When you visit a gynaecologist after a positive pregnancy test, one of the first questions they ask is: "What was the date of your last menstrual period?" From that date — and sometimes from an ultrasound — your doctor calculates your Estimated Due Date (EDD).
Understanding how the EDD is calculated helps you know how reliable it is, why it might change after an early ultrasound, and what "term" actually means.
Why "Due Date" Is an Estimate, Not a Deadline
Only about 5% of babies are born on their exact EDD. Normal term delivery spans a window from 37 weeks 0 days to 42 weeks 0 days. The EDD represents the midpoint of this window — statistically, half of first-time mothers deliver before it and half after.
The EDD is critical for:
- Monitoring fetal growth against gestational age milestones
- Scheduling antenatal tests (NT scan at 11–13 weeks, anomaly scan at 18–20 weeks)
- Identifying pregnancies that may need early delivery or induction
- Planning maternity leave
Method 1: Naegele's Rule (LMP-Based)
Naegele's Rule is the oldest and most widely used method for calculating EDD. It was described by German obstetrician Franz Karl Naegele in 1812 and remains the default in Indian hospitals.
Formula: EDD = Date of Last Menstrual Period + 7 days + 9 months
Or equivalently: EDD = Date of Last Menstrual Period + 280 days (40 weeks)
Example
LMP: September 1, 2025
- Add 7 days: September 8, 2025
- Add 9 months: June 8, 2026 ✓
Assumptions of Naegele's Rule
This rule assumes:
- A 28-day menstrual cycle (the average)
- Ovulation occurs on Day 14 of the cycle
- Fertilisation occurs on the day of ovulation
For women with irregular cycles or cycles significantly longer or shorter than 28 days, the LMP-based EDD will be inaccurate.
Adjusted Rule for Longer/Shorter Cycles: EDD (adjusted) = Standard EDD ± (Actual cycle length − 28 days)
Example: If your cycle is 35 days, add 7 days to the standard EDD.
Method 2: Conception Date Method
If you know your conception date (from ovulation tracking, IVF transfer date, or a timed cycle), the EDD is:
EDD = Conception Date + 266 days (38 weeks)
This is more accurate than the LMP method for women who ovulate later or earlier than Day 14.
IVF Pregnancies: For IVF, the EDD is calculated from the egg retrieval date:
- Day 3 transfer: Add 266 days from retrieval
- Day 5 transfer (blastocyst): Add 261 days from retrieval
Method 3: Ultrasound Dating (Most Accurate)
The most accurate method of establishing gestational age is an early ultrasound — ideally between 7 and 13 weeks. During this window, the sonographer measures:
- Crown-Rump Length (CRL) between 7–13 weeks — the distance from the top of the fetal head to the bottom of the spine. This has a margin of error of only ±5–7 days.
After 13 weeks, the CRL becomes less reliable because fetal growth begins to vary more between individuals. Instead, doctors measure:
- Head Circumference (HC) — after 14 weeks
- Biparietal Diameter (BPD) — the width of the skull
When Does Ultrasound Change the EDD? Indian obstetric guidelines (similar to RCOG guidance followed by most Indian hospitals) recommend revising the LMP-based EDD with the ultrasound EDD when:
- The first-trimester CRL is available, and
- The ultrasound EDD differs from the LMP EDD by more than 7 days (first trimester) or 14 days (second trimester)
The Three Trimesters and Key Milestones
| Trimester | Weeks | Key Events |
|---|---|---|
| First | 0–13 weeks | Implantation, embryo formation, heartbeat detectable at ~6 weeks, NT scan at 11–13 weeks |
| Second | 14–27 weeks | Anomaly scan at 18–20 weeks, fetal movements felt, growth acceleration |
| Third | 28–40 weeks | Rapid weight gain, lungs mature, GBS screening, delivery preparation |
Term milestones:
- Preterm: Before 37 weeks
- Early term: 37–38 weeks 6 days
- Full term: 39–40 weeks 6 days
- Late term: 41–41 weeks 6 days
- Post-term: 42 weeks and beyond (requires medical evaluation for induction)
Calculate Your Due Date
Our Pregnancy Due Date Calculator calculates your EDD from your LMP using Naegele's Rule, shows your current gestational week, and lists key trimester milestone dates. No sign-in required.
For IVF pregnancies or when you know your conception date, use the "Conception Date" mode in the calculator.
This article is for informational purposes only. Consult your gynaecologist or obstetrician for personalised medical guidance.