Free EDD Calculator - Estimated Date of Delivery
Enter the first day of your last menstrual period to calculate your estimated due date.
Understanding the Estimated Date of Delivery (EDD)
The EDD calculator, commonly referred to as a pregnancy due date calculator, helps expectant parents determine when their baby might arrive. This tool provides two distinct estimates: one based on the first day of your Last Menstrual Period (LMP) and another derived from an ultrasound (US) examination. By presenting both results side by side, the calculator allows you to compare them and identify any significant discrepancies early in the pregnancy.
To make full use of this free EDD calculator online, you will need the following information:
- The first day of your LMP.
- The date of your most recent ultrasound.
- The gestational age reported during that ultrasound.
- The average length of your menstrual cycle.
The LMP Method and Naegele’s Rule
The LMP method relies on a well‑known formula called Naegele’s rule:
This calculation assumes a standard 28‑day menstrual cycle and a pregnancy lasting 280 days. In the formula, each month is treated as 30 days long. Despite its simplicity, the LMP method has clear limitations. Women with irregular cycles, those who are unsure of their LMP date, or those who conceived soon after a prior pregnancy may receive a less accurate estimate. In such situations, the ultrasound‑based due date is often considered more reliable.
The Ultrasound (US) Method
Ultrasound dating involves measuring specific parts of the fetus during routine scans:
First trimester
- Crown‑rump length (CRL) — from the top of the head to the buttocks.
Second trimester
- Biparietal diameter (BPD) — the width of the fetal head.
- Head circumference (HC).
- Femur length (FL).
- Abdominal circumference (AC).
- Fundal height (also recorded manually).
Third trimester
- Biparietal diameter, head circumference, femur length, and abdominal circumference are again assessed.
These measurements are compared against large population databases to estimate the gestational age. As the pregnancy advances, the reliability of ultrasound dating gradually declines. For this reason, doctors are cautious about changing the EDD during the third trimester; a large difference at that stage may indicate growth problems rather than a wrong due date.
Interpreting the Results
You will receive two gestational ages: one from your LMP and one from the ultrasound. A small difference between them is normal and not a cause for worry. However, a significant gap can signal something that needs attention.
- First trimester: A large difference often means the LMP date was misremembered or that ovulation occurred on an unexpected day. This is common among women with irregular cycles.
- Second trimester: The same logic applies. Any concerns should be discussed with your healthcare provider.
- Third trimester: A notable discrepancy is known as intrauterine growth restriction (IUGR) or, conversely, fetal overgrowth. Slower growth may be due to naturally smaller parents, but it can also stem from placental insufficiency, viral infections, or genetic disorders. Increased fetal size may be linked to substantial weight gain during pregnancy or maternal diabetes. Regular check‑ups allow your doctor to monitor the baby’s development and intervene if necessary.
When Should the EDD Be Redated?
The American College of Obstetricians and Gynecologists (ACOG) provides clear guidelines for when it may be appropriate to change the EDD based on the discrepancy between LMP and ultrasound dating:
| Gestational Age (weeks) | Discrepancy That Supports Redating (days) |
|---|---|
| < 9 | > 5 |
| < 16 | > 7 |
| < 22 | > 10 |
| < 28 | > 14 |
| ≥ 28 | > 21 |
Before any redating is performed, the doctor must rule out pathological causes. The table above is a reference; clinical judgment remains essential.
Important Reminder
This Estimated Date of Delivery calculator is intended as an educational and planning aid. It should never replace a professional evaluation by an obstetrician or midwife. If you have any concerns about your pregnancy, consult a qualified healthcare provider.
FAQ
1. How is the EDD calculated using the Last Menstrual Period method?
The LMP method uses Naegele's rule: EDD = LMP + 1 year - 3 months + 7 days. This assumes a 28‑day cycle and a 280‑day pregnancy. If your cycles are irregular, the result may be less reliable, and the ultrasound estimate is often preferred.
2. What measurements are taken during an ultrasound to determine gestational age?
In the first trimester, the crown‑rump length (CRL) is measured. During the second and third trimesters, the biparietal diameter, head circumference, femur length, and abdominal circumference are assessed. These values are compared with population norms to estimate fetal age.
3. What does a significant difference between the LMP‑based and ultrasound‑based due dates indicate?
A small difference is normal. A large gap in the first trimester often suggests an inaccurate LMP date or unexpected ovulation. In the third trimester, a significant discrepancy may point to intrauterine growth restriction (IUGR) or fetal overgrowth, possibly due to placental issues, infection, or maternal diabetes. Always consult your doctor.
4. When should a doctor consider redating the EDD according to ACOG?
ACOG recommends redating when the discrepancy exceeds a specific number of days depending on gestational age: more than 5 days before 9 weeks, more than 7 days before 16 weeks, more than 10 days before 22 weeks, more than 14 days before 28 weeks, and more than 21 days at 28 weeks or later. Pathological causes must be excluded first.
How to Use
- Enter the first day of your last menstrual period (LMP) in MM/DD/YYYY format.
- Select your average menstrual cycle length (defaults to 28 days).
- View your estimated due date, gestational age, current trimester, and estimated conception date instantly.