What Is an Expected Date of Delivery (EDD)?
The Expected Date of Delivery — also called the due date, EDD, or EDC (Estimated Date of Confinement) — is the predicted date on which a baby is expected to be born.
It is calculated based on the first day of your Last Menstrual Period (LMP) and assumes a standard 28-day menstrual cycle with ovulation occurring on Day 14.
How Is the Due Date Calculated?
This calculator uses Naegele’s Rule — the standard formula used by obstetricians worldwide since the 19th century:
EDD = First Day of LMP + 9 months + 7 days
Or equivalently: EDD = First Day of LMP + 280 days (40 weeks)
Example: If your last period started on January 1, 2026:
- Add 9 months → October 1, 2026
- Add 7 days → October 8, 2026
- Your EDD = October 8, 2026
Understanding Your Pregnancy Week by Week
A normal, full-term pregnancy is divided into three trimesters spanning 40 weeks from the LMP:
First Trimester (Week 1–12) This is the most critical period of organ development. The embryo develops a heartbeat around Week 6. By Week 12, all major organs are formed. Morning sickness, fatigue, and breast tenderness are most common in this phase. Miscarriage risk is highest in the first trimester.
Second Trimester (Week 13–26) Most women feel the best during this phase. Morning sickness typically subsides. You’ll feel the baby’s first movements (quickening) between Weeks 18–22. This is also when the anatomy scan (anomaly scan) is performed, usually between Weeks 18–20.
Third Trimester (Week 27–40) The baby gains most of its weight in this phase. You may experience backache, shortness of breath, and difficulty sleeping as the uterus expands. The baby typically moves into a head-down position by Week 34–36.
Is the Due Date Exact?
No — and this is important to understand.
Only about 4% of babies are born on their exact due date. The EDD is an estimate based on statistical averages. A normal full-term birth can occur anywhere between Week 37 and Week 42.
- Before 37 weeks = Preterm birth (requires NICU monitoring)
- 37–38 weeks = Early term
- 39–40 weeks = Full term (optimal)
- 41–42 weeks = Late term
- After 42 weeks = Post-term (induction is usually recommended)
When Is the Due Date Adjusted?
Your calculated EDD may be revised by your doctor based on:
Ultrasound measurements: A first-trimester ultrasound (done between Weeks 8–13) is the most accurate way to date a pregnancy. If the ultrasound-based date differs from the LMP-based date by more than 7 days, most OB-GYNs will adjust the EDD.
Irregular menstrual cycles: Naegele’s Rule assumes a 28-day cycle. If your cycle is shorter (e.g., 24 days) or longer (e.g., 35 days), ovulation happens earlier or later than Day 14, which shifts the actual conception date. Your doctor will adjust accordingly.
IVF pregnancies: For IVF, the EDD is calculated from the egg retrieval date or embryo transfer date — not the LMP — making it more precise.
Important Milestones to Watch For
| Week | Milestone |
|---|---|
| 6–8 | First ultrasound — heartbeat detection |
| 11–13 | NT scan (Nuchal Translucency) — Down syndrome screening |
| 16–20 | Quadruple marker test (maternal serum screening) |
| 18–20 | Anomaly scan (detailed anatomy scan) |
| 24–28 | Glucose challenge test — gestational diabetes screening |
| 28 | Anti-D injection for Rh-negative mothers |
| 32–34 | Growth scan |
| 36 | GBS (Group B Streptococcus) swab |
| 40 | Due date — if no labour, induction discussion begins |
A Word From Dr. Aman
“New mothers often feel anxious when they go past their due date. I always remind my patients — the EDD is a landmark, not a deadline. Your baby’s lungs, brain, and gut are still maturing right up to 40 weeks and beyond. Unless there’s a medical indication, patience in those final days is usually the right call. That said, always stay in close contact with your OB-GYN after Week 40.”
Frequently Asked Questions
Q: My cycle is not 28 days. Is this calculator still accurate? Naegele’s Rule works best with a regular 28-day cycle. If your cycle is consistently different, mention this to your doctor. A first-trimester ultrasound will give you a more accurate date than any calculator.
Q: Can I use this calculator if I had IVF? For IVF, the EDD is calculated differently — from egg retrieval or embryo transfer date. Your fertility clinic will provide a specific EDD. This calculator is designed for natural conception based on LMP.
Q: What if I don’t remember my exact LMP date? An early ultrasound (before 13 weeks) can estimate gestational age based on the size of the embryo (crown-rump length). This is actually more accurate than LMP-based calculation in many cases.
Q: Is it safe to deliver before my EDD? Deliveries before 37 weeks are considered preterm and carry additional risks for the baby, including breathing difficulties and feeding problems. Between 37–39 weeks is early term and may be appropriate in certain medical situations. The safest window for delivery is 39–40 weeks unless medically indicated otherwise.
Q: My doctor’s due date is different from this calculator’s result. Which should I trust? Always trust your doctor’s date — especially if it’s based on an ultrasound. The calculator gives a useful starting estimate. Your doctor’s EDD, particularly if confirmed by first-trimester ultrasound, is the one to follow.
Content reviewed by Dr. Aman, MBBS. This calculator is for informational purposes only. Always consult your obstetrician or gynecologist for personalized pregnancy care and guidance.