trimester ultrasound reduces this margin to plus or minus 5 days.
- Professional disclaimer: This calculator provides an estimate for informational purposes only. It does not replace prenatal care. All pregnant individuals should confirm their due date with a qualified obstetric provider, especially in cases of irregular cycles, recent contraceptive use, or assisted reproduction.
Your Next Step
The number you just calculated is a planning anchor, not a promise. Book your first prenatal appointment, bring your LMP date, and let your provider confirm or adjust the EDD with an early ultrasound. From there, every screening, milestone, and preparation step falls into place around that single date.