Health Tools

Pregnancy Due Date Calculator

Calculate your estimated due date from LMP or conception date. Includes trimester milestones and key appointment windows.

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Estimated due date & trimester milestones

First Day of Last Period

Average Cycle Length (days)

Pregnancy FAQ

How accurate is the due date estimate?

Only about 5% of babies are born on their exact due date. Most arrive within 2 weeks before or after. The estimate is most accurate when based on a known LMP with a regular 28-day cycle. Ultrasound dating in the first trimester is the most reliable method.

When does each trimester start?

The first trimester runs from week 1 to 13, the second from week 14 to 27, and the third from week 28 until birth at approximately week 40. Full-term pregnancy is considered 39–40 weeks.

What is Naegele's Rule?

Naegele's Rule is the standard formula for calculating due date: take the first day of your last menstrual period, add one year, subtract three months, and add seven days. This assumes a standard 28-day cycle and 280-day (40-week) pregnancy.

Health Guide

Due Date Calculator Guide: Naegele's Rule, LMP Dating & What an EDD Really Means

A practical guide to pregnancy due date math — Naegele's rule, the difference between LMP and conception dating, why ultrasound dating sometimes wins, and the statistical reality of when babies actually arrive.

Written by Calculixy Editorial TeamReviewed by an OB-GYN physicianUpdated: May 2026

From the Underwriting Desk

The "due date" sounds precise. It isn't. Only about 4% of babies arrive on their estimated due date, and full-term pregnancy spans a four-week window from 37 to 41 weeks. A due date calculator gives you the median best-guess based on a 19th-century formula that assumes a 28-day cycle with ovulation on day 14 — assumptions that don't match every pregnancy. Understanding what the number means and what it doesn't sets realistic expectations for an event that resists scheduling.

What this calculator does

The Due Date Calculator estimates your Estimated Due Date (EDD) using Naegele's Rule, the standard obstetric formula since 1812. Given the first day of your Last Menstrual Period (LMP), it adds 280 days (40 weeks) to produce the EDD. The calculator can also work backward from a known conception date or estimate gestational age and trimester transitions.

This calculator uses LMP-based dating, the most common approach in early pregnancy. For pregnancies confirmed via early ultrasound (especially before 14 weeks), the ultrasound-based EDD may be more accurate and is what your healthcare provider will use. If LMP and ultrasound dates disagree by more than a week, the ultrasound typically wins.

When to use this calculator

To estimate your due date before your first prenatal appointment
To plan ahead for maternity leave, baby prep, and major life decisions
To estimate gestational age at a future date (e.g., "how far along will I be at our anniversary?")
To track which trimester you're in and when transitions happen
To estimate when fetal milestones occur (heartbeat, movement, viability)
To time pre-pregnancy planning (when to conceive for a specific target due date)

Naegele's Rule — the standard formula

Developed by German obstetrician Franz Naegele in 1812, this formula is still the default for due date estimation worldwide.

EDD = LMP + 280 days Or equivalently: EDD = LMP − 3 months + 7 days + 1 year

Variable definitions

EDD = Estimated Due Date
LMP = First day of last menstrual period (last bleeding before pregnancy began)
280 days = 40 weeks; the average duration from LMP to delivery for a 28-day cycle

Critical assumptions: Naegele's Rule assumes a 28-day menstrual cycle with ovulation on day 14. For people with longer cycles, the formula gives an EDD that's too early; for shorter cycles, too late. Adjustments for non-standard cycles add or subtract days based on the difference from 28.

Step-by-step example

Say your last menstrual period started on January 15, 2026. Using Naegele's Rule:

Method 1 (add 280 days):

January 15 + 280 days = October 22, 2026

Method 2 (subtract 3 months, add 7 days, add 1 year):

January 15 − 3 months = October 15, 2025
October 15 + 7 days = October 22, 2025
October 22 + 1 year = October 22, 2026

Both methods yield October 22, 2026 as the estimated due date.

Pregnancy timeline from this EDD:

Week 1 (gestational age, by convention): Week of LMP (Jan 15)
Week 4: Conception likely occurred in this week (around Jan 29)
Week 12 (end of 1st trimester): Approximately April 9, 2026
Week 20 (anatomy scan): Approximately June 4, 2026
Week 27 (start of 3rd trimester): Approximately July 23, 2026
Week 37 (full term begins): Approximately October 1, 2026
Week 40 (EDD): October 22, 2026
Week 42 (post-term): November 5, 2026

LMP vs. conception dating

LMP-based (standard)

Counts pregnancy from the first day of the last period. By this convention, "week 1" is actually the week before conception. Most obstetric and pediatric standards use this counting method, partly because LMP is usually known and conception date is usually estimated.

