Macrosomia Myths: What You Should Really Be Concerned About
Have you been told your baby is “too big'”? The term macrosomia, often used to label larger babies, is surrounded by misinformation. In this blog post, we’ll break down what macrosomia really means, how common it is, and why your baby’s size may not be as risky as you’ve been told.
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As you enter your third trimester of pregnancy, there’s a lot to do and there’s a lot that can happen.
Unfortunately, the third trimester is also a time when many women are diagnosed with—or at least suspected to have—conditions that can drastically change their birth experience…and not in a positive way.
The thing is, a lot of the time the suspicion is wrong and everything is perfectly normal when the baby is born.
I’ve recently written about fetal growth restriction, the “aging” placenta, and breech positioning, to name just a few of these possible complications.
Today I want to cover another big one…pun intended…
I want to explain what’s really going on when a provider tells a woman she has a “big baby” and therefore needs to give birth sooner rather than later.
The fancy term for a big baby is “macrosomia.” And it’s becoming more and more common for women to be induced or get a c-section simply because a doctor suspects a baby is going to be “too big.”
But macrosomia literally can not be diagnosed until after birth! The only way to know for sure how big a baby is is to weigh him or her once they are born.
And that’s just one element of the misinformation and confusion surrounding “big babies.”
So let’s get into it.
The (Not-So-Helpful) Definition of Macrosomia
Macrosomia is most commonly defined as a baby who weighs more than 8 pounds, 13 ounces (or a nice round 4000 grams). Severe macrosomia is defined as a baby who weighs more than 9 pounds, 15 ounces (4500 grams).
In other words, medical professionals have stated that any baby who grows to be more than 8 pounds and 13 ounces is abnormal and has an increased risk of complications and injury during birth.
As would be expected, many people take issue with that. How can one arbitrary weight limit determine whether or not birth will proceed normally?
It can’t.
As put simply by one group of authors:
“Absolute weight thresholds are not useful for identifying…macrosomic [babies].”
Those authors recommend a percentile method of identifying macrosomia (where the biggest 5% or 2.25% of babies are diagnosed).
But even then, it isn’t necessarily true that just because one baby is much larger than another that that baby will have a harder time being born. For one thing, the first baby’s mother may simply be larger in stature than the other baby’s mother!
My point is that “macrosomia” as it is usually defined and diagnosed is unhelpful. It creates fear in doctors and women alike and can result in more harm done than good.
Macrosomia is Real
That said, I do want to make sure to state that macrosomia is a real thing. Babies can be born at a birth weight that is bigger than what is healthy for them. In other words, babies can be overweight. And they will look it when they are!
But true macrosomia is very rare.
My point in writing about it is to help you see through the misinformation and fear that so often clouds the topic of big babies so you and your baby can have the best birth experience possible.
How common is “macrosomia”?
Though the 4000–4500 gram limit is somewhat arbitrary, we do have statistics on how common it is for a baby to be in that range.
Nearly 90% of women (who aren’t diabetic, which we’ll talk about in a moment) give birth to a baby who weighs less than 8 pounds, 13 ounces.
About 9% give birth to a baby who weighs between 8 pounds, 13 ounces and 9 pounds, 15 ounces.
And about 1% give birth to a baby who weighs more than 9 pounds, 15 ounces.
But how common is it for a birth to be difficult because of a big baby? That depends on what we mean by “difficult.”
Big babies and their mothers are at increased risk of some complications.
HOWEVER.
Most, if not all, of the complications are likely not a result of the baby’s size but how mom and baby are treated because of the baby’s size.
Overall, there are two main things that we need to discuss when it comes to complications caused by a “big baby”:
- The possibility of shoulder dystocia
- How doctors will treat a mom with a suspected big baby
The Truth About Shoulder Dystocia
Dystocia literally means “difficult birth.” A shoulder dystocia describes an instance where the baby’s shoulders are the reason a birth is difficult.
