Emily Oster

9 min Read Emily Oster

Emily Oster

How Accurate Are Fundal Height Measurements?

Why they're done, and what they can—and can't—tell you

Emily Oster

9 min Read

It’s been a few years, but when I think back on my doctor’s visits during pregnancy, the most vivid memories I have are of lying on the table and having my stomach measured. There was something so simple and physical about the idea that you’d just use a regular measuring tape, and somehow that would speak volumes. Having said that, I was never entirely sure what they were measuring or what they were getting from the measurements. Sometimes, in my less charitable moments, I wondered whether this was just a meaningless activity intended to convince me that the 45 minutes to an hour I’d just spent in the waiting room was worth it.

This measurement is called a “fundal height” measurement. Despite my grumpy feelings, it provides useful — if quite noisy — information about fetal growth. The other approach to estimating fetal size, more commonly used at the end of pregnancy, is ultrasound. With both of these metrics, there are natural questions about accuracy. If your belly is measuring big or small, how much do you learn from that? If the ultrasound says the baby is 10 pounds, are they? How sure can you be?

Unlike many of the topics I write about, this is one with a lot of good data. We can answer these questions. So let’s do it.

Fundal height measurements

The tape measure your OB is running over your belly is measuring fundal height — this is basically the size of the uterus measured from the top of the pubic area to the top of the uterus (that’s the “fundus”). The goal of measuring fundal height is to detect abnormal fetal growth, typically after 24 weeks.

To do this measurement, your doctor just … measures. The trick — and this is where the physical expertise comes in — is to be able to feel the points to measure between. Best practice is to measure using the unmarked side of the tape measure, to avoid bias. If we know what we are looking for, we may be biased to see it (this is why somehow many men seem to think they are 6 feet tall rather than, say, 5 foot 10. I digress).

After 24 weeks of pregnancy, the height of the fundus approximates gestational weeks. So it should be 26 centimeters around 26 weeks, etc. Data suggest a margin of error of plus or minus 3 centimeters, so within 3 centimeters of the expected number, we assume things are fine. If the measurement is 4 or more centimeters off what is expected, that’s a marker for possible fetal growth issues. The bigger concern is if the figure is much smaller than expected, which could be a sign of fetal growth restriction. Fetal growth restriction means the fetus is small, which could be a sign of a placental or other issue (or it could just be that they are small).

It is important to emphasize that phrase: could be a sign. It is a fairly crude approach to this — I say that in a positive, not a normative, way. There is information here, and it’s an easy test, so it’s impressive that we can get anything out of it! But you cannot make a diagnosis of growth restriction based on fundal height alone. This measurement is influenced by a variety of factors, including maternal weight, fibroids, whether you have to pee, if you drank a lot of water, and so on.

In terms of accuracy, there is a variety of data. One large study in Sweden used data from 42,000 women and asked whether small fundal height measurements were predictive of a baby being small for gestational age at birth. The researchers found that, in general, fundal heights were predictive, but much more so later in pregnancy. There are a few ways to summarize their results, but here is one take:

  • If we take a measurement below the 10th percentile (a common threshold) as a cutoff to be concerned, about 25% of pregnant women will fall below this threshold at least once during their pregnancy.
  • For 2 out of 10 “positive” findings on this test, the infant will be small for gestational age.
  • For 9 out of 10 negative findings, the infant will not be small for gestational age.

The second bullet here might be the most relevant. For 8 out of 10 people who get a concerning result on this screen, they will have a normal-size baby. Again, this screening provides quite a lot of information, but it needs follow-up. You would not want to act, or panic, based on a worrisome result.

Typical follow-up if there is a concern with fundal measurement is an ultrasound to assess fetal growth. It is also common — even without any concern — to use an ultrasound to assess the fetus size near the end of pregnancy. How accurate are those? Let’s turn to that next.

Measuring fetal size with ultrasound

The first thing to say here is that you cannot measure fetal weight directly with an ultrasound. What you can do with an ultrasound is take various body measurements. The main measurements used are head diameter, head circumference, abdominal circumference, and femur length. These can all be measured on an ultrasound, but with error. As you might imagine, reflecting on the circumstances of non-fetal humans, these measurements are not going to perfectly indicate weight. They’re likely to be a bit better with a fetus than an adult, for many reasons, but ultimately there is error.

The general system for estimating fetal weight involves taking these measurements, or a subset of them, and running them through a formula. These formulas are developed based on taking data on the measurements and then comparing them to actual fetal weight — researchers ask how they could combine the measurements to get the most accurate prediction.

