Big Babies: When Your Your Fetus is Above the 90th Percentile

There is no one in the world who is more concerned about how your baby is going to fit through your pelvis than you. Being told your fetus is in the 90th percentile is nerve-wracking. Which is why I get so upset when providers approach a conversation about a suspected big baby based on ultrasound measurements. I'm not talking about folks with other medical diagnoses, like gestational diabetes. I'm talking about perfectly healthy, low-risk folks. I've had this conversation twice this week with two different clients, so now I'm just going to go off.

What does big baby even mean?

A baby might be labeled as "big" if they are measuring at or above the 90th percentile.

• The technical term is large for gestational age

• To get a true diagnosis of fetal macrosomia your baby must weigh 4000 grams, or about 8lb 13oz at birth regardless of gestational length; this diagnosis cannot be given from ultrasounds even if they are estimating your baby's weight is above 4000 grams

  • Average birth weight is around 7lb 6oz according to the WHO

• BUT risks don't rise until the baby weighs >4500 grams, or 9lb 15oz, about the 97th percentile at birth

• An extremely large baby is over 5000 grams or 11lbs

  • Fun fact: my colleague just caught a 12lb 5oz baby for someone who was unmedicated at a birth center; your body grows a baby that is the right size for your body!

Anecdotally, I have had many clients who have been told they have a suspected big baby who then go on to deliver a slightly larger (around 8lb), but definitely not macrosomic, baby. I feel frustrated on behalf of my clients who spend the last weeks of their pregnancy worrying extra over something that generally they are already worried about. What pregnant person isn't constantly thinking, "How am I going to get this baby out???"

The largest baby I have seen born was 9lb 12oz and that baby was born peacefully into water at a birth center during an unmedicated labor. No one suspected that the baby was big. There was no pressure from the midwives about pushing quickly or forcefully. My client listened to her body and birthed her baby in her own time. She had a small first degree tear (most common tear type is second degree). I was so proud of that person for choosing a team of providers that aligned so well to her desire for a low-intervention birth. She may have had a different experience in another setting.

What Increases Your Risk of Having a Big Baby?

You are more at risk of having a baby that is large for gestational age if:

• Big babies run in your family, or you yourself were a big baby (genetics)

• The baby's sex is male

• You have diabetes, either before pregnancy and/or during

• You've had other big babies prior to this baby

• All the risk factors docs love to bring up: higher BMI, older age, past your due date

According to the NHS, "most babies, even large babies, will have no problems."

We grow babies that are the right size for our bodies. So what is all the fuss about?

The fuss is about providers and their nervous systems. I'm in my "everything is about the nervous system" era and I will not apologize for it. There are some extremely rare complications associated with babies over 4500 grams (9lb 13oz). Providers see a LOT of birth and so chances are that they have seen those complications.

One complication is shoulder dystocia, which occurs just as often in big babies as in smaller babies, but doctors tend to really worry about it. Add onto that the fact that injuries related to shoulder dystocia are one of the most common reasons for litigation against doctors. Then, add onto that that giving birth on your back - the preferred position for most docs - also closes your pelvis and makes it difficult to do some of the maneuvers that help baby's shoulders make it past your pubic bone.

You put all of those factors together and you get providers who really want to avoid vaginal birth if they suspect it will be hard or complicated. A larger baby stresses them out. In fact, the biggest risk associated with having is a suspected big baby is just that - your provider suspecting it! In fact, docs are so stressed about big babies and shoulder dystocia that they just ran a study in the UK where they looked to see if induction for suspected big babies would lower the risk of shoulder dystocia. The study was cut short because there were no differences between the folks who were induced and the folks who went into labor spontaneously BUT the researchers re-analyzed the data until they found a difference and then made a big fuss about that.

Bottom line: many providers have a bias against suspected big babies.

Babies heads mold to help them fit - that is where you get the cone head!

How Likely is a Big Baby?

One out of 3 people are told at the end of their pregnancy that their baby is big. At minimum, every third person in America is told they will have a big baby. But if it is the 90th percentile, how could that be possible? Your baby is supposed to be the largest out of 10. The math is not mathing.

From Evidence Based Birth:

Although only one in ten babies is born large, researchers found that ...one out of three families in the study were told that their babies were too big. In the end, the average birth weight of their suspected “big babies” was only 7 lbs., 13 oz. (Declercq, Sakala et al. 2013).