In the case of a shoulder dystocia, a baby’s shoulders will get lodged somewhere in their mother’s pelvis. Obviously, if a baby’s shoulders are stuck, he or she won’t come out as easily as most babies will after their head is born.
Beyond those basics, shoulder dystocia gets…confusing.
I want to cover 4 things to help us all make sense of it. First, let’s look at how common it actually is.
How common is true shoulder dystocia?
It’s common for care providers to label a birth with “shoulder dystocia” when that wasn’t actually the case. True shoulder dystocia meets three criteria:
- Baby’s head is out but the shoulders aren’t following
- At least one minute has passed since baby’s head came out
- Some sort of intervention is required for the baby’s body to be born
True shoulder dystocia is rare. And it’s hard to know exactly how common it is because it is likely overdiagnosed, as I mentioned. But it seems to occur for 5–9% of babies who weigh more than 8 pounds, 13 ounces. (That’s referring to their actual weight at birth, not their estimated weight in the womb.)
Said another way, even in babies who may be labeled “big,” 91–95% of them will be born without experiencing shoulder dystocia.
For comparison, let’s look next at how common shoulder dystocia is for small babies.
Small babies can have a shoulder dystocia, too.
Another important thing to know about shoulder dystocia is that it can happen to a baby of any size. While it is less common overall for small babies, it does happen in about 0.6–1.4% of babies who weigh between 5 pounds, 8 ounces and 8 pounds, 13 ounces at birth.
In other words, 98.6–99.4% of smaller babies will not experience shoulder dystocia.
What does that mean for you? Simply that your baby’s size is not a good predictor of shoulder dystocia.
We can’t accurately estimate a baby’s size before birth.
The next thing to know is that ultrasounds late in pregnancy are not great at predicting the size of a baby. When they happen after 28 weeks of pregnancy, ultrasounds are known to be less accurate. They may be off by as much as 30 days of growth or 2 pounds!
In the case of a big baby, researchers have concluded:
“The prediction of birth weight by ultrasonography or clinical measurement is imprecise. For suspected macrosomia, the accuracy of estimated fetal weight using ultrasound…is no better than that obtained via abdominal palpation [feeling mom’s belly to estimate baby’s size].”
A separate study indicated that “Fetal biometrics [measurements from an ultrasound] had limited ability to predict shoulder dystocia and [therefore] lack clinical usefulness.”
(Interestingly, that same study found that using an epidural seemed to be the only thing associated with a clear increase in chances of shoulder dystocia.)
So not only is shoulder dystocia because of a big baby nearly impossible to predict, we can’t even know for sure that a baby is big!
RELATED >> Understanding Ultrasound Safety: Both Sides of the Story
Few cases of shoulder dystocia result in serious problems.
Finally, let’s look at what shoulder dystocia is like—and how serious it is or isn’t—in those rare cases that it does happen.
Shoulder dystocia, if not resolved within a few minutes, can be life-threatening. The baby’s chest or cord may be compressed, cutting off oxygen supply.
However, most cases of shoulder dystocia are resolved easily and quickly. And when resolved in the right way, injury to mom and baby is minimal or non-existent.
This is one area where midwifery care is far superior to typical medical care.
Shoulder Dystocia in the Hospital
In the hospital, most women give birth while lying on their back. This decreases the width of the pelvic opening and keeps the pelvis still rather than allowing it to be mobile, which would help the baby to navigate his or her way through.
Because mom is on her back and because doctors tend to intervene rather than let the woman manage birth, they use maneuvers such as the McRoberts’ maneuver to get the baby out manually.
(The McRoberts’ maneuver entails pulling mom’s knees all the way to her armpits and, often, applying external pressure right above her pubic bone to push the baby out.)
Shoulder Dystocia with a Midwife
A midwife would handle a shoulder dystocia very differently. First, she would encourage mom to move all throughout labor, even if it was only to walk to the bathroom and back.