There are many possible formulas that can be used and, to be frank, looking at them isn’t going to really help one understand anything. For example, here is a popular one (the “Hadlock three-parameter formula”):

EFW = 10(1.326 + 0.0107 × HC + 0.0438 × AC + 0.158 × FL −  0.00326 × AC × FL))

If you parse this, you see that estimated weight is increasing in head circumference (HC), abdominal circumference (AC), and femur length (FL), but also there is a downward adjustment for the interaction between femur length and abdominal circumference. The goal of these formulas isn’t to be interpretable, it’s to get the best fit to the data.

In practice, all of these formulas perform similarly, and all have the same problem. They are all pretty accurate on average. That is: if your question was what the size of the average baby is, these formulas do pretty well. This is more or less mechanical: these formulas are explicitly developed in a way that would make that true. But they all come with a lot of error. This is true for a few reasons. First, there is error in the measurements that are used as inputs — it’s not that easy to measure head circumference with ultrasound. Second, even if the measurements were perfect, there is variation across babies.

The size of the errors are pretty large. A 2023 study of more than 30,000 babies in Sweden looked at estimated weight on an ultrasound within two days before birth and compared it to actual birth weight. Between 70% and 75% of the fetuses had a birth weight within 10% of estimated. But this means that as many as 30% of the babies were over or under by at least 10%. That’s the difference between 7 pounds and 6 pounds 5 ounces, which is … big. As many as 6% of measurements were off by more than 20%, which renders them basically useless.

The errors are larger for very small and very large babies. This is not surprising, since there are less data on those, so the models have less information to fit to. It’s unfortunate, though, since those are precisely the situations in which these data are most useful. It actually isn’t that important to know if your baby is going to be 7 pounds 12 ounces or 8 pounds 6 ounces. When this matters more is if you’re expecting a very small baby (say, under 3 pounds) and want to know what to be prepared for, since 3 pounds and 2 pounds are very different, or if you’re expecting a very large baby, which might necessitate different delivery approaches.

There are various ways that the accuracy situation might be improved, including better measurements with 3-D ultrasound or MRI, and possibly better models. More data might allow better prediction at the end of the distributions, but maybe not. Ultimately, it’s a hard problem.

Given the large error, there are questions raised by some about whether these measurements should be taken at all. On one hand, more information is (usually) better, and there is clearly some information provided by these measurements. On the other hand, there are decisions made based on these data, and bad measurement may lead to clinical error.

For example: there is evidence that when fetal weight is overestimated, women are more likely to have a C-section for labor arrest. Put simply, if labor is slow, and the estimated fetal weight suggests the baby is big, doctors (in this study at least) were more likely to move to a C-section than with the same length of labor and the same actual size of baby but a higher estimated weight.

There’s no consensus here, unfortunately, and in most practices, estimating fetal weight near the end of pregnancy is standard. I should note that ultrasound is also used, and also has significant error, for detecting fetal growth restriction during pregnancy. The same problems that plague the estimation of weight near birth come up here. It is just not very precise.

Bottom line

  • There are no precise methods for estimating fetal size in utero.
  • Fundal measurements are simple to do, and provide a signal about growth, but the majority of concerns raised by these measurements are unfounded.
  • Ultrasound-based measurements are accurate on average, but about 30% of people will have errors of more than 10% in either direction.
  • For most people, these measurements never play a role in your care, so their lack of precision doesn’t matter. If you are asked to make decisions based on estimated fetal weight (for example, a decision about an induction or a C-section) consider asking for a second measurement.
0 Comments
Inline Feedbacks
View all comments
A baby's profile is seen on a sonogram.

4 min read

Does Extra Prenatal Care over 35 Matter?

And a lesson on regression discontinuity

Emily Oster
An ultrasound view of the bottoms of a baby's feet.

Jul 19 2023

2 min read

How to Understand My Baby’s Clubfoot Diagnosis

Do you or your network have advice for parents dealing with clubfoot? At our 20-week ultrasound, we found out our Read more

Emily Oster
View of a baby's head during a 37-week ultrasound.

Aug 09 2023

3 min read

How Worried Should I Be About Microcephaly If My Baby’s Head Is Measuring Small?

My two recent ultrasound head circumference measurements have shown my baby’s head to be in or under 1%, which technically Read more

Nathan Fox
An ultrasound picture, a stethoscope and a readout of a heart monitor are seen on a physician's desk.

Aug 23 2023

4 min read

Are Ultrasounds Safe?

Have you done any research into the safety of ultrasounds (both scans and their use to monitor how things are Read more

Emily Oster

Instagram

left right
Happy Father’s Day to the Fathers and Father figures in our ParentData community! 

Tag a Dad who this holiday may be tricky for. We’re sending you love. 💛

Happy Father’s Day to the Fathers and Father figures in our ParentData community!

Tag a Dad who this holiday may be tricky for. We’re sending you love. 💛
...

