What Is the Estimated Due Date (EDD)?
The Estimated Due Date (EDD) — also called the due date or expected delivery date — is the predicted date your baby will be born. It corresponds to 40 weeks of gestation counted from the first day of your last menstrual period (LMP).
The 40-week count includes the 2 weeks before ovulation. Actual conception occurs around week 2, but medicine counts from the LMP because it is an objective, easily identifiable date that every person knows.
Naegele's Rule: The EDD Formula
Naegele's Rule, proposed by German obstetrician Franz Karl Naegele in 1812, remains the standard method for calculating the EDD:
EDD = LMP + 280 days
Practical calculation
To manually calculate your EDD:
- Take the first day of your last menstrual period (LMP)
- Add 1 year
- Subtract 3 months
- Add 7 days
Example:
- LMP: January 1, 2026
- Add 1 year → January 1, 2027
- Subtract 3 months → October 1, 2026
- Add 7 days → EDD: October 8, 2026
EDD and Menstrual Cycle Length
Standard Naegele's Rule assumes a 28-day cycle with ovulation on day 14. Cycles vary, so the EDD must be adjusted:
| Cycle length | EDD adjustment | |---|---| | 21 days (short cycle) | −7 days | | 28 days (standard) | No change | | 35 days (long cycle) | +7 days |
Our calculator automatically adjusts the EDD based on your cycle length.
Methods to Calculate Your EDD
1. From Last Menstrual Period (LMP)
The most widely used method. Requires knowing the exact first day of your last period. Accurate when cycles are regular.
2. From Conception Date
If you know the conception date (e.g., after a single intercourse or insemination), the EDD is calculated by adding 266 days (38 weeks):
EDD = Conception date + 266 days
3. First-Trimester Ultrasound
An ultrasound between weeks 10 and 13 is the most accurate method to confirm or revise the EDD. The crown-rump length (CRL) of the baby is measured. If it differs by more than 7 days from the calculated EDD, the doctor will update the official date.
EDD vs. Actual Birth Date
The EDD is an estimate, not an exact prediction:
- Only 4–5% of babies are born on their exact due date
- 90% of births occur between weeks 37 and 42
- Normal delivery happens between week 37 (early term) and week 41+6 (post-term)
If you reach week 41–42 without going into labor, your doctor will evaluate whether induction is necessary. It doesn't mean something is wrong — every baby has its own timeline.
Prenatal Care Based on Your EDD
Knowing your EDD lets you schedule key prenatal appointments:
| Week | Checkup / Exam | |---|---| | 6–10 | First prenatal visit, EDD confirmation | | 10–13 | First-trimester ultrasound, nuchal translucency | | 16 | Serum markers (optional) | | 18–22 | Anatomy ultrasound (morphology scan) | | 24–28 | Gestational diabetes screening | | 32–34 | Third-trimester ultrasound | | 36+ | Weekly checkups, birth preparation |
Frequently Asked Questions
Yes. EDD (Estimated Due Date), due date, expected delivery date, and FPP (Fecha Probable de Parto in Spanish) are all synonymous terms referring to the same 40-week calculation.
If you don't remember your exact last menstrual period date, a first-trimester ultrasound can estimate the EDD with great precision by measuring the size of the embryo. See your doctor as soon as possible.
Twin or multiple pregnancies may be managed differently. Although the EDD is calculated the same way, delivery is often planned earlier (weeks 37–38) to reduce risks. Your doctor will set the date based on your type of pregnancy.
Gestational age is the time elapsed since the first day of the last menstrual period. At 40 weeks gestational age, it coincides with the EDD. The baby actually has 38 weeks of real development (embryonic age), because conception occurs around gestational week 2.