Told Your Baby Is Measuring Big? Here’s What the Research Actually Shows
I saw a post on social media this morning—just a few lines, tucked into a busy feed—but it stopped me in my tracks. The person who shared it sounded anxious and unsure, sharing that their doctor told them their baby was measuring “big.” The comments were full of people echoing the same worry:
“Mine said that too.”
“I didn’t feel like I had a choice.”
“I wish I’d known more.”
Reading it brought me back to so many conversations I’ve had with clients over the years—people told they’re growing a “big baby” and that they need to change their birth plans because of it. Often, the suggestion comes with urgency but without much clarity.
So let’s slow things down. Let’s talk about what a “big baby” actually means, what the evidence says, and how you can make decisions from a place of knowledge—not fear.
First things first: what is a “big baby”?
In clinical terms, a baby is considered big—or macrosomic—when they weigh 9 pounds, 15 ounces or more at birth (Rouse et al., 1996). That’s the official definition. But here’s the thing: most of the time, when someone is told they’re having a big baby, that prediction is based on an ultrasound estimate—and those estimates aren’t as precise as we might hope.
In fact, ultrasound weight estimates are typically off by about 15% in either direction. So if you’re told your baby is measuring 9 pounds, the actual weight could be as low as 7 lbs 10 oz or as high as 10 lbs 5 oz (Declerq et al., 2013). That’s a big range. And studies show that when ultrasounds try to predict if a baby will be big, they’re only right about half the time (Chauhan et al., 2005).
Let that land for a second: if someone tells you your baby is big based on an ultrasound, there’s a 50/50 chance they’re wrong.
Even researchers agree—there’s “no clear consensus with regard to the prenatal identification, prediction, and management of macrosomia.” Why? Because diagnosing a big baby before birth is extremely difficult; it's essentially a diagnosis that can only be confirmed after birth (Rossi et al., 2013).
And here’s another important piece: only about 1 in 10 babies in the U.S. is actually born weighing more than 9 lbs 15 oz (U.S. Vital Statistics, 2019). That means most people being told they’re growing a “giant baby” aren’t actually growing a baby that meets the clinical definition of macrosomia at all.
So why does this matter?
Because those conversations about big babies often lead directly to discussions about induction or planned c-sections—decisions that come with real implications. More often than not, they’re framed around fear. Fear of something going wrong. Fear of the baby getting stuck. Fear of injury.
This fear changes how people are treated. Research shows that when a big baby is suspected, families are more likely to experience shifts in how their care providers approach labor and birth. That suspicion alone can lead to higher rates of interventions, especially cesareans. People are more likely to be told their labor is “taking too long” or that their baby “doesn’t fit.”
In fact, research consistently shows that a provider’s perception that a baby is big can be more harmful than the baby actually being big (Sadeh-Mestechkin et al., 2008).
Let’s talk about shoulder dystocia
One of the biggest concerns raised in these conversations is shoulder dystocia—a complication where the baby’s shoulders get stuck after the head is born. It’s serious, and it’s understandably frightening.
So here’s what the evidence actually says:
For babies who are actually large, the risk of shoulder dystocia is between 7% and 15% (Beta et al., 2019; Rouse et al., 1996).
If a baby is only suspected of being large, that risk drops to around 7% (Morrison et al., 1992; Nath et al., 2015; Foster et al., 2011).
Inducing labor in these cases may reduce the risk slightly, down to about 4%—but it doesn’t change the risk of permanent nerve injury or NICU admissions.
So yes, induction might reduce the chance of shoulder dystocia a little. But it doesn’t reduce the more serious outcomes people are often most worried about. And induction itself comes with its own risks—especially when it’s done before the body is ready.
What do the guidelines say?
In 2016, the American College of Obstetricians and Gynecologists (ACOG) released a committee opinion stating that induction is not recommended for suspected big babies, because it doesn’t improve outcomes for birthing people or babies. In 2020, they reiterated that more research is needed before recommending induction before 39 weeks solely to prevent shoulder dystocia (ACOG, 2020).
Why? Because early induction has risks of its own—risks that may outweigh the potential benefits when the only indication is a guess that the baby might be big.