“Whilst googling things like ‘new dad sad’ and ‘why am I crying new dad,’ I came across an article written by a doctor who had trouble connecting with his second child. I read the symptoms and felt an odd sense of relief.” Today we’re bringing back an essay by Kevin Maguire of @newfatherhood about his experience with paternal postpartum depression. We need to demystify these issues in order to change things for the better. Comment “Link” for a DM to read his full essay.

#parentdata #postpartum #postpartumdepression #paternalmentalhealth #newparents #emilyoster

“Whilst googling things like ‘new dad sad’ and ‘why am I crying new dad,’ I came across an article written by a doctor who had trouble connecting with his second child. I read the symptoms and felt an odd sense of relief.” Today we’re bringing back an essay by Kevin Maguire of @newfatherhood about his experience with paternal postpartum depression. We need to demystify these issues in order to change things for the better. Comment “Link” for a DM to read his full essay.

#parentdata #postpartum #postpartumdepression #paternalmentalhealth #newparents #emilyoster
...

What does the data say about children who look more like one parent? Do they also inherit more character traits and mannerisms from that parent? Let’s talk about it 🔎

#emilyoster #parentdata #parentingcommunity #lookslikedaddy #lookslikemommy

What does the data say about children who look more like one parent? Do they also inherit more character traits and mannerisms from that parent? Let’s talk about it 🔎

#emilyoster #parentdata #parentingcommunity #lookslikedaddy #lookslikemommy
...

SLEEP DATA 💤 We asked you all about your kids’ sleep—and got nearly 15,000 survey responses to better understand kids’ sleep patterns. Comment “Link” for an article that breaks down our findings!

This graph shows sleeping location by age. You’ll notice that for the first three months, most kids are in their own sleeping location in a parent’s room. Then, over the first year, this switches toward their own room. As kids age, sharing a room with a sibling becomes more common. 

Head to the newsletter for more and stay tuned for part two next week on naps! 🌙

#parentdata #emilyoster #childsleep #babysleep #parentingcommunity

SLEEP DATA 💤 We asked you all about your kids’ sleep—and got nearly 15,000 survey responses to better understand kids’ sleep patterns. Comment “Link” for an article that breaks down our findings!

This graph shows sleeping location by age. You’ll notice that for the first three months, most kids are in their own sleeping location in a parent’s room. Then, over the first year, this switches toward their own room. As kids age, sharing a room with a sibling becomes more common.

Head to the newsletter for more and stay tuned for part two next week on naps! 🌙

#parentdata #emilyoster #childsleep #babysleep #parentingcommunity
...

Weekends are good for extra cups of ☕️ and listening to podcasts. I asked our team how they pod—most people said on walks or during chores. What about you?

Comment “Link” to subscribe to ParentData with Emily Oster, joined by some excellent guests.

#parentdata #parentdatapodcast #parentingpodcast #parentingtips #emilyoster

Weekends are good for extra cups of ☕️ and listening to podcasts. I asked our team how they pod—most people said on walks or during chores. What about you?

Comment “Link” to subscribe to ParentData with Emily Oster, joined by some excellent guests.

#parentdata #parentdatapodcast #parentingpodcast #parentingtips #emilyoster
...

Humility. That’s why. That’s the whole reason.

#emilyoster #secondbaby #parentingjokes #parentinghumor

Humility. That’s why. That’s the whole reason.

#emilyoster #secondbaby #parentingjokes #parentinghumor
...

Bug season is upon us. Besides annoyance, this can bring up safety concerns, particularly with ticks. They are carriers of diseases, most notably Lyme disease. So what’s the best course of action?

Prevention is key! I suggest:
⭐ Regular tick checks
⭐ Using bug sprays with DEET 
⭐ Wearing long sleeves and pants in the woods

Some parents worry about DEET, but repellants with up to 30% DEET are recommended by both the CDC and AAP. The data says you’re in the clear, so go for it. Enjoy your summer!

#parentdata #emilyoster #tickseason #bugbites #bugspray

Bug season is upon us. Besides annoyance, this can bring up safety concerns, particularly with ticks. They are carriers of diseases, most notably Lyme disease. So what’s the best course of action?

Prevention is key! I suggest:
⭐ Regular tick checks
⭐ Using bug sprays with DEET
⭐ Wearing long sleeves and pants in the woods

Some parents worry about DEET, but repellants with up to 30% DEET are recommended by both the CDC and AAP. The data says you’re in the clear, so go for it. Enjoy your summer!

#parentdata #emilyoster #tickseason #bugbites #bugspray
...

The list of what not to do while pregnant feels longer than a CVS receipt. At ParentData, we want to empower you to make the right decisions for you. 

What an amazing group of women, and an honor to speak at the #MomsFirstSummit debunking parenting myths. 

What are some pregnancy rules you chose to bend after being empowered by data?