Being upright allows gravity to help baby move down. And movement keeps the pelvis mobile, making movement through the pelvis easier for baby.
If a shoulder dystocia does occur despite being upright and mobile during labor, the first thing most midwives will do is help a mother get on her hands and knees and then get her into a runner’s lunge position (one foot flat on the ground and kneeling with the other).
Frequently, just this movement and the asymmetrical position helps to shift the baby enough for them to be born.
If that isn’t enough, a midwife will likely apply pressure above the pubic bone. And if that doesn’t work, a midwife will reach inside a mother’s vagina to help the baby turn manually, using well-practiced maneuvers to get the baby out.
Risks of True Shoulder Dystocia
The most common negative outcome of shoulder dystocia (though it is still quite rare) is brachial plexus nerve injury. The brachial plexus is a cluster of nerves that goes from the spinal cord in the neck down through the shoulder and into the arm.
Damage to a baby’s brachial plexus can happen in the case of shoulder dystocia because many doctors try to get the baby out by pulling gently on the baby’s head. But even gentle pressure can damage those delicate nerves.
You can see why in this video (click on image to go to YouTube):
If a baby’s shoulder is stuck behind a bone in their mother’s pelvis, pulling on their head will only stretch the neck and shoulder and can do damage to those nerves. Usually, the nerves will heal and the arm will return to full function (though sometimes it takes months or years). Rarely, the damage is severe and permanent.
Brachial plexus injury occurs in about 3% of shoulder dystocia cases.
Other injuries that may occur because of a shoulder dystocia are even more rare but include broken bones (2% of cases) and brain damage (0.3%).
A Doctor’s Fear Is Dangerous
With that understanding of shoulder dystocia, it’s time to talk about the other big risk factor for “big baby” births: doctors.
While I do believe that most obstetricians have good intent and are trying their best to provide safe care for mothers and babies, I also know they have a different perspective than midwives. They see birth as a high-risk situation. So that is how they treat it.
In this specific case, evidence has shown repeatedly that a doctor’s suspicion that a baby is big is more dangerous to a mother and her baby than an actual big baby.
3x the Risk
A study published in 2008 made this truth clear. The researchers looked at outcomes for moms and babies when baby was suspected to have macrosomia (and actually was big) compared to babies who weren’t labeled as macrosomic but did end up being over 8 pounds, 13 ounces at birth.
I think Evidence Based Birth explains it best (emphasis added by me):
“The end results were astonishing. Birthing people who were suspected of having a big baby (and actually ended up having one) had triple the induction rate, more than triple the Cesarean rate, and a quadrupling of the maternal complication rate, compared to those who were not suspected of having a big baby but had one anyway.
“Complications were most often due to cesareans and included bleeding (hemorrhage), wound infection, wound separation, fever, and need for antibiotics. There were no differences in shoulder dystocia between the two groups.”
In other words, babies of the same big size at birth had vastly different outcomes simply because some were thought to be too big and some were big but nobody knew it.
When NOT to Talk About Risk
Another study backed up what the 2008 study had concluded. The study took place in a hospital where staff routinely warned women about the risks of birthing a big baby if their baby was suspected to weigh more than 8 pounds, 13 ounces.
Researchers found that when women received warnings about having a big baby and were counseled on the “risks,” significantly more of them had c-sections (and faced the associated risks) compared to women who birthed babies of the same size but weren’t given any warnings.
The researchers recommend not telling women about the “risks” of big babies unless the baby is suspected to weigh at least 9 pounds, 15 ounces.
Women’s Experience is Worse
Finally, beyond medically worse outcomes, women have worse experiences when their care provider suspects their baby is big.
Researchers carried out a study to demonstrate that this is a very real concern.
They concluded, “The prediction of a ‘large’ baby in pregnancy has undeniably negative impacts on women. [These women] struggle with fear and guilt as they experience their pregnancies as sites of risk and are constituted as failed mothers who are responsible for their large babies.”
