Gestational Age Calculator

Created by Joanna Michałowska, PhD candidate
Reviewed by Bogna Szyk and Jack Bowater
Last updated: Oct 27, 2021

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 estimated due date, but also how to do it, and why knowing fetal age is so important.

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 the gestation can be also estimated by other methods. This particular calculator will show you how to calculate gestational age from estimated due date.
Not sure if you know your estimated due date? Use our pregnancy due date 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 of the point in time that is used as the basis of calculating gestational age, which is usually one of the following:
  • last menstrual period (LMP).
  • corresponding time determined by obstetric ultrasonography.
  • adding 14 days to a 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 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 to estimate the gestational age on the basis of the ultrasonography.
  • when the exact date of fertilization is known (in 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 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, postterm)
  • monitor pregnancy weight gain
  • identify infants at risk of adverse health events.

Other pregnancy calculators

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. Are you having problems with counting days, or are you not sure if everything is adding up correctly? Don't hesitate and use the calendar calculator! If you had a c-section in the past and you want to know your chances of a successful vaginal birth, try the VBAC calculator.

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