EDD Calculator - Estimated Date of Delivery

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Bogna Szyk and Jack Bowater
Based on research by
American College of Obstetricians and Gynecologists’ Committee on Obstetric Practice Methods for Estimating the Due Date (May 2017)See 2 more sources
Butt K, Lim K; DIAGNOSTIC IMAGING COMMITTEE. Determination of gestational age by ultrasound. Journal of Obstetrics and Gynaecology Canada (February 2014)Savitz DA, Terry JW Jr, Dole N, Thorp JM Jr, Siega-Riz AM, Herring AH. Comparison of pregnancy dating by last menstrual period, ultrasound scanning, and their combination. American Journal of Obstetrics and Gynecology (December 2002)
Last updated: Jan 09, 2023

EDD calculator helps you discover your Estimated Date of Delivery (EDD) with two different methods, and compares the results for you.

Once you finally got that positive pregnancy test, on the day estimated with the pregnancy test calculator, the next thing on your mind must have been - when will the baby arrive?

There are two main techniques for obtaining the EDD. Both are common medical practice, and both are used in this Estimated Date of Delivery calculator. In essence, the first one is based on the date of your Last Menstrual Period, and the second one relies on the results of an Ultrasound (Ultrasonography, US) examination. With the EDD calculator, you can compare these two estimations and check if your baby is healthy.

How to calculate EDD?

To take advantage of all the functions of this Estimated Date of Delivery calculator, you will need the following data:

  • First day of you Last Menstrual Period (LMP);
  • Date of your last Ultrasound (US) examination;
  • Gestational age predicted by your last Ultrasound examination; and
  • The average length of your menstrual cycle.

Don't remember when your last period was? Try our period calculator!

The LMP method is older and is based on a simple equation. The second technique uses the result of an ultrasound examination, and is more complicated as it starts with ultrasound measurements of different parts of the fetus's body to roughly determine its age.

How far along am I?

Upon providing all the data, you will receive two results: the fetus' LMP and US gestational ages. There might be a slight difference between them, which you shouldn't be worried about. However, if the disparity is significant, it might be a sign of a growth problem, or another severe illnesses. This issue is more severe the later the pregnancy is. You will find all the relevant details in the section below.

Update your pregnancy calendar!

Last Menstrual Period (LMP) EDD calculator

This EDD calculator uses Naegele's rule:

EDD = LMP + 1 year - 3 months + 7 days

The rule assumes that all the months last 30 days, and the pregnancy lasts 280 days.

Despite its simplicity, it has a few limitations. If your periods are irregular, the result might be misleading. The same problem applies to women who do not follow their menstrual cycle, or who became pregnant right after their previous pregnancy.

Ultrasound(US) EDD calculator

Gestational age is determined with the use of ultrasound equipment. Your OB/GYN will gather the following data during your pregnancy:

First trimester

  • crown-rump length (CRL) - measures the length of the fetus, from head to buttocks.

Second trimester

  • the biparietal diameter - the width of the fetus; head;
  • head circumference;
  • fundal height;
  • the femur length; and
  • abdominal circumference - the circumference of the fetus' belly.

Third trimester

  • the biparietal diameter;
  • head circumference;
  • the femur length; and
  • abdominal circumference.

An ultrasound machine can find all of the data required to answer to the vital question: "how far along am I"? The information discussed above is compared with population statistics, which allows for the precise estimation of the fetus' gestational age. The more advanced the gestational age of the fetus, the lower the measurement's reliability.

When using the EDD calculator in pregnancy, we shouldn't carelessly redate the EDD in the third trimester, as different EDD dates in this period may be indicative of growth issues, and may require an in-depth diagnosis.

Why is there a difference between my baby's gestational ages?

The Estimated Date of Delivery calculator supplies you with two different results - what does it mean if they're inconsistent?

A small difference between the two EDD is normal and shouldn't worry you.

  1. If the difference is significant and you're in your first trimester, it indicates that you probably miscalculated your Last Menstrual Period date, or that the ovulation happened on a different day than expected. It's not uncommon among women whose periods are irregular.

  2. If your first ultrasound check was during your second trimester, the general rule stays the same. Always consult your doctor about any doubts you may have!

  3. A significant difference in the third trimester is called an IntraUterine Growth Restriction (IUGR). In most cases, this state is caused by growth naturally slowing down -smaller parents tend to have smaller children. In some cases, however, a decreased fetus growth rate may inform us of a pathological process taking place in the uterus, such as:

  • placenta insufficiency;
  • viral infection; and
  • genetic disorders.

Increased size of a fetus may be caused by the fact that the baby had taller parents, but may also be caused by:

  • a significant pregnancy weight gain; and
  • maternal diabetes.

Remember, if your check-ups are frequent, your doctor will be able to track your baby's development. This allows the physician to intervene immediately in case of any abnormality.

Why my doctor had to redate my EDD?

The American College of Obstetricians and Gynecologists released guidelines for redating EDD:

Weeks of gestation

Discrepancy between LMP and US EDD that supports 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

Keep in mind that the doctor must first rule out all the possible pathological processes!

Łucja Zaborowska, MD, PhD candidate
Calculate your Esimated Date of Delivery with two different methods and compare the results! 👶
Current date
1. Based on the Last Menstrual Period
Your last period
2. Based on an Ultrasound
Date of the US
US weeks
US days
Menstrual cycle length
Your gestational age
LMP age
US age
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