#emilyoster #parentdata #pregnancyproblems #pregnancymyths

The list of what not to do while pregnant feels longer than a CVS receipt. At ParentData, we want to empower you to make the right decisions for you.

What an amazing group of women, and an honor to speak at the #MomsFirstSummit debunking parenting myths.

What are some pregnancy rules you chose to bend after being empowered by data?

#emilyoster #parentdata #pregnancyproblems #pregnancymyths
...

Looking for Memorial Day Weekend plans? Might be the perfect time to give potty training a shot. Potty training is notoriously difficult, and we unfortunately don’t have a lot of evidence-based guidance on what works best. So I asked the ParentData community to fill out a survey and share their knowledge — about 6,000 people responded.

👉Comment “Link” for a DM to an article that summarizes all of the best potty training advice we collected. 

Remember, you are not alone in the potty training struggle! It can be incredibly challenging, so please give yourself some grace.

#emilyoster #parentdata #pottytraining #pottytrainingtips #toddlertips

Looking for Memorial Day Weekend plans? Might be the perfect time to give potty training a shot. Potty training is notoriously difficult, and we unfortunately don’t have a lot of evidence-based guidance on what works best. So I asked the ParentData community to fill out a survey and share their knowledge — about 6,000 people responded.

👉Comment “Link” for a DM to an article that summarizes all of the best potty training advice we collected.

Remember, you are not alone in the potty training struggle! It can be incredibly challenging, so please give yourself some grace.

#emilyoster #parentdata #pottytraining #pottytrainingtips #toddlertips
...

We’re hiring an Associate Editor at ParentData! More details at my link in bio. Please share with the great writers and data-loving people in your network. 📊💻

We’re hiring an Associate Editor at ParentData! More details at my link in bio. Please share with the great writers and data-loving people in your network. 📊💻 ...

Do you brand things a certain way to get your kid to accept it? Like calling carrots “rabbit popsicles”? Or telling them to put on their “super speed socks” in the morning? Share your rebrands in the comments below! You never know who you might be helping out 👇

#emilyoster #funnytweets #relatabletweets #parentingjokes #kidssaythedarndestthings

Do you brand things a certain way to get your kid to accept it? Like calling carrots “rabbit popsicles”? Or telling them to put on their “super speed socks” in the morning? Share your rebrands in the comments below! You never know who you might be helping out 👇

#emilyoster #funnytweets #relatabletweets #parentingjokes #kidssaythedarndestthings
...

Have you ever panic-googled a parenting question when everyone else is asleep? If so, you’re not alone. 

Today is the first episode of a new biweekly series on my podcast: Late-Night Panic Google. On these mini-episodes, you’ll hear from some familiar names about the questions keeping them up at night, and how data can help. First up: @claireholt!

Listen and subscribe to ParentData with Emily Oster in your favorite podcast app 🎧

#parentdata #emilyoster #claireholt #parentingstruggles #parentingtips #latenightpanicgoogle

Have you ever panic-googled a parenting question when everyone else is asleep? If so, you’re not alone.

Today is the first episode of a new biweekly series on my podcast: Late-Night Panic Google. On these mini-episodes, you’ll hear from some familiar names about the questions keeping them up at night, and how data can help. First up: @claireholt!

Listen and subscribe to ParentData with Emily Oster in your favorite podcast app 🎧

#parentdata #emilyoster #claireholt #parentingstruggles #parentingtips #latenightpanicgoogle
...

Sun safety is a must for all ages, especially babies! Here are my tips for keeping your littlest ones protected in the sunshine:
☀️ Most importantly, limit their time out in hot weather. (They get hotter than you do!)
☀️ Keep them in the shade as much as possible when you’re out.
☀️ Long-sleeve but lightweight clothing is your friend, especially on the beach, where even in the shade you can get sunlight reflecting off different surfaces.
☀️ If you want to add a little sunscreen on their hands and feet? Go for it! But be mindful as baby skin tends to more prone to irritation.

Comment “Link” for a DM to an article on the data around sun and heat exposure for babies.

#sunsafety #babysunscreen #babyhealth #parentdata #emilyoster

Sun safety is a must for all ages, especially babies! Here are my tips for keeping your littlest ones protected in the sunshine:
☀️ Most importantly, limit their time out in hot weather. (They get hotter than you do!)
☀️ Keep them in the shade as much as possible when you’re out.
☀️ Long-sleeve but lightweight clothing is your friend, especially on the beach, where even in the shade you can get sunlight reflecting off different surfaces.
☀️ If you want to add a little sunscreen on their hands and feet? Go for it! But be mindful as baby skin tends to more prone to irritation.

Comment “Link” for a DM to an article on the data around sun and heat exposure for babies.

#sunsafety #babysunscreen #babyhealth #parentdata #emilyoster
...