That is not how it should be!
Regardless of how common or uncommon shoulder dystocia and big babies are, no woman should be set up to feel like a failure for how her pregnancy and birth went.
What to Do
One of the best things you can do to avoid a suspicion of macrosomia is to decline a third trimester ultrasound (when no other factors indicate one is needed).
Third trimester ultrasounds are very rarely necessary. If your baby was fine at your 20 week scan, they are almost certainly fine now.
That said, if you feel like you should get an ultrasound, do it! Following your intuition (which I believe is one and the same with divine inspiration) is your best bet for a healthy pregnancy and a safe birth.
If you want an ultrasound because it helps you feel connected to your baby and it’s fun to see him or her, I get it. But maybe give it a second thought because the chance you take in getting that ultrasound has very real consequences.
If you choose to accept a third trimester ultrasound, you must also accept the high chance that your doctor will think your baby is too big (or too small), your amniotic fluid levels are not what they should be, or that some other factor indicates that you should get induced early.
Induction and C-sections Have Risk, Too
While there is so much I could explain about shoulder dystocia and what increases your risk of it and macrosomia during birth and what doesn’t, it’s far too much to cover in one blog post. But one simple thing that is important and easy to remember is this:
No option is risk-free.
Sure, a large baby may mean more risk than an average-size baby. And shoulder dystocia can create complications.
But so can an induction. And, certainly, so can a c-section.
One of the problems with the way women are “informed” in a hospital setting is that the information is often presented without giving the full picture of all options.
A woman will be told about the risks of her baby getting “too big” but not about how induction can increase the chances of her baby being premature or needing to be born by c-section.
She may be told that shoulder dystocia could lead to severe brachial plexus injury but she might not be told that getting a c-section may lead to placental problems in future pregnancies.
The thing that needs to change isn’t how care providers handle shoulder dystocia (though that does need improvement).
The change we need is for care providers to give women ALL the information in an unbiased way so they can make a decision based on their circumstances and their preferences, not because they feel pressured into something out of fear.
Other Things to Know About “Big Babies”
There are a few final things I want to share on the topics of macrosomia and shoulder dystocia so that you have the information you need to go back to your care provider and make a decision you feel good about.
These are in no particular order.
Perineal Tears and Postpartum Hemorrhage
You will likely hear that birthing a big baby vaginally increases your risk of severe perineal tears and postpartum hemorrhaging. Let me break down a few things about both.
Severe Perineal Tearing
First, the two biggest risk factors for severe perineal tearing are 1) episiotomies and 2) assisted birth, both of which doctors may use if they suspect a big baby.
Pair that with the fact that I know of women who have given birth at home to 10+ pound babies and had no tearing. It seems evident that a baby’s size isn’t the issue here.
In other words, maybe the likelihood of tearing goes up not because the baby is big but because of what a doctor does to intervene in the birth of a big baby.
Postpartum Hemorrhaging
Second, people often cite postpartum hemorrhaging as another complication of giving birth to a big baby. Let’s first establish that postpartum hemorrhaging has many causes and so is a complicated risk to look at.
In the case of a bigger-than-average baby, it doesn’t really make sense that postpartum hemorrhaging would be a direct complication.
If birth proceeds normally, the uterus will contract to control bleeding and no severe injuries should be present that would lead to excessive bleeding.
One possible reason for increased bleeding after the birth of a big baby may be that suspected big babies are more likely to be born after induction or by c-section, both of which inherently increase the risk of excessive bleeding.
Finally, it’s important to consider the role of hormones in preventing hemorrhaging after birth. The moment of birth and breastfeeding in the time after triggers the release of oxytocin in mom. Oxytocin helps the uterus to contract down and close the wound left by the placenta.
