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What's the deal on suspected 'Big Babies'?

Updated: Oct 17, 2022

By the third trimester, it's routine for care providers to measure the “fundal height” during prenatal visits. Typically, “normal growth” is considered when the fundal height is equivalent to gestational weeks (ie. at 35 weeks gestation we’d expect to see the fundal height measuring 33-36cm). It's common for your baby's size to be off by a centimeter or two. Measurements that are consistently high or low, often raise a red flag for providers to suggest additional testing (typically in the form of ultrasound)

When fundal height measures high, providers want to understand the cause; Is it a variation of normal? Is it excess fluid? Is it baby?

In a normal, healthy pregnancy without conditions such as Gestational Diabetes, it is best to dig a little deeper, gather the evidence and understand your options. So lets talk about the evidence and the guidelines on suspected big babies!

What defines a 'big baby'?

Fetal Macrosomia is defined by the ACOG as,

“ Fetal growth beyond a specific weight, usually 4,000 g (8 lb, 13 oz) or 4,500 g (9 lb, 4 oz), regardless of the fetal gestational age. Results from large cohort studies support the use of 4,500 g as the weight at which a fetus should be considered macrosomic.”

How accurate are ultrasounds at determining a big baby?

Ultrasounds have a 50% accuracy rate (meaning they are right half the time and wrong half the time) Results can be anywhere between 15% above or 15% below your baby’s actual weight. For example, if your baby’s actual weight was 8 lbs, the ultrasound could estimate the baby’s weight to be anywhere between 6 lbs., 13 oz and 9 lbs 3 oz!

"Weighing the newborn after delivery is the only way to accurately diagnose macrosomia (Big baby)"

-ACOG Guidelines

Will I require an induction if they suspect a big baby?

Not necessarily! Induction may lower the chance of shoulder dystocia from 7% to 4% and the chance of birth fractures from 2% to 0.4%. The ACOG states that 'suspected fetal macrosomia is not an indication for induction of labor, because induction does not improve maternal or fetal outcomes'.

Induction increases the risk of other complications and research is still not clear when the best timing is for induction.

"Many studies have shown that the “suspicion” of a big baby typically increases the risk of Cesarean without improving the health of mother or baby. If doctors think you have a big baby, they are more likely to diagnose your labor as stalled, or pressure you into a Cesarean, compared to a woman who has a big baby but it wasn’t suspected. About half of the time when they suspect a big baby, the prediction will have been wrong." -Evidence Based Birth

So what does all of this mean?

You have choices and options for your care and the right to informed consent and informed refusal of any recommended treatment.

Its important to gather the information you need to make an informed choice, knowing the risks, benefits and alternative treatments (with the option to do nothing)

Do your research and talk with a trusted care provider about your unique circumstances and your options.

To read the ACOG guidelines on fetal macrosomia as well as other evidence based resources;

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