Pregnancy Due Date Calculator: When Is Your Baby Really Coming?

By Ankit Gupta Published May 1, 2026

The first question every newly pregnant woman asks her doctor and Google is "When will my baby arrive?" The Estimated Due Date (EDD) is not a fixed deadline but a 40-week projection from your last menstrual period (LMP). A free Pregnancy Due Date Calculator gives you the same number an OB-GYN would calculate, in seconds, using LMP, conception date, IVF transfer date, or ultrasound dating. In this guide you'll learn the three standard methods, see two worked examples, and understand why only about 4% of babies actually arrive on their predicted date.

Why Due-Date Estimation Trips Most People Up

The biggest confusion is that pregnancy is measured from the first day of the last menstrual period (LMP), not from conception. So when a doctor says you're "8 weeks pregnant," the embryo is really about 6 weeks old. Naegele's Rule the formula used since 1812 assumes a 28-day cycle with ovulation on day 14, which doesn't fit every woman. If your cycle is 32 days, Naegele's Rule overestimates due date by 4 days. Ultrasound dating in the first trimester is more accurate but still has a 57 day window.

What Is an Estimated Due Date?

The Estimated Due Date (EDD), also called the Expected Date of Delivery, is the projected day a full-term baby (40 completed weeks) will be born. The American College of Obstetricians and Gynecologists (ACOG) defines full-term as 37 to 41 weeks and 6 days, so any day in that window is healthy and normal. The World Health Organization uses the same 40-week reference but notes that EDD is a midpoint, not a guarantee.

Only about 5% of babies arrive on the predicted date. About 50% arrive within a week before or after, and 90% within 2 weeks of the EDD. Babies born before 37 weeks are preterm; after 42 weeks, postterm both are tracked carefully by the obstetrician. The EDD is therefore a planning tool for maternity leave, baby showers, and clinical scheduling, not a delivery promise.

The Formula and Method

Three formulas dominate clinical practice:

1. Naegele's Rule (LMP-based)

EDD = LMP + 280 days
    = LMP  3 months + 7 days + 1 year
    ```
    
    ### 2. Conception-Date Method
    
    ```
    EDD = Conception date + 266 days (38 weeks)
    ```
    
    ### 3. Ultrasound Dating (CRL-based)
    
    ```
    EDD = ultrasound date + (40 weeks  gestational age measured)
    ```
    
    Variables you'll work with:
    
    | Variable | Meaning | Notes |
    |---|---|---|
    | LMP | First day of last menstrual period | Most common starting point |
    | Cycle length | Typical days between periods | Adjust if not 28 |
    | Conception date | Day of ovulation/fertilization | Often LMP + 14 |
    | CRL | Crown-rump length (ultrasound) | Used in weeks 614 |
    | Gestational age | Weeks since LMP | "How pregnant you are" |
    | Fetal age | Weeks since conception | LMP age minus 2 |
    
    Follow these seven steps to estimate accurately:
    
    1. Note the **first day** of your last period  not the last day.
    2. 2. Confirm your typical cycle length (most are 28; adjust if 2535).
    3. 3. If using conception date (IVF transfer, known ovulation), add 266 days.
    4. 4. Subtract 3 calendar months from LMP, then add 7 days and 1 year.
    5. 5. If LMP is uncertain, prioritize first-trimester ultrasound dating.
    6. 6. Recheck EDD against ultrasound at 813 weeks  that's the gold standard.
    7. 7. Track week-by-week milestones, not exact-day expectations.

## Worked Example #1: LMP-Based EDD with Naegele's Rule

LMP: **February 14, 2026**, cycle length 28 days.

| Step | Calculation | Result |
|---|---|---|
| Start | LMP | 14 Feb 2026 |
| Subtract 3 months |  | 14 Nov 2025 |
| Add 7 days |  | 21 Nov 2025 |
| Add 1 year |  | **21 November 2026** |

So the EDD is **21 November 2026**. The "term window" runs from 31 October 2026 (37 weeks) to 5 December 2026 (42 weeks). At a routine 12-week ultrasound around 9 May 2026, the doctor will measure CRL and may shift the EDD by a few days if needed. For a woman with a 32-day cycle, Naegele's Rule would overestimate by 4 days, so a corrected EDD would be 25 November 2026.

## Worked Example #2: IVF Transfer Date

For an IVF pregnancy with a Day-5 blastocyst transfer on **15 March 2026**, the conception date is treated as 5 days before transfer = 10 March 2026.

| Step | Calculation | Result |
|---|---|---|
| Conception date | Transfer  5 days | 10 Mar 2026 |
| Add 266 days |  | **1 December 2026** |

The EDD for this IVF pregnancy is **1 December 2026**. IVF dating is the most accurate of all methods because both conception date and embryo age are known exactly. The term window is 9 November to 14 December 2026.