As for planned c-sections, ACOG has been clear: cesarean to prevent shoulder dystocia may be considered for suspected big babies weighing over:
11 lbs (5,000g) in birthing people without diabetes
9 lbs 15 oz (4,500g) in those with diabetes
That’s significantly higher than the 9-pound estimates many people hear during pregnancy.
And here's a number that might surprise you:
To prevent one case of permanent injury from shoulder dystocia in babies suspected of weighing 9 lbs 5 oz, we’d need to perform nearly 3,700 cesareans (Rouse et al., 1996).
That’s thousands of major surgeries—with all the physical, emotional, and financial impacts they carry—to prevent one rare outcome.
So what does this mean for you?
It means that if someone tells you your baby is “too big,” it’s not a closed door. It’s an invitation to ask more questions—to understand the nuances and to weigh the real risks and benefits.
Here are a few questions you might start with:
How certain are we about the baby’s size?
What are the risks and benefits of waiting for labor to start naturally?
What are the risks of induction or cesarean for me, specifically?
Can we talk through the numbers together?
You’re not asking for too much. You’re advocating for yourself—and that’s exactly what you should be doing.
That pregnant person on social media was looking for answers, for reassurance, for clarity. What they got was a flood of shared fear—but not much in the way of evidence.
If you’re in their shoes—if you’ve been told your baby is big and that your options are limited—I want you to know this:
You are allowed to pause.
You are allowed to ask questions.
You are allowed to choose the path that feels right for you—not the one pushed by fear or vague estimates.
Your birth is not a foregone conclusion.
And you are not powerless in this process.
TL;DR: What to Know If You’re Told Your Baby Is “Big”
A “big baby” means 9 lbs 15 oz or more—but only about 1 in 10 babies are actually born that size.
Ultrasound weight estimates are often off by 15% in either direction and are only accurate about half the time.
Suspected big babies don’t automatically mean complications. The risk of shoulder dystocia is 7–15% for actual big babies—and less for suspected ones.
Induction might slightly lower the risk of shoulder dystocia, but doesn’t reduce the risk of serious complications, and early induction carries its own risks.
Planned c-sections for suspected big babies aren’t routinely recommended—it would take 3,700 surgeries to prevent one case of permanent injury for a baby estimated at 9 lbs 5 oz.
Looking for more info on big babies?
I chatted about big babies, and the research on c-sections and inductions with my friend Ashley Cooley on the Bringing Up Baby podcast!
References:
The American College of Obstetricians and Gynecologists, Committee on Practice Bulletins. (2020). ACOG Practice Bulletin: Macrosomia.
Beta, J., Khan, N., Khalil, A., et al. (2019). Maternal and neonatal complications of fetal macrosomia: a systematic review and meta-analysis. Ultrasound Obstet Gynecol. [Epub ahead of print].
Chauhan, S. P., W. A. Grobman, et al. (2005). “Suspicion and treatment of the macrosomic fetus: a review.” Am J Obstet Gynecol 193(2): 332-346.
Committee on Practice Bulletins, American College of Obstetricians and Gynecologists [ACOG] (2002; reaffirmed 2008). “Fetal Macrosomia: Practice Bulletin No. 22.” ObstetGynecol 96(5).
Committee on Practice Bulletins, American College of Obstetricians and Gynecologists [ACOG]. (2016). “Fetal macrosomia. Practice Bulletin No. 173.” American College of Obstetricians and Gynecologists. Obstet Gynecol 128: e195–209.
Declercq, E. R., C. Sakala, et al. (2013). “Listening to mothers III: Pregnancy and childbirth.”
Hehir, M. P., McHugh, A. F., et al. (2015). “Extreme macrosomia- Obstetric outcomes and complications in birthweights >5000 g.” Aus NZ J Obstet Gynecol 55: 42-46.
Rouse, D. J., J. Owen, et al. (1996). “The effectiveness and costs of elective cesarean delivery for fetal macrosomia diagnosed by ultrasound.” JAMA 276(18): 1480-1486.
Sadeh-Mestechkin, D., A. Walfisch, et al. (2008). “Suspected macrosomia? Better not tell.” Arch Gynecol Obstet 278(3): 225-230.