EDD Calculator — Estimated Date of Delivery
Calculate your estimated due date using your last menstrual period (LMP) or ultrasound examination data. Find out how far along you are and which trimester you're in.
What Is the EDD Calculator?
The Estimated Date of Delivery (EDD) calculator helps expectant mothers and healthcare providers estimate when the baby is likely to be born. While only about 5% of babies are born exactly on their due date, the EDD serves as an important planning milestone and helps track fetal development.
How to Calculate EDD
There are two main methods to calculate the estimated date of delivery:
1. Last Menstrual Period (LMP) Method — Naegele's Rule
This is the most common method, using Naegele's Rule:
This rule assumes that pregnancy lasts 280 days (40 weeks) from the first day of the last menstrual period and that all months last 30 days. For cycles different from 28 days, an adjustment is made:
2. Ultrasound (US) Method
Ultrasound provides a more precise estimate, especially in the first trimester. It determines gestational age through measurements including:
- First trimester: Crown-rump length (CRL)
- Second trimester: Biparietal diameter, head circumference, femur length, abdominal circumference
- Third trimester: Same as second trimester, but less reliable
The EDD is calculated as:
How Far Along Am I?
Gestational age is measured from the first day of your last menstrual period. This means that at the time of conception (about 2 weeks after LMP), you are already considered to be 2 weeks pregnant. Gestational age is expressed in weeks and days (e.g., "12 weeks and 3 days" or "12+3").
When Should My Doctor Redate My EDD?
According to the American College of Obstetricians and Gynecologists (ACOG), the EDD should be redated if the discrepancy between the LMP-based and ultrasound-based dates exceeds certain thresholds:
| Gestational Weeks | Discrepancy for Redating |
|---|---|
| < 9 weeks | More than 5 days |
| < 16 weeks | More than 7 days |
| < 22 weeks | More than 10 days |
| < 28 weeks | More than 14 days |
| ≥ 28 weeks | More than 21 days |
Why Is There a Difference Between Gestational Ages?
A small difference between LMP-based and ultrasound-based gestational ages is normal. However, significant discrepancies may indicate:
- First trimester: Miscalculated LMP date or ovulation on a non-typical day
- Second trimester: Similar reasons; consult your doctor
- Third trimester: Possible IntraUterine Growth Restriction (IUGR), which could be caused by placental insufficiency, viral infection, or genetic disorders
Frequently Asked Questions
How accurate is the due date?
Only about 5% of babies are born on their exact due date. Most babies are born within 1-2 weeks before or after the EDD. The due date is most accurate when calculated using first-trimester ultrasound measurements.
Can my due date change?
Yes. Your doctor may redate your EDD based on ultrasound findings if the discrepancy exceeds ACOG guidelines. However, redating in the third trimester should be done cautiously, as different dates may indicate growth issues.
What if I don't know my last period date?
If you don't remember your LMP, an early ultrasound (preferably in the first trimester) can provide an accurate estimate of gestational age and due date. First-trimester ultrasound dating within the first 13 weeks is the most reliable.
What are the three trimesters?
The first trimester spans weeks 1–12, the second trimester weeks 13–26, and the third trimester weeks 27–40. Each trimester involves distinct developmental milestones for the fetus.