6 Things to Know About Your Fetal Growth Ultrasound

You’ve had or you’re having a growth ultrasound where they’re telling you how large your baby is in comparison to other babies their age. What do you need to know about it, and your results.

Why I’m writing this post: I run a LOT of pregnancy social media and outreach and I constantly get people saying — I’m 27 weeks and my baby is 2 pounds 1 ounce, what does that mean moving forward? Today I want to share all that I can about what you need to know about those ultrasounds, so you can make your best steps moving forward.

As a note: when your provider starts doing more testing that NEEDS to be a trigger in your mind to be SURE to take a prenatal class (also called birth classes) is extra important in your case. I quickly learned that hard hospital chairs on Tuesdays a 6 pm (or that insane 10-hour marathon class on Saturdays) was NOT the way forward and created this. You can get it done in just a few hours. It’s so easy, you’ll be prepared before you know it!

And finally, this is something you need to talk with your provider about. It really is SO specific to you, your history, your current circumstances and needs I really can’t comment on your growth ultrasound on social media — but I thought I’d explain a few things here that will give you some good information as you head into talking with your provider.

Ok, here are 6 things to know about a fetal growth ultrasound:

WHY did they order the ultrasound?

The first thing to ask yourself is WHY they ordered the ultrasound.

Most often it is:

Did you measure big at your appointment
Do you have a higher risk reason to check on baby’s growth (placental issues, diabetes, etc)

Knowing that reason gives you some input as we move forward here.

For instance, if we KNOW you have a placental issue, but baby is small that triggers other thoughts vs someone who just measured off one time).

BTW when you’re doing stuff like this, kick counts are EXTRA important too — so be sure to grab my cheat sheet here:

Babies Grow at the Same Rate Early On: not true later though!

Providers try to get you in for an ultrasound early on to get your due date set. That’s because all babies grow at pretty much the same rate the first trimester or so.

That way we can know that if your baby is 6.5 cm long it’s X number of weeks — right? Does that make sense. That’s how they come-up with those early due dates. Of course, they also use your last menstrual period and that can be used as well. Every clinic seems to have their own way of coming up with your final due date, but that early ultrasound can be very helpful.

For instance, I’m a late ovulater. So — that ultrasound almost always pushes my due date back a few days.

HOWEVER once babies pass that first trimester, they grow at different rates — just like regular children. When you walk into a Kindergarten class you’ll see kids of all heights, even though they’re all about 5 years old. And obviously, when you go to Jr high it’s even MORE pronounced.

We have genes that make us a certain size. Some of my friends have big babies and some have smaller babies — of course, hard to know on your first.

BUT it’s important to know that later on in pregnancy, if your baby is measuring a bit big or a bit small it doesn’t mean they’re coming early — it just means they’re a bit big or a bit small compared to the other kids in the wombs at the same age.

Check out some other posts on ultrasounds if you’re interesed:

Ultrasounds Get Less Good The Larger Your Baby Is

It SEEMS like this is science and it should be pretty easy to predict how big your baby will be at birth.

BUT an ultrasound is tricky.

I use a room as an example — and you’re looking through a small window on the outside.

If there’s only a few people in the room (aka, early on in pregnancy where there’s lots of room in the womb) it’s easy to spot your friend.

BUT if it’s a busy party and LOADS of people — it’s much harder to find your friend through that small window.

Same goes for pregnancy — as that uterus fills up it’s harder to get as good of a picture as to how big baby is (and an even smaller chance of guessing how big baby will be).

Sad news, it’s really our best and only indicator/test. I’m a huge fan of an experienced provider giving a good guess as well after they put their hands on you. One of my favorite things as a labor nurse was to do Leopold’s maneuvers (where you check how baby is facing on a patient’s abdomen) and then guess how big baby would be. I will say the more I did it, and the more I saw weights at delivery the better I got at it. However, much of that also has to do with your own body’s habitus and other things, so that’s not perfect either.

When an ultrasound report comes back they give a fetal weight (so like 2 lbs 3 oz. plus or minus a certain # of pounds based on the machine and the # of weeks you are at the ultrasound testing).