## Common Mistakes to Avoid

- **Counting from "missed period" instead of LMP.** The LMP is the start of the period, not the day it was due to start.
- - **Assuming a 28-day cycle.** Add or subtract days proportional to your actual cycle length.
- - **Ignoring first-trimester ultrasound.** It's more accurate than LMP if your cycles are irregular.
- - **Treating EDD as a deadline.** Only 5% of babies arrive on the predicted day.
- - **Mixing fetal age and gestational age.** Most apps and doctors use gestational age (LMP-based).
- - **Adjusting for time zones.** Conception isn't precise enough for time-zone math; calendar days are fine.

## How to Use the AllSmartCalculators Due Date Tool

Open the [Pregnancy Due Date Calculator](https://allsmartcalculators.com/health/pregnancy-due-date-calculator), pick your calculation method (LMP, conception, or ultrasound), enter the relevant date, and adjust cycle length if needed. The tool returns EDD, current gestational week, trimester, and what's happening with baby's development this week. You'll also see the full-term window (3741w6d), key prenatal appointment dates, and a one-tap export to your calendar. It's the same arithmetic your OB-GYN uses but with the cycle-length adjustment Naegele's Rule misses.

## Related Calculators You'll Find Useful

- [Ovulation Calculator](https://allsmartcalculators.com/health/ovulation-calculator)  find your fertile window
- - [Pregnancy Week Calculator](https://allsmartcalculators.com/health/pregnancy-week-calculator)  track weeks and milestones
- - [Conception Date Calculator](https://allsmartcalculators.com/health/conception-date-calculator)  reverse-calculate from EDD
- - [BMI Calculator](https://allsmartcalculators.com/health/bmi-calculator)  pre-pregnancy weight assessment
- - [Age Calculator](https://allsmartcalculators.com/utility/age-calculator)  for the baby's age post-birth

## Frequently Asked Questions

### How accurate is a pregnancy due date calculator?

LMP-based calculators are accurate within 1 week if your cycle is regular and you know your LMP precisely. First-trimester ultrasound dating is the gold standard, accurate within 57 days. Only about 5% of babies arrive exactly on the EDD, but 90% arrive within 2 weeks of it. Treat EDD as a midpoint, not a deadline.

### What's the difference between gestational and fetal age?

Gestational age is measured from the first day of your last menstrual period (LMP)  what doctors and apps use. Fetal age is measured from conception, which is roughly 2 weeks later. So "8 weeks pregnant" means 8 weeks gestational age but about 6 weeks fetal age. All routine pregnancy milestones use gestational age.

### Can I calculate due date from conception date?

Yes. If you know the conception date (often from IVF or an LH test confirming ovulation), add 266 days to get the EDD. This is more accurate than LMP-based estimates if your cycle is irregular. For IVF transfers, count back from transfer day based on embryo stage (5 days for blastocyst, 3 days for cleavage stage).

### How is the EDD adjusted if my cycle is not 28 days?

For each day longer than 28, add that many days to the EDD; for each day shorter, subtract. So a 32-day cycle adds 4 days to a Naegele-based EDD. The AllSmartCalculators tool does this automatically when you set cycle length. After 13 weeks, ultrasound dating overrides LMP if there's more than a 7-day discrepancy.

### When should I get my first ultrasound to confirm the due date?

Most OB-GYNs schedule the first dating ultrasound between 8 and 13 weeks of gestation. The crown-rump length (CRL) measured during this window is the most accurate predictor of due date  within 5 days for ages under 9 weeks and within 7 days for 914 weeks. After 14 weeks, ultrasound dating becomes less precise.

### Is the due date the same as the delivery date?

No. The Estimated Due Date is a midpoint of a 5-week window in which the baby is considered full-term (3741 weeks). Only about 5% of babies are born exactly on their EDD. Half arrive in the week before or after, and almost all arrive within 2 weeks of the EDD. Anything before 37 weeks is preterm and may need medical attention.

## Final Thoughts & Next Steps

Your due date is a planning anchor, not a calendar entry. Pick the most reliable input (IVF transfer, ultrasound, or LMP if cycles are regular), use a calculator that adjusts for cycle length, and treat the EDD as the middle of a 5-week window. Open the [Pregnancy Due Date Calculator](https://allsmartcalculators.com/health/pregnancy-due-date-calculator) now to see your EDD, the trimester you're in, and what's happening with baby this week. Bookmark the [Pregnancy Week Calculator](https://allsmartcalculators.com/health/pregnancy-week-calculator) for weekly check-ins.

> **Disclaimer:** This article is for general educational purposes only and is not medical advice. Always consult a qualified obstetrician or healthcare provider for prenatal care, due-date confirmation, and any pregnancy-related concerns.