Decoding the Meaning of GA vs AUA and EDD : Understanding Ultrasound Measurements, Fetal Age, and Estimated Due Dates

When navigating the exciting yet often confusing world of pregnancy, you’ll encounter terms like AUA, GA, EDD, and ultrasound.

These terms are pivotal in assessing fetal age, predicting due dates, and ensuring a healthy pregnancy journey.

But what do these abbreviations EDD AUA and GA mean, and how do they differ from one another?

This comprehensive guide will demystify these terms for you, providing a clearer understanding of what’s happening during those critical 40 weeks.

What Does AUA Mean on an Ultrasound?

AUA stands for “Adjusted Ultrasound Age.” This term is often used to describe the age of the fetus based on ultrasound measurements.

During an ultrasound, the size and development of the fetus are measured to calculate an average ultrasound age.

This is particularly useful if you’re not sure about the date of your last menstrual period, making it a reliable resource for dating your pregnancy.

What is GA (Gestational Age), and How is it Different from AUA?

Gestational Age (GA) is usually calculated from the first day of your last menstrual period (LMP). In contrast, AUA is based on ultrasound measurements.

GA is the actual time elapsed since the beginning of your last period, whereas AUA may vary based on the growth and development of the fetus.

In short, GA is more related to menstrual history, and AUA is derived from the ultrasound done to assess the fetus’s growth.

How is the Estimated Due Date (EDD) Determined?

The Estimated Due Date (EDD) is usually calculated based on your LMP or the most accurate estimate from an early ultrasound.

It often employs a basic formula that assumes a menstrual cycle of 28 days and ovulation occurring around 14 days from the first day of your last period.

The typical pregnancy lasts about 280 days, so this formula provides an estimated due date.

What Role Does the Last Menstrual Period (LMP) Play in Pregnancy Dating?

The LMP provides a foundation for calculating GA and, by extension, the EDD. An accurate LMP date can help healthcare providers manage your pregnancy with precise interventions, focusing care on the baby’s development.

However, LMP isn’t always reliable for everyone; in those cases, a dating ultrasound will be used to estimate date baby is due.

How Does Fetal Age Compare to GA vs AUA ?

While GA is calculated from the first day of your last menstrual cycle, fetal age is measured from the actual date of conception.

This age is usually two weeks less than the GA. AUA and fetal age are closer but may differ depending on the fetus’s growth rate.

How Accurate is the Conception Date?

The conception date is often a best guess unless a pregnant woman knows the exact day of ovulation and fertilization.

It’s an approximation used to offer additional insight into fetal development but is not usually considered as reliable as GA or AUA for due date calculations.

Why Do GA and AUA Sometimes Don’t Match?

GA and AUA may not match due to various factors like irregular menstrual cycles or uncertainties in the actual ultrasound age.

In such cases, healthcare providers may use the ultrasound date for a more accurate estimate if it varies significantly from the GA based on LMP.

How Do Ultrasound Measurements Affect Management of Pregnancy?

If your baby’s development doesn’t meet the ‘expected’ averages based on GA or AUA, care providers may put into place various interventions and management plans.

These could range from more frequent ultrasounds to medical treatment for the mother.

When Should You Have Your First and Subsequent Ultrasounds?

Your first ultrasound is usually done between 8 and 11 weeks gestation. Subsequent ultrasounds may be recommended based on the specific needs of your pregnancy, such as growth scans to assess the baby’s development further.

What are the Limitations and Issues That May Arise?

Ultrasound and other dating methods have limitations. For instance, ultrasounds later in pregnancy may not be as accurate in determining the fetus’s age because babies have a lot of growing and developing at different rates.

Below is a table showing the approximate Adjusted Ultrasound Age (AUA), Gestational Age (GA), and Estimated Due Date (EDD) per trimester.

The table assumes that the first day of the last menstrual period (LMP) was accurate and that ovulation occurred around day 14. It also assumes a 40-week pregnancy.