So, you might hear that your baby, at 36 weeks measured 5 pounds 4 ounces — but you should ask the provider the varying certainty of it — is that plus or minus a pound, or two pounds?

Just know that measurement isn’t always “right on” but is their best guess.

BTW if you’re doing any other testing — jump into my free beginning prenatal class where we talk about all of that even more (it’s also a sample of my full class, so a great way to get started on that too!).

Growth is On a Curve

You’ll learn more about this as baby gets older, but there is a growth curve.

baby growth curve / what is a growth curvbe and how does it apply top regnancy ultrsaounds (this baby is small but growing well) — *As you can tell baby is growing as it gets further along, but is still growing (but is smaller compared to other babies)*

They’ll weigh your baby and then plot them out according to how many weeks or months old they are. This is in comparison with other babies of their age/gender in the area you live.

My kids were always low — but they STAYED low. They’d be 22% for weight for most of their growing-up years. And about 75% for height. As long as they STAY on the curve you’re good. I have tall’ish skinny kids. They continue to grow and at every visit they were around the 22% mark. And now, as adults that is still what they are.

However, let’s say we go in for the 9 month check up and she dropped to the 5% mark — that’s a reason to get worried. Now you’re wondering if she’s eating enough, if she’s having any other issues, etc.

The same goes for pregnancy. Let’s say your baby is measuring small — but at every ultrasound they are growing and they’re making sure there is good blood flow to the baby through the cord (often called doppler studies). At that point you may have frequent’ish ultrasounds to make sure baby is still growing on that curve — just like my skinny kids. You may just have a smaller baby.

With a larger baby you’re seeing if they stay in their percentage. Let’s say your baby is in the 75th percentile — if they measure again next week and they’re in the 90th that’s more concerning than if they just stayed on that 75th percentile curve.

Knowing the curve overall can really help with ultrasounds (and frankly, as baby is having well-check ups too).

I wish providers could explain stuff like this to all their patients in the office, but understanding the BASICS makes those visits SO much more effective. Come join me in here to get those basics out of the way!

which seems better hard hospital chairs, ugly commute, tricky schedules or prenatal class on your couch by the fire?

What does this mean moving forward?

So, your baby’s small. Are they just a small baby, or are they not growing and thriving in the womb?

Or, your baby’s big? Are they just a big baby (and have you had big babies before vaginally, we call that a “tested pelvis”)? Or are they growing too fast and we need to check for something like diabetes or perhaps consider a cesarean if we think they’re just too big.

Soapbox Moment: Growth ultrasounds get a lot of bad rap online because of their inaccuracies. However, that combined with their own feelings on the baby (how they feel baby through your abdomen) are their best indicator of fetal size. Trying to deliver a baby that is truly too large can have REALLY bad consequences, and it is their JOB to give you the issues you could have and help you make a choice moving forward. I’d be 100% extra mad if I had a VERY large baby and my doctor said nothing, and I’d hope you’re in that same camp too.

If you want more information on this — check out my post on what to do if your provider thinks you have a big baby. That’s a great post with tons more info on informed consent at that point.

Here’s a few other posts you might find helpful:

Does this mean you’ll go early?

This is the #1 question I get surrounding these ultrasounds. My baby is measuring 2 weeks ahead, does this mean I’ll go early?

Fetal growth scans, in general, are not a crystal ball. They’re a nice highly-educated guess on baby’s weight and how they’re growing.

That’s it.

It doesn’t mean you’ll go early, or late. It often isn’t even super accurate at baby’s weight at delivery — it is just information you’re going to use going forwards.

Baby size is thought to be a “trigger” that tells the uterus to get them OUT of there. However, it’s not well-understood and isn’t always “accurate” (because some people still end-up having 13 pound babies).

It MAY be a reason your provider recommends an induction, or perhaps further testing.

It is definitely your right (and duty) to ask more questions about what they think it means. Don’t just hear “well, she’s measuring 2 weeks ahead) and leave it at that.