The “Big Baby” Dilemma
The trouble with using ultrasound to determine baby size.
If you’re like me, you eagerly anticipated your first prenatal ultrasound. It’s a sneak peek at your growing baby. There’s a nose! And toes! Of course, you get adorable photos, but an ultrasound is also intended to assess baby’s development, as well as the health of the placenta and uterus.
Ultrasound is clinically used as a screening tool for various conditions. It’s also often used to determine fetal size. In fact, two out of three American women report receiving an ultrasound at the end of pregnancy to get a size estimate before birth.
Ultrasound Sizing, Not a Precise Science
In my first pregnancy, my second trimester ultrasound showed my daughter measuring unusually large, what is known as macrosomia. (The American College of Obstetricians and Gynecologists, ACOG, confusingly defines macrosomia as a baby weighing more than either 8 lbs, 13 ounces, or 9 lbs, 4 ounces.) My doctor ordered a follow-up ultrasound closer to my due date.
What was she worried about? In short, shoulder dystocia, the “nightmare of obstetricians” per this 2010 study. This happens when a baby gets stuck in the birth canal. Even if well-managed, shoulder dystocia is one of the most highly litigated types of cases in obstetrics, due to birth-related injuries to both mom and baby. One of the biggest risk factors for shoulder dystocia is having a “big baby.”
To avoid these kinds of complications, medical teams rely on ultrasound to provide size estimates. However, there’s one significant problem: ultrasound sizing is shockingly inaccurate.
In a 2012 study, researchers found prenatal ultrasound has an error margin of up to 15 percent for an average-sized baby. So that estimated 7-pound baby could actually weigh 1 pound more or less.
When ultrasound is used to attempt to identify “big babies” larger than 8 pounds, 13 ounces, it’s correct less than 50% of the time.
That means if you had 10 babies predicted to be “big”, five or more will actually weigh less than the predicted weight. That’s no better than tossing a coin!
Ultrasound Sizing and Intervention
Why does it matter if using ultrasound to size babies is imprecise? Because numerous studies have shown that if a care provider even suspects the possibility of a big baby, that suspicion alone leads to higher induction rates and higher Cesarean rates.
Yet, according to ACOG and many research studies, these interventions for big babies “do not improve maternal or fetal outcomes,” and they do not recommend them.
When my baby continued to look large on my third trimester ultrasound, my care team, which included a well-respected maternal fetal medicine specialist, recommended a planned induction on my due date. At the time, I didn’t know that ACOG recommended against this approach. I trusted my doctors, who had guided me through a high-risk pregnancy full of health scares.
A previous family experience also weighed on my decision. My nephew was unexpectedly large, and he experienced shoulder dystocia during birth. He was pinned in the birth canal for a lengthy period of time and stopped breathing. Ultimately, the attending physician broke his collarbone to get him out, saving his life. A terrifying story for a mom carrying a “big baby.”
But the data shows that my chances of experiencing the same situation weren’t as high as my doctors led me to believe. Research estimates shoulder dystocia occurs in 0.6 to 3 percent of all births. This risk increases to a rate of 5 to 9 percent among babies weighing between 8 pounds, 13 ounces, and 9 pounds, 3 ounces, and born to mothers without diabetes. The risk goes up for larger babies, but study estimates vary widely on the precise number.
Weighing Your Choices
So what should you do? Natural birth advocates, including the famous midwife Ina May Gaskin, would tell you adamantly not to induce for a suspected large baby. A 2016 systematic literature review found increased risk of perineal damage for women induced due to large baby weight concerns. The same review also found some benefits (decreased numbers of birth fractures and shoulder dystocia) for inducing near term for babies who were actually larger than normal. However, the authors also point out that many women who ended up having average-sized babies likely received unnecessary inductions because of ultrasound inaccuracy.
In short, this is your baby and your decision.
You make your own judgement call based on what you believe is best for you and your baby.
For me, I chose to be induced on my due date for a variety of health concerns, including size. Luckily, I had a healthy baby girl after a routine vaginal delivery. Was she a 10-pounder like the ultrasound predicted? Not even close.
She weighed a perfectly normal 7 pounds, 7 ounces.
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