How to Calculate Pregnancy Due Date (EDD)
Estimated Delivery Date calculation guide: Naegele's rule, LMP method, ultrasound dating, and gestational age.
Understanding Due Date Calculation
A normal human pregnancy lasts approximately 280 days or 40 weeks from the first day of your last menstrual period (LMP). The estimated due date (EDD) is calculated using this timeframe.
It's important to understand that the due date is an estimate. Only about 5% of pregnancies end on the exact due date. Most babies are born within 2 weeks before or after the due date.
Naegele's Rule (LMP Method)
The Formula:
Due Date = LMP + 280 days (or + 40 weeks)
Alternative: Add 9 months + 7 days to your LMP date
Example: If LMP is January 10, due date is October 17
Ultrasound Dating
During the first trimester ultrasound (8-14 weeks), doctors can accurately date the pregnancy within ±3-5 days. This is considered more accurate than LMP-based calculation, especially if you have irregular cycles.
Second and third trimester ultrasounds are less accurate (±2-3 weeks) as individual variation in fetal growth increases.
Gestational Age vs Chronological Age
Gestational Age:
Calculated from LMP (first day of last period). Typically 1-2 weeks higher than chronological age.
Chronological Age:
Actual age from conception. Usually determined by ultrasound.
Pregnancy Timeline Overview
| Trimester | Weeks | Key Events | Dating Accuracy |
|---|---|---|---|
| First Trimester | 0-13 weeks | Implantation, organ formation, first heartbeat | ±3-5 days (ultrasound) |
| Second Trimester | 14-26 weeks | Fetal growth, movement felt, viability starts | ±1-2 weeks (ultrasound) |
| Third Trimester | 27-40 weeks | Rapid growth, position changes, labor prep | ±2-3 weeks (ultrasound) |
| Post-term | 41-42 weeks | Labor induction usually recommended | Risk increases after 42 weeks |
Early Pregnancy Dating Methods Comparison
LMP Only (Naegele's Rule)
First Trimester Ultrasound
Second Trimester Ultrasound
Third Trimester Ultrasound
Combined LMP + Ultrasound
Important Adjustments to Due Date
Irregular menstrual cycles
Can shift due date by 1-3 weeks. Ultrasound dating is more reliable than LMP calculation.
Cycle longer than 28 days
Due date is pushed back. Use actual cycle length in Naegele's rule instead of standard 28 days.
Conception date known (assisted fertility)
Use conception date, not LMP. Add 266 days from conception (vs 280 from LMP).
First ultrasound shows discrepancy
Ultrasound becomes authoritative. Ignore LMP if ultrasound differs by >1 week in first trimester.
PCOS or other fertility conditions
Longer cycles expected. Work closely with doctor for accurate dating.
What to Expect Throughout Pregnancy
Weeks 4-8
Missed period, nausea, breast tenderness, fatigue
Weeks 8-12
Morning sickness peaks, frequent urination, mood swings
Weeks 12-16
Morning sickness improves, energy returns, baby bump visible
Weeks 16-20
Quickening (baby movement), cravings, back pain begins
Weeks 20-28
Braxton Hicks contractions, swelling, increased appetite
Weeks 28-36
Heartburn, shortness of breath, difficulty sleeping
Weeks 36-40
Baby drops lower, frequent urination returns, contractions increase
Common FAQs
What if I don't remember my LMP?
Early ultrasound (8-12 weeks) can date your pregnancy accurately. This becomes the reference for all future calculations. Ask your doctor for dating ultrasound.
Can I deliver before my due date?
Yes. Full-term pregnancy is 37-40 weeks. Any delivery after 37 weeks is considered full-term and safe. Doctors may induce labor after 40 weeks or immediately if complications arise.
What is the normal delivery window?
Most babies arrive between 37-42 weeks. Delivery before 37 weeks is premature. After 42 weeks, doctors typically induce labor due to increased risk of complications.
Does the due date change after ultrasound?
Yes, if ultrasound shows significant difference from LMP calculation. First trimester ultrasound is the most reliable dating method and often becomes the new reference.
Is my due date exact or a range?
It's an estimate. Only 5% of babies are born on exact due date. Think of it as a target week, not a specific day. Normal range is ±2 weeks from due date.
How often should I have ultrasounds?
Typically 3-4: dating scan (8-12 weeks), anatomy scan (18-22 weeks), growth scan if needed (28+ weeks). More frequent if high-risk or complications.
Related Concepts
Related Tools
Pregnancy Due Date Calculator
Calculate your estimated due date based on LMP or conception date.
Pregnancy Week Calculator
Track which week of pregnancy you're in and remaining days to due date.