Conception-based

Counts from the estimated conception date (typically about 14 days after LMP). This produces gestational ages 14 days less than LMP-based dating. A fetus at "10 weeks gestational age" by LMP is at "8 weeks from conception."

When healthcare providers reference fetal weeks, they almost always mean gestational age (LMP-based) unless they specifically say "post-conception." This is the convention you'll see in most pregnancy literature and on ultrasound reports.

Ultrasound dating and when it overrides LMP

First-trimester ultrasound (especially 8–13 weeks) can measure fetal crown-rump length with enough precision to estimate gestational age within ±5–7 days. When the ultrasound EDD differs from the LMP EDD, modern obstetric practice typically follows specific criteria for which date to use as the "official" EDD:

Before 9 weeks gestational age (LMP): Ultrasound dating used if it differs from LMP by more than 5 days
9–15 weeks 6 days: Ultrasound used if difference exceeds 7 days
16–21 weeks 6 days: Ultrasound used if difference exceeds 10 days
22–27 weeks 6 days: Ultrasound used if difference exceeds 14 days
After 28 weeks: LMP retained even with larger differences (later ultrasounds become less reliable for dating)

If you don't remember your LMP date or it's uncertain, an early ultrasound is the most accurate way to establish a due date.

Adjusting for non-standard cycles

Naegele's Rule assumes a 28-day cycle. Cycles longer than 28 days mean ovulation occurs later, so conception was later than the formula assumes — meaning the due date should be later too.

Adjusted EDD = Standard EDD + (Cycle Length − 28)

A person with a 32-day cycle has an EDD approximately 4 days later than Naegele's Rule predicts. A person with a 24-day cycle has an EDD 4 days earlier. The calculator can adjust for cycle length if your cycles are consistently outside the standard range.

Note for irregular cycles: If your cycles vary substantially (PCOS, hormonal issues, recent contraception, breastfeeding), LMP-based dating is unreliable. Early ultrasound is the better dating method.

Real-world use cases

1. Maternity/paternity leave planning

The EDD anchors leave planning, but realistic planning assumes a four-week window. If you need to give an employer a single date, the EDD is reasonable. If you can offer a range (37–41 weeks), that's more accurate.

2. Childcare arrangements

Most daycares require a few months of advance notice. Knowing your EDD lets you secure a spot starting 6–12 weeks after delivery, accounting for the actual arrival being a couple of weeks earlier or later than predicted.

3. Medical appointment scheduling

Prenatal visit schedules follow standard intervals from gestational age: every 4 weeks until 28 weeks, every 2 weeks until 36 weeks, then weekly. Knowing your due date lets you map out the appointment cadence in advance.

4. Anatomy scan timing

The "anatomy scan" — a detailed second-trimester ultrasound — is typically scheduled between 18 and 22 weeks. This is also when sex can be determined and structural development is best evaluated. Knowing your EDD lets you target this window.

5. Glucose tolerance and Tdap timing

Routine pregnancy tests have specific gestational windows: glucose tolerance test typically 24–28 weeks, Tdap vaccine 27–36 weeks, Group B strep screening 36–38 weeks. The calculator helps anticipate when these will occur.

6. Trying to conceive for a specific target

Working backward: if you want a baby born in spring, you'd conceive in summer. The calculator can run in reverse from a target due date to suggest conception/LMP timing.

The statistical reality of due dates

A due date isn't a prediction of arrival — it's the median of a distribution. Research from large cohort studies shows:

4–5% of babies are born on their exact due date
~70% of babies arrive within the week before or after the due date
~10% arrive before 37 weeks (preterm)
~5% arrive after 42 weeks (post-term, when induction is usually offered)
First pregnancies tend to deliver slightly later than later pregnancies (by about 5 days, on average)

This is why mature obstetric care treats the EDD as a midpoint, not a deadline. The "full-term" window of 37–41 weeks is the relevant range; deliveries within it are not early or late, just normal variation.