Of the people who were told that their baby was getting big, two out of three said their care provider discussed inducing labor because of the suspected big baby, and one out of three said their care provider talked about planning a Cesarean because of the big baby.

In other words, you are very likely to be told you have a big baby and then have your provider recommend induction or cesarean. Even though you may in fact not have a big baby on your hands, and inductions and cesareans have no been shown to decrease risks.

FAQs

Is there anything I can do to prevent a big baby?

There isn't much that you can do. It is hard to accurately predict a big baby. As with pregnancy in general, a nutritious diet and regular exercise will help. Get screened for gestational diabetes.

If you would like to prevent the suspicion of a big baby, you can forgo the ultrasounds at the end of pregnancy that are meant to measure baby's size. They are fairly inaccurate anyway. If you have no medical reason for an ultrasound (remember, suspected big baby is not a medical reason), then you can opt out. Sure, you won't get to see your baby on the screen, but you may save yourself a whole lot of stress and worry.

What are the risks associated with a big baby?

The number one risk of a suspected big baby is having an unplanned cesarean related to provider bias. Other risks include:

• Perineal tears

• Postpartum hemorrhage

• Newborn complications like low blood sugar

• Stillbirth, but varies quite a lot from situation to situation

The risks do seem to be higher with gestational diabetes and that is why many providers encourage or even demand an induction in the 39th week for folks with GDM.

What is shoulder dystocia?

After the baby's head is born, next are the shoulders. They typically come one at a time. Shoulder dystocia means that one of them is stuck, usually under the birthing person's pubic bone.

Half of all shoulder dystocia cases happen with babies who are under 8lb 13oz. Shoulder dystocia is just as common in big babies as it is in smaller babies, but providers worry about it more with big babies. You may want to talk to your provider about their experience with shoulder dystocia, but know that even bringing it up will put them on edge.

There are many other risk factors that are associated with shoulder dystocia, like induction and oxytocin augmentation (pitocin). You can read more here.

Shoulder dystocia is no necessarily a bad outcome, but there are injuries related to trying to resolved a shoulder dystocia that are a concern.

How accurate are the ultrasounds to predict big babies?

Out of 10 people who are told that their babies are big, 5 will be born weighing over 8lb 13oz. The other 5 will be born weighing less than 8lb 13oz. So about 50% accurate. We just know that ultrasounds are inaccurate.

My favorite study to quote looked at three measures of baby size to gauge the most accurate: they used ultrasound technology, they had a trained provider feel the baby from the outside, and they asked the pregnant person what they thought. What do you think was the most accurate?

The pregnant person's guess was most accurate. The ultrasound was the least accurate of the three.

Are there better outcomes for people who have an induction for a suspected big baby?

A Cochrane review found little difference between those assigned to induction and those who went into labor spontaneously.

• Inductions saw a slight decrease in shoulder dystocia

• But also lead to an increase in severe tears

There is no known best timing for induction for a suspected big baby. Often, providers will say things like, "Well, your baby isn't getting any smaller." Inductions are a big deal: a long list of medical interventions that you are up front agreeing to at least consider and likely to have administered to you. When it is medically necessary, there is no question that induction is the right call. Making sure that the benefits outweigh the risks for your specific case is important, and for many that means understanding if induction is medically necessary.

I highly recommend that you look at the information on inducing for big babies from Evidence Based Birth. Overall, more research is needed before suspected big baby can be considered a medically indicated reason for induction.

Should I just have a cesarean?

It would take 3,700 unnecessary cesareans to prevent one case of permanent injury due to shoulder dystocia. Again, shoulder dystocia can happen in any size baby. No way of giving birth is risk-free. Cesareans have their own risks and benefits, and they do have increased risks for subsequent pregnancies.

Note: ACOG does support cesarean for babies that are suspected of being 11lbs or greater.

Ultimately, no one can make the choice for you because no one has the exact same risk profile as you.

I have had clients who are told, right off the bat, that since their baby is likely in the 90th percentile they may want to consider an elective cesarean. Not only does this undermine my client's confidence in themselves and their bodies, it also undermines their confidence in the care team to safely deliver the baby. You need to feel safe and empowered in order for the labor hormones to flow and a suspected big baby does not do you any favors. This is where choosing a place to birth at that has both providers AND policies that support your preferences is key.

Want to learn more? Try listening to this Evidence Based Birth podcast, which summarizes the recent research.

As with all of my blog, I am writing from my perspective. Doulas don't give medical advice - we help clients achieve true informed consent through information and data. I am a doula and therefore none of this is medical advice.