Can Ultrasound Really Predict the Weight of My Baby?

On Pregnancy & Childbirth, Robin Elise Weiss asks one of the most common questions seen repeatedly on pregnancy message boards: Can they tell how big the baby is by ultrasound? The question is usually followed by some variation of “…because I’m 36 weeks and the baby is already 8 pounds and my doctor says I’ll probably need a c-section.”

Ultrasound exams are notoriously inaccurate for predicting the weight of your baby. The ultrasound gives an estimate of the weight of your baby, but this estimate can be off a pound or more in either direction. There are multiple ways to predict the weight via ultrasound, making it not advised for most to make decisions about induction and planned cesarean section based solely on estimated fetal weight.

While it makes patients uncomfortable to think that a) care providers routinely rely on inaccurate measures of fetal weight when making recommendations of the safety of vaginal birth and that b) care provider bias plays a significant role in determining whether or not a woman will have an unnecessary cesarean, Weiss writes briefly about one study that suggests that a bedside ultrasound will increase the chance of surgical birth.

One thing that is show in some studies is that an estimation of a big baby via ultrasound can increase the likelihood that you will have a cesarean. This is thought to be because your practitioner has the “big baby” seed planted in his or her mind already and is less tolerant of variances in labor.

Weiss is referring to the study, Overestimation of fetal weight by ultrasound: does it influence the likelihood of cesarean delivery for labor arrest?, the abstract for which is pasted below:

Sean C. Blackwell MD, Jerrie Refuerzo MD, Rati Chadha MD and Carlos A. Carreno MD; Division of Maternal Fetal Medicine, Department of Obstetrics, Gynecology and Reproductive Sciences, University of Texas Health Science Center at Houston, Houston, TX, Foothills Medical Center, University of Calgary, Calgary, AB, Canada


We sought to determine whether the overestimation of ultrasound-derived estimated fetal weight (EFW) is associated with increased diagnosis of labor arrest.

Study Design

This is a historical cohort study of nulliparous women with term pregnancies who underwent bedside ultrasound examination for EFW before labor induction. Labor outcomes of women with EFW overestimation > 15% the actual birthweight were compared with those with EFW not overestimated.


Overestimation of EFW occurred in 9.5% of cases (23/241). The rate of cesarean delivery (CD) for labor arrest was higher for those with EFW overestimation (34.8% vs 13.3%; P = .01) even though there were no differences in length of the induction duration. After adjusting for confounding factors, EFW overestimation remained associated with CD for labor arrest (odds ratio, 4.8; 95% confidence interval, 1.5-15.2).


Our finding suggests that an overestimation of EFW may be associated with a lower threshold for CD for labor arrest.


On Weiss’ post, readers were giving the chance to weigh in on fetal weight estimates with their own experiences.

“Readers Respond: How big was your baby supposed to be via ultrasound?”

“your lucky if she weighs 7 pounds”

Hinting that she was small, the Doc said that at the required-to-stay-eligble-ultrasound the day before Bobbie was born, and weighed in at 10 pounds 13 ounces.


Estimated to be over 8.5 pounds… my baby was delivered 6lbs 6oz. My midwife (CNM) said “push hard… this is going to be a big baby”. I did and she came out in one push… and I had a 2nd degree tear.

One right, one wrong

They said my first son was going to be somewhere around 9 lbs. but wound up being 7 lbs 13 oz. With my second I had my last ultrasound at 28 weeks, they guessed he was about 5 lbs at that time. I thought this was really big and kinda laughed knowing how wrong ultrasounds can be. Funny thing is that he was born at 11 lbs. 2 oz. I guess it was a bit of a better guess with my second.

Off by a pound

My twins were supposed to be ‘within an ounce of one another’ just a bit before I went into labor. At birth, they were a full pound different. So rarely do OB’s notify their patients that 3rd trimester ultrasounds have an acceptable variance of + / - two weeks!

Off by a pound +

The ultrasound said 6lb 6 oz, He was born at 5lbs even by VBAC at home. He was a scrawny little guy, but healthy. As a doula I see the inaccuracies all the time. Unfortunitly some DRs do not let their clients know that and moms make decisions based on faulty info. I HATE THAT!

Bigger Baby

I only had one u/s at 20 weeks and then manual assessment. All along they told me my baby was very avg, maybe 7 1/2 lbs…my daughter was born at 10.5 oz. and 24”. Not unusual for in my family but maybe good that doctors didn’t use the big baby scare to push early interventions. Induction was never mentioned even though I birthed at 41.6 weeks (also very normal in our family) and there was never a mention of a possible section.

It happens!

I had an ultrasound in labor and they suggested that my baby was really big. I was told 10 lbs 13 oz and 23 in. They convinced me I needed a c-section. He wound up weighing in at 7.5 lbs. I was really sad. I turned around and had a vaginal birth with my next baby who was over 8 lbs - so the weight wasn’t an issue, it was my doctor’s concern about the weight that made me have a c-section.

Estimate Was Incorrect

My baby’s final ultrasound was two days before she was born, during a biophysical profile. The ultrasound guessed her at 8lbs and it was over a pound off! Thankfully, I had been warned by my practitioner and my childbirth educator that ultrasounds are not a good way to estimate weight. She was born vaginally with no complications and I was not induced.

Waaaay off.

I was really surprised because my doctor acted like this ultrasound was just as accurate as the scale. Because they thought my baby was about 8 lbs. they said my due date was off. So we induced. My 6 lb baby spent over a week in NICU with breathing problems. The NICU doctors said she was born too early.


To get a sense of how pervasive the recommendation of cesarean for suspected macrosomia based on inaccurate ultrasound measurements has become, set up a Google Alert for “ultrasound big baby c-section” or “ultrasound big baby cesarean,” or perform a Google search for the same terms.


If you had an ultrasound estimate of fetal weight performed during your pregnancy, how did it compare to actual birth weight?


Related Reading:

Can My Doctor Really Predict Shoulder Dystocia?

I Had Shoulder Dystocia Once. Will It Happen Again?

Iatrogenic Shoulder Dystocia?

Misdiagnosis of Shoulder Dystocia: Bed Dystocia and Snug Shoulders

Brave Message Board Mom Questions Necessity of First Cesarean

Our Bodies, Ourselves: “Do Not Intervene for a Big Baby”

Suspected Macrosomia? Better Not Tell the Admitting Doctor.

Amanda’s Story- 10 lbs., 4 oz.- Hospital VBAC


Massachusetts’ Wicked High Cesarean Rates


(Photo credit)


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