Common mistakes

Treating the EDD as a deadline. Less than half of babies arrive within 3 days of their EDD in either direction. The EDD is a midpoint, not a target.
Using LMP dating with irregular cycles. Naegele's Rule assumes a 28-day cycle. If yours varies by more than a few days month to month, an early ultrasound is more reliable.
Confusing gestational age with conception age. "12 weeks pregnant" (gestational, LMP-based) is the same as "10 weeks post-conception." Both are correct; they measure different things.
Forgetting the LMP date. If you're trying to conceive or just discovered pregnancy, write down your LMP date. It's the most important input for early prenatal care. If you don't remember, an early dating ultrasound provides equivalent information.
Comparing your timeline to other people's. Every pregnancy varies. Some women feel movement at 16 weeks, others at 24. Some show at 12 weeks, others at 20. Neither extreme is automatically a concern — variation is normal.
Skipping early ultrasound when LMP is uncertain. If you're not sure when your last period was, an early dating ultrasound is the standard of care and should be requested.
Booking non-refundable travel near the EDD. Babies arrive in a window. Booking expensive travel for the EDD week is a recipe for missed plans. Either travel well before 36 weeks or after the baby has arrived.

Frequently asked questions

How accurate is a due date calculator?

The calculator is mathematically exact — it gives a specific date from your inputs. The real-world accuracy depends on cycle regularity and the accuracy of the LMP date. Most LMP-based EDDs are within 7–10 days of actual delivery for women with regular cycles. Ultrasound-based EDDs in the first trimester are typically within 5–7 days.

What if I don't remember my LMP?

Tell your healthcare provider as soon as possible. They'll order an early dating ultrasound, which establishes the EDD nearly as accurately as known-LMP dating. Don't guess at the LMP — an inaccurate guess can cause later confusion in care.

How is pregnancy 40 weeks if conception is 38 weeks ago?

Convention. Obstetric practice counts pregnancy from LMP, which is roughly 14 days before conception. This is called "gestational age." The "post-conception" or "fertilization age" is 2 weeks less. Both are correct measurements; they differ in their starting point.

What's the difference between EDD and EDC?

EDD (Estimated Due Date) and EDC (Estimated Date of Confinement) mean the same thing. EDC is the older term, still used by some healthcare providers. Both refer to the predicted delivery date.

Can twins arrive on the EDD?

Most twin pregnancies deliver before 37 weeks, even when the EDD calculation suggests 40. Twin and higher-order multiple pregnancies require specialized prenatal care and different gestational age expectations.

What if my baby is late?

Most healthcare providers continue routine monitoring through 41 weeks and discuss induction options around 41–42 weeks. Some discomforts in the final weeks are normal; warning signs to contact your provider include reduced fetal movement, bleeding, severe headache, vision changes, or abdominal pain.

Should I trust LMP or ultrasound dating?

In early pregnancy (before 14 weeks), trust the ultrasound if it differs from LMP by more than 5–7 days. After 14 weeks, dating becomes less precise. After 28 weeks, LMP-based dating is usually retained even if ultrasound suggests a different date. Your healthcare provider will assign one "official" EDD that's used throughout the pregnancy.

What does "full term" mean now?

The American College of Obstetricians and Gynecologists (ACOG) categorizes term deliveries: Early Term (37 0/7 to 38 6/7 weeks), Full Term (39 0/7 to 40 6/7 weeks), Late Term (41 0/7 to 41 6/7 weeks), Post-Term (≥42 0/7 weeks). The "39 weeks rule" reflects that babies born at 39+ weeks generally have better outcomes than those at 37 or 38 weeks — so elective inductions before 39 weeks are now discouraged without medical indication.

Related Calculixy calculators

From the editorial desk: Pregnancy resists scheduling, and a due date calculator handles that resistance by giving you a single date that's the best guess available. Use it for planning. Don't use it for arrival expectations. The most useful thing an EDD can do is anchor your prenatal care timeline — appointment schedules, screening tests, milestone scans — while leaving the actual birth to happen somewhere in a four-week window around it.

Calculixy's Due Date Calculator provides estimates for educational use only and does not constitute medical advice or diagnosis. Pregnancy dating involves clinical judgment that combines LMP, ultrasound measurements, and individual factors. Consult a licensed obstetric provider (OB-GYN, midwife, or family practice with obstetric training) for personalized pregnancy care. If you experience any concerning symptoms during pregnancy, contact your provider immediately. All calculations run locally in your browser; Calculixy does not collect or transmit the values you enter.