What is Gestational Age in Pregnancy?

What is Gestational Age in Pregnancy?

Your pregnancy week by week

Curious about gestational age and how it’s worked out? Read our article to find out more about what it means for you and your growing baby.
What is gestation?

Well, gestation is essentially the amount of time between when you conceive and when you give birth.

So, when we talk about gestational age, we mean how far along a pregnancy is.

Knowing the gestational age of your baby helps your doctor to estimate your due date.

And it gives them a guideline for working out if your little one is growing as they should be.

So, let’s find out more.

In this article: 📝

  • What is the gestational age?
  • How do you calculate gestational age?
  • What is fetal age vs gestational age?
  • What is a “normal” gestational age?
  • Why is gestational age important?
  • Can you calculate gestational age when your baby arrives?

What is the gestational age?

Gestational age is the medical term for how far along your pregnancy is.

Your gestational age is measured in weeks, starting from the point of your last period.

Yep, that’s right, your gestational age actually starts before you’re technically pregnant.

How do you calculate gestational age?

To work out the gestational age of your baby, your doctor can use a combination of the date of your last period and a first-trimester ultrasound (also called the “dating” ultrasound!).

They’ll describe your baby’s gestational age in weeks and days – for example, 23 weeks and 4 days.

Not sure how to count pregnancy weeks? We’ve got you covered.

Or, if you’re trying to figure out your due date, all you need is the date of your last period and the length of your average menstrual cycle, and use our free due date calculator!

And once you’ve worked out your gestational age, you can then figure out how big baby is from our baby size calculator ‒ so cute!

Things are a little different if you’ve conceived via IVF or another form of ART (assisted reproductive technology).

In this case, the embryo’s age and the date of conception are used to calculate the gestational age instead

And because conception actually happened outside your body, under lab conditions, these calculations have a high level of accuracy.

The fertility team who helped on your pregnancy journey can use various formulas to calculate your baby’s gestational age, depending on the type of IVF you’ve had.

Is gestational age accurate?

Interestingly, gestational age as calculated by a medical professional can be off by as much as 18-36 days, according to this study in 2020.

But while there may be inaccuracies, using a first-trimester dating ultrasound to figure out your gestational age is still the most accurate method for now.

Can gestational age be off by 2 weeks?

Yes, having a gestational age calculated by a medical professional can still be inaccurate by 2 weeks.

And it’s not unheard of to be off by 18-36 days, either.

How does ultrasound determine gestational age?

Many doctors use a dating ultrasound to work out your gestational age by looking at the size of baby.

This is done between weeks 8-12 because baby’s development and growth is pretty uniform at this point ‒ toward the end of the second trimester and the third, that’s when baby’s growth pattern can change.

What is fetal age vs gestational age?

Gestational age, as we’ve seen, is worked out from the date of your last period.

It’s the number most of us use to describe how long a pregnancy is.

As in “I’ve hit 35 weeks – nearly there!”

But fetal age is different: it’s actually two weeks behind gestational age.

That’s because it starts at the time of conception, and conception happens around ovulation (about two weeks after your last period).

So fetal age specifically tells you the age of the fetus – it’s your baby’s actual age.

What is a “normal” gestational age?

On average, the total gestational period lasts between 37 and 42 weeks.

However, some babies can be born prematurely ‒ before 37 weeks.

So, gestational age is key to tracking where you are on your pregnancy journey, and when to pack your hospital bag!

Why is gestational age important?

Gestational age is important because it can help your healthcare practitioner work out if your baby is growing as they need to.

If your baby seems to be growing differently to their gestational age, they may need extra monitoring.

Gestational age also helps doctors to know when to do any prenatal tests and screenings.

Can you calculate gestational age when your baby arrives?

When your baby is born, your healthcare practitioner will carry out some health checks, and they’ll also check your baby’s gestational age.


To do this, they’ll look at some key things, like your baby’s length, weight, head circumference, skin condition, and vital signs.

They’ll also consider the condition of their hair, muscle tone, posture, and reflexes.

Depending on their results, your baby will be noted as: “appropriate for gestational age” (AGA), “small for gestational age” (SGA), or “large for gestational age” (LGA).

If they’re “small” or “large” that just means that your healthcare practitioner will keep an extra eye on them, in case of complications.

Because calculating your due date from your last period isn’t always accurate, confirming gestational age from an ultrasound in your first trimester can be really useful.

Saying that, it’s possible that your due date could actually be changed later on in pregnancy by your healthcare practitioner.

But, essentially, the more information you have and the more accurate it is, the more you’ll know what to expect for each milestone of your pregnancy.

Why not connect with other mamas-to-be on Peanut to be your Bump Buddies, due around the same time as you and share those milestones together?

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