What is Gestational Age in Pregnancy?

What is Gestational Age in Pregnancy?

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 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 does gestational age mean?
  • How to calculate gestational age?
  • Is gestational age accurate?
  • Why do doctors add 2 weeks to pregnancy?
  • Why is gestational age important?
  • What is AGA in pregnancy?

What does gestational age mean?

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

Your gestational age by LMP is basically measuring the weeks starting from the first day of your last menstrual period (LMP).

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

How to calculate gestational age?

To work out your GA in pregnancy, 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.

Don’t worry, we’ve got all the deets on how to count pregnancy weeks here.

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. Cute!

IVF gestational age calculator

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

To determine IVF gestational age, the embryo’s age and the date of conception are used instead.

In other words, the gestational age of the embryo is measured from when the sperm and egg unite.

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.

Gestational age calculator by ultrasound

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.

Is gestational age accurate?

So, how accurate is gestational age?

Well, interestingly, when calculated by a medical professional, GA 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.

How accurate are ultrasounds in determining gestational age?

Studies place the accuracy of ultrasound GA at +/- 5 to 7 days

But again, this is strictly for ultrasounds performed in the first trimester (between weeks 9 and 13).

From the second trimester, the accuracy is far more variable.

Still, a first-trimester ultrasound is not always on the cards for every mama-to-be.

If that’s the case for you, experts recommend getting your scan as early in the second trimester as possible for the most accurate estimation.

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.

Why do doctors add 2 weeks to pregnancy?

It’s because GA is typically determined from the first day of the last menstrual period rather than the point of conception (when sperm actually fertilizes the egg).

And the reason for this is that LMP is a far more obvious starting point for parents-to-be than the date of conception.

In a neat 28-day cycle, ovulation would generally happen around day 14 (and conception soon after)—which is where you get the two-week ‘cushion.’

But menstrual cycles vary, much like fertile windows.

In fact, a recent study found that out of 1.5 million women, only 16.32% had a 28-day cycle.

Another cohort study found an even smaller percentage again, with only 13% of women fitting into the 28-day bracket.

Other reasons for discrepancy between lMP and ultrasound gestational age include being highly overweight or underweight, having diabetes, or even a young maternal age.

Really, the best way of measuring gestational age according to the American College of Obstetricians and Gynecologists (ACOG) is that first-trimester ultrasound.

This is where doctors can measure the embryo or fetus and determine the fetal age.

What is gestational age vs fetal age?

Gestational age, as we’ve seen, is worked out from the date of your last period, and it’s the number most of us use to describe how long a pregnancy is.

But fetal age is different: it’s actually measured from when conception happens (around ovulation).

Basically, fetal age (or embryonic age) is your baby’s actual age.

What is the average gestational age?

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

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

Why is gestational age important?

Basically, gestational age can help your healthcare practitioner work out if your baby is growing as expected and adjust your medical care as needed.

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.

And it enables you to track where you and baby are on your pregnancy journey (and when to pack your hospital bag).

Let’s be real, seeing baby’s changing size week by week is one of the most exciting parts of pregnancy.

What is AGA in pregnancy?

When your baby is born, your healthcare practitioner will carry out some health checks, including checking your baby’s gestational age.

To do this, they’ll look at some key things: 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:

If they’re “small” or “large,” it 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.

And the closer you are to finding your bump buddy on Peanut.


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