Overview

Ultrasounds are a key tool for estimating your baby’s weight and size during pregnancy, but how accurate are these measurements? In this episode, we respond to a listener whose baby is measuring in the 97th percentile and address concerns about giving birth to a larger baby. We explain how ultrasounds calculate weight, explore their reliability, and discuss what it means for babies who are small or large for their gestational age. Learn about the potential risks and what interventions your doctor or midwife might recommend based on ultrasound results. Whether your baby is measuring small, large, or somewhere in between, this episode will give you the confidence to understand ultrasound accuracy and navigate your options.

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Hi Vanessa, the Pregnancy Podcast has been such an amazing resource during my pregnancy. Thank you for all the helpful info you share. I’m 36 weeks pregnant, and at my latest ultrasound, my doctor told me my baby is measuring in the 97th percentile for weight. I’m feeling really nervous about delivering such a big baby. Does this mean I’ll need a c-section, and how accurate are these weight and growth estimates from ultrasounds? Any advice on preparing for this possibility would really put my mind at ease. Thank you again for all the great episodes! -Sarah

How Ultrasounds Calculate Weight

The only way to estimate your baby’s weight before birth is by plugging measurements from an ultrasound into a formula to calculate your baby’s weight. Your care provider or the ultrasound technician will take four key measurements: head circumference, biparietal diameter (width of the baby’s head), abdominal circumference, and femur length. Then they enter these measurements into algorithms that compute the estimated weight.

How Accurate Are Ultrasounds?

There are a lot of questions about how accurate ultrasounds are for measuring your baby. One study  found ultrasounds accurate (within 10%) 84% of the time. Another study found that mothers predicted weight within 10% in 59% of cases and ultrasounds in 65% of cases. This means that mothers were almost as accurate at estimating the size of their babies as ultrasounds were. These two studies are consistent with other research in that ultrasounds have a considerable margin of error. Although, in the absence of any other accurate way to predict the size of a baby before birth, we rely on these measurements.  

To make things even more challenging, there is no universal calculation to take ultrasound measurements and apply them to height and weight. One study looked at several formulas used to estimate fetal weight. Researchers found that all of them either over or under estimate the fetal weight. The researchers concluded that the interpretation of ultrasound estimated fetal weight should be made cautiously, especially in small for gestational age babies.  

Fetal Growth Percentiles

Fetal growth percentiles are created by comparing an individual baby’s measurements (such as weight, length, and head circumference) to a reference population of babies of the same gestational age. These percentiles provide a way to understand where a baby’s growth falls relative to others. The 50th percentile represents the median, where half the babies measure above and half below. 10th percentile indicates babies who measure smaller than 90% of the population. The 90th percentileindicates babies who measure larger than 90% of the population.

ACOG considers newborns whose birth weight is less than the 10th percentile to be small for gestational age. Large for gestational age is generally used to describe a birth weight equal to or greater than the 90th percentile for a given gestational age. However, there is no universal agreement on these definitions. Some have proposed the 15th and 97th percentiles as more optimal cut-offs to define small or large for gestational age.

Accuracy of Ultrasounds to Diagnose Small or Large for Gestational Age Babies

A study that looked at third-trimester ultrasounds to evaluate small for gestational-age babies found that they were not very accurate. Ultrasounds miss the diagnosis of many small for gestational age newborns. A study examining over 26,000 pregnancies with an ultrasound between 35 and 38 weeks had 2,241 babies deemed large for gestational age. When they compared birth weight to ultrasound measurements, 8.6% of the time, the ultrasound falsely identified a baby as being large when they weren’t. Ultrasounds were more accurate at predicting large for gestational age babies in a subgroup of pregnancies scanned for a suspected large baby.

This is a theme we see often when interventions that may not apply to everyone, including low-risk mothers, become routine care. With more frequent ultrasounds, more babies will be diagnosed as small or large for gestational age. Not all of those diagnoses will be accurate. The first thing you need to understand is that the ultrasound measurements indicating your baby is in the 97th percentile may not be 100% accurate, so do not panic.

The Risk of Shoulder Dystocia

Typically, once a baby’s head is out the rest of their body comes out quickly. Shoulder dystocia is a complication where a baby’s shoulders get stuck. This is often linked to bigger babies and is the main reason cesareans for a large baby are recommended. Risks to the baby include injury to the brachial plexus (nerves in the shoulder), fractures, or oxygen deprivation if the umbilical cord is compressed for too long. Risks to the mother include significant tearing, heavy bleeding, or injury to the uterus. The good news is that there are techniques doctors and midwives can utilize to manage shoulder dystocia.

Inducing Labor and Planned Cesarean for Large Babies

Some care providers may suggest inducing labor or a planned cesarean for a big baby. Inducing labor aims to give birth before the baby grows any larger. You can induce labor in many ways, and each method has risks and benefits. See this episode for a deep dive into the evidence on inducing labor. There are also many methods you can use to try and naturally induce labor. See this episode for the research on each method and to learn which are actually evidence-based.

Similar to induction, a cesarean is also an intervention that comes with risks and benefits. Every expecting mother should be knowledgeable about cesarean birth, even if that is not your primary birth plan. In the United States, one in three babies is born via cesarean. Labor is an intense experience. A lot can happen that puts you in a position where you or your care provider has to make quick decisions and may not have time to explain your options at length. Knowing what to expect in a cesarean birth and what your options are will better prepare you for labor and give you confidence to navigate any scenario.

Check out these episodes on cesarean birth for more in-depth information:

Talking to Your Doctor or Midwife

A great tactic to help calm your nerves about anything related to birth is to educate yourself. Learn about induction and cesarean so you can have a productive and informed conversation about these possibilities with your doctor or midwife. You may also ask your care provider how comfortable they are managing shoulder dystocia and have them walk you through that scenario.

It is entirely understandable to be nervous about giving birth to a big baby. There is also the perspective that your body will not build a baby bigger than you can birth. You may also consider exploring different labor positions. Some positions can open your pelvis and allow more room for your baby to make an exit. Overall, ultrasounds are not 100% accurate in predicting the size or weight of a baby before birth. I encourage you to talk to your doctor or midwife to understand their recommendations and your options. Consider scheduling an additional appointment so you can have adequate time to ask questions and discuss the pros and cons of interventions or different scenarios.

Additional Resources

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