But fear and stress fight against oxytocin. If mom is unhappy with her birth situation or worried about her baby’s well-being, her body isn’t going to release as much oxytocin, thus potentially allowing the body to bleed more than is safe.
Induction Doesn’t Improve Outcomes
Next let’s address the question of whether or not induction for a “big baby” improves outcomes.
Multiple studies have provided evidence that induction because of suspected macrosomia does not improve outcomes for baby or mom.
A review of 11 studies about induction for suspected macrosomia versus waiting concluded that “labor induction for suspected fetal macrosomia results in an increased cesarean delivery rate without improving perinatal outcomes [outcomes for baby].”
The American College of Obstetricians and Gynecologists (ACOG), which provides recommendations for how OBGYNs should practice, recommends against induction before 39 weeks because of suspected macrosomia.
They state: “There is insufficient evidence that benefits of reducing shoulder dystocia risk would outweigh the harms of early delivery.”
Diabetes and Weight Gain are Correlated with Macrosomia
Finally, let’s talk about how diabetes comes into play with macrosomia. While macrosomia is very hard to predict accurately, there are a few things that increase the chances of having an extra-big baby…two things, actually:
- Poorly-controlled diabetes
- Excessive weight gain during pregnancy
Because of the way diabetes (chronic or gestational) affects a mother’s body, it can lead to a baby growing larger than they “should” or would have if their mother didn’t have diabetes.
A lot of weight gain during pregnancy (or being overweight before getting pregnant) can have the same effect on a baby.
How Diabetes Affects a Baby
When a mother has diabetes or is heavier, her body produces extra insulin, a hormone that helps regulate blood sugar levels and gives a person energy. But diabetes means your body doesn’t use that extra insulin properly.
The American Diabetes Association explains how this affects a woman’s baby:
“When you have gestational diabetes, your pancreas works overtime to produce insulin, but the insulin does not lower your blood glucose levels. Although insulin does not cross the placenta, glucose and other nutrients do. So extra blood glucose goes through the placenta, giving the baby high blood glucose levels. This causes the baby’s pancreas to make extra insulin to get rid of the blood glucose. Since the baby is getting more energy [from the insulin] than it needs to grow and develop, the extra energy is stored as fat.”
All that said, even a woman with gestational diabetes or who is gaining a lot of weight during pregnancy isn’t necessarily going to have a macrosomic baby.
In a group of nearly 13,000 women, 30–40% of women with diabetes had a baby who was large for their gestational age. That means that 60–70% of women with diabetes didn’t have an extra-big baby.
Put simply, even the best predictors of macrosomia that we know of still only have a 30–40% accuracy.
Conclusion: Mother Knows Best
To end, I want to quote a study which I admire for stepping away from scientific and medical norms to acknowledge the power of a mother’s intuition.
After concluding that ultrasound and clinical estimations of a baby’s weight aren’t very good at predicting macrosomia, the researchers stated:
“Merely asking a [pregnant] woman for her estimate of [her baby’s] birth weight may provide an estimate as accurate as any other.”
Hats off to those researchers.
Because that’s the truth: mom is going to know best. If something is wrong, a mother who is in tune with her body, her baby, and God is going to know, long before a doctor tells her so.
When it comes to macrosomia, I love one midwife’s motto: “Our bodies grow babies that will fit.”
So, trust your body and your baby. Lean into your intuition.
If you are taking care of yourself, you will know if something is wrong. Otherwise, trust that everything will be alright…because you were created for this.
You were created to give life.
Until next time,
Allison
P.S. ACOG lists a few iatrogenic (meaning caused by a doctor’s actions) risk factors associated with shoulder dystocia. Those include:
- Induction with Pitocin
- Using an epidural
- Poor use of maneuvers
- Assisted birth (forceps or vacuum)
So if you want to further decrease your chances of macrosomia if your baby seems big, avoid those things.
READ MORE:
The Cascade of Interventions [Explained]
Fetal Growth Restriction: What It Is and What It’s Not