Trimester Weeks in Trimester Approximate AUA (Weeks) Approximate GA (Weeks) Approximate EDD (Weeks Remaining)
First Trimester 1-13 1-11 3-13 27-39
Second Trimester 14-26 12-24 14-26 14-26
Third Trimester 27-40 25-38 27-40 0-13


  1. AUA: The AUA may change based on ultrasound measurements and is typically close to GA but may differ based on individual growth rates of the fetus.
  2. GA: This is counted from the first day of your last menstrual period and is usually used as the primary method for dating the pregnancy.
  3. EDD: This is calculated by adding 40 weeks to the first day of your last menstrual period. The ‘Weeks Remaining’ is how many weeks are left until the estimated due date based on GA.
  4. The table is an approximation and individual cases may vary. Always consult with your healthcare provider for accurate information.


  • AUA is the Adjusted Ultrasound Age based on ultrasound measurements.
  • GA (Gestational Age) is calculated from the first day of your last menstrual period.
  • The Estimated Due Date (EDD) is derived either from LMP or an early, accurate ultrasound.
  • Fetal Age is the actual age of the fetus from the time of conception and is different from AUA and GA.
  • Conception Date is often less reliable for determining due dates.
  • Management of Pregnancy may change if AUA or GA indicate that the baby’s development is not on track.
  • First Ultrasound is generally done between 8 and 11 weeks, with additional scans as needed.
  • Both GA and AUA have limitations, and care providers may rely more heavily on one over the other based on individual circumstances.

Understanding these terms will not only make you well-versed in the language of pregnancy but also empower you to make informed decisions along the journey.

Frequently Asked Questions (FAQs)

What does AUA mean in an ultrasound?

AUA stands for “Adjusted Ultrasound Age.” It is the age of the fetus calculated based on measurements taken during an ultrasound scan. AUA provides a basis for evaluating the fetus’s growth and development.

How is GA different from AUA?

GA stands for “Gestational Age,” and it is calculated based on the first day of your last menstrual period (LMP). AUA, on the other hand, is derived from ultrasound measurements. GA measures the total time since the first day of the last period, while AUA may change based on the fetus’s rate of growth and development.

What is EDD, and how is it calculated?

EDD stands for “Estimated Due Date.” It is usually calculated using the date of the first day of your last menstrual period (LMP). The general formula assumes a menstrual cycle of 28 days with ovulation around day 14. Starting from the first day of the last period, 280 days (or 40 weeks) are added to get the EDD.

Can LMP be used for accurate dating?

LMP can be a reliable method for dating a pregnancy if the menstrual cycle is regular and the date of the last period is known accurately. However, if the menstrual cycle is irregular or the LMP is uncertain, an early dating ultrasound is usually recommended for more accurate dating.

Is fetal age the same as AUA or GA?

No, fetal age is calculated from the actual date of conception, which makes it usually about two weeks less than the gestational age. AUA and fetal age might be closer in terms but can still vary depending on individual growth rates of the fetus.

How reliable is the conception date?

The conception date is often a best guess and is less reliable than GA or AUA for determining due dates. Unless you know the exact date of ovulation and fertilization, the conception date is typically an approximation.

Why do GA and AUA sometimes not match?

GA and AUA can differ due to various reasons like irregular menstrual cycles or discrepancies in fetal growth. If there is a significant difference between the two, healthcare providers may rely more on the AUA obtained from an ultrasound for more accurate dating.

How can ultrasound measurements affect pregnancy management?

If ultrasound measurements suggest that the baby’s growth and development do not align with ‘expected’ averages, healthcare providers may initiate various interventions. This could range from additional ultrasounds to dietary and medical adjustments for the mother.

When should the first ultrasound be done?

Typically, the first ultrasound is performed between 8 and 11 weeks of gestation. However, the timing may vary based on medical necessity and individual circumstances.

Are there limitations to using ultrasound for dating?

Yes, ultrasounds later in pregnancy are generally less accurate for dating because babies grow and develop at different rates. That’s why early pregnancy ultrasounds are often more reliable for calculating EDD.

Understanding these FAQs can give you a better grasp of key terms and practices related to pregnancy dating, empowering you to engage more actively in your healthcare



This post is written and edited by Sandy who is a clinical pharmacist with over 20 years of experience specializing in pre-natal and post-natal care.