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Gestational Age Calculator

Created by Joanna Michałowska, PhD candidate and Dominika Śmiałek, MD, PhD candidate
Reviewed by Bogna Szyk and Jack Bowater
Based on research by
Committee Opinion No 700: Methods for Estimating the Due Date; Obstetrics and gynecology; May 2017See 1 more source
Gardosi, J., Vanner, T., & Francis, A. (1997). . Gestational age and induction of labour for prolonged pregnancy; British journal of obstetrics and gynaecology; July 1997
Last updated: Jan 18, 2024

Gestational age calculator is a simple pregnancy calculator that will help you to determine how far along you are using just one piece of information - your estimated due date (EDD). It should be determined by your doctor or can be easily calculated.
In the paragraphs below, you will find information on not only how to calculate gestational age from the estimated due date but also how to do it and why knowing fetal age is so important.

We try our best to make our Omni Calculators as precise and reliable as possible. However, this tool can never replace a professional doctor's assessment. All information on this website is for informational purposes only and is not intended to serve as a substitute for medical consultation. Always consult your results with a health care provider.

What is gestational age?

Gestational age (GA) is a common term used to describe the age of a pregnancy. In other words, this is the amount of time that the baby has been in the mother's belly. It is measured in weeks, usually by calculating the time difference between the first day of the woman's last menstrual cycle and the current date. According to this method, a baby that has just been conceived (approximately two weeks after the last period) is already two weeks old in gestational age. The corresponding date of gestation can also be estimated by other methods. This particular calculator will show you how to calculate gestational age from the estimated due date.
Not sure if you know your estimated due date? Use our due date calculator or IVF due date calculators.

What is estimated due date?

This term defines the estimated date of delivery for a pregnant woman. Calculating due date by gestational date can be done in just two steps:

  1. Determine the point in time that is used as the basis for calculating gestational age, which is usually one of the following:
  • last menstrual period (LMP).
  • corresponding time determined by obstetric ultrasonography.
  • adding 14 days to the date of fertilization.
  1. Add the estimated gestational age at childbirth to this date, which, on average, is equal to 280 days (40 weeks).

Once you have established the time of delivery of your baby, you can calculate gestational age from the estimated due date!

How do we calculate gestational age?

Different gestational age calculators use different information to estimate the duration of pregnancy. The most popular methods include:

  • Counting the days since the beginning of the last normal period;
  • Assessing the gestational age by obstetric ultrasonography - our mean sac diameter and crown rump length calculator are really helpful in estimating the gestational age on the basis of the ultrasonography;
  • When the exact date of fertilization is known (in the case of in vitro), calculating the number of days since that point in time and adding 14 days;
  • Assessing gestational age by due date - this is the method this calculator uses and runs on the following calculations:

EDD - 280 days = date of LMP

current date - date of LMP = GA


  • EDD is the estimated due date.
  • LMP is the last menstrual period.
  • GA is the gestational age.

Once the estimated date of delivery has been calculated using one of the methods described above, we can calculate gestational age by the due date. First, by subtracting 280 days (average duration of the pregnancy) from the estimated due date, we can establish the date of the last menstrual period. Next, to determine the gestational age, calculate the difference between the current date and the date of the last menstrual period.

Please note that the end date (so today's date) is also included in the calculation.

You can also perform the calculations the other way round - if you know the fetal age (thanks to your doctor), you can calculate the due date using the gestational age.

Why should I use the GA calculator?

This fetal age calculator will allow you to obtain vital information which is needed to monitor your baby's development. Once the GA is known, it can be used to:

  • Control prenatal development;
  • Correctly schedule prenatal care;
  • Calculate the results of prenatal tests;
  • Classify childbirth (preterm, term, post-term);
  • Monitor pregnancy weight gain;
  • Identify infants at risk of adverse health events.

This estimated gestational age calculator is not the only tool that might be useful to monitor the course of your pregnancy. The conception calculator will help you to estimate the conception date of your child.


How do we measure the gestational age?

We can calculate the gestational age of a baby:

  • Using the last menstrual period date;
  • Assessing it with the ultrasound machine and fetal measurements; or
  • Applying the fertilization date, if we know it.

How do we measure the time in gestational age?

We usually measure the gestational age in weeks.

The entire pregnancy lasts around 40 weeks or 280 days.

What was the gestational age of the youngest preemie?

The youngest baby that survived premature birth was the Alabama boy named Curtis - his birth took place at 21 weeks and 1 day of gestation (148 days)!

The brave boy weighed only 420 g (14.8 oz) and set a Guinness World Record for the most premature baby.

Joanna Michałowska, PhD candidate and Dominika Śmiałek, MD, PhD candidate
Estimated due date
Current date
Gestational age
Time until the baby is born 👶
Time remaining
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