Macrosomia | 5 Myths About Having A Big Baby

When thinking about giving birth, we often jump to thinking about the size of baby. There is so much joking and fear about huge babies in the movies, on commercials and even amongst friends.

We think having a big baby means pain, complications and birth interventions, such as a c-section.

But is baby’s size something we really need to worry about?

Firstly, it helps to understand some basics about the definition of a big baby, and what it really means.

The Definition Of Macrosomia

Macrosomia literally means ‘large body,’ and is the medical term for a big baby. A baby is considered macrosomic by some if they are 4kg (8lb 13oz) or larger. Other professionals define macrosomia as a baby weighing 4.5kg (9lb 15oz) or larger.

The average size of a full term baby is 3.4kg (7lb 8oz). Keeping in mind that an average is just that, an average. Some babies will be smaller than that, others much larger. That’s how we calculate an average, by working out the mid figure. So there is a wide range of normal.

How Common Are Big Babies?

If we define a large baby as ≥4.5kg, around 1.8% of Australian babies are considered macrosomic, or big. Similar statistics are seen in the US with 1.8% of babies being born ≥4.5kg.

If we define large babies as ≥4kg around 8.7% of US babies are macrosomic.

While having a large baby can increase the risk of certain birth complications, a large baby does not automatically equal a high risk or painful birth. We hear of growth scans, percentages and weights before baby is even born, which leads to the idea that baby’s body size is extremely important. While baby’s size can impact labour, there are many myths surrounding just how much it impacts it.

What are the myths about macrosomia babies and birth:

Macrosomia Myth #1: A Baby Can Be Accurately Diagnosed As Macrosomic Before Birth

While we have ways to estimate baby’s size in utero, they are only educated guesses.

The only way to know baby’s actual size is to weigh and measure them after birth.

Measuring the fundal height (the size of the uterus) is a good way to track general growth. It tells us that pregnancy is progressing. While it has its role in monitoring growth, fundal height can’t give us baby’s exact size. Fluid, the size of the placenta and baby’s positioning can all impact measurements.

The Leopold’s maneuver, a specific way to palpate or feel baby, can offer some assistance in estimating the baby’s size. Feeling the baby, figuring out the baby’s position and the fundal height can act as one piece to the puzzle of guessing baby’s size.

Ultrasounds offer a peek inside the uterus, a way to check out baby before it is born. It’s a wonderful diagnostic tool, but it still has limitations. Early in the first trimester, an ultrasound offers pretty accurate size estimates, as early baby development is pretty standard. But as pregnancy progresses and genetics come into play, the accuracy in measuring baby is less. A third trimester weight prediction can vary from the actual birth weight by 10-15%.

Each method of measuring baby provides an estimate. When all are used together it can provide an educated guess. However, a measurable error rate exists, so we must remember that it truly is only an estimate.

Macrosomia Myth #2: Having Gestational Diabetes Means I Will Birth A Big Baby

While certain risk factors — like gestational diabetes — can increase the chance of a big baby, it does not guarantee it.

In the US, almost 9% of babies are born weighing more than 4kg, so about 1 in 10 babies. With different providers using varying definitions of macrosomic, it’s a good idea to ask your care provider which weight they use to define a big baby. If we use 4.5kg as the defining weight, then only 1.7% of babies born are considered big.

Overall, women have a 1 in 10 chance of having a baby weighing more than 4kg. If you have gestational diabetes, then there is a 13.7% chance of having a baby over 4kg and a 2.6% chance of having a baby over 4.5kg. So while the risk increases, it is still far from a guarantee. Women who treat their gestational diabetes may even cut their risk of having a big baby by 50%.

Perhaps the bigger risk comes from undiagnosed or untreated gestational diabetes. Mothers diagnosed with gestational diabetes have the ability to watch their diet, remain active and exercise, and monitor their blood sugars. If a mother is borderline, doesn’t watch her sugars and doesn’t follow a diabetic friendly diet, she is at greater risk for giving birth to a large baby. Some studies even indicate a 20% increased risk of having a large baby. For this reason, regular exercise and a healthy whole foods diet is ideal for all expectant mamas.

For mothers with type I or type II diabetes the risk of a big baby is higher. There is a 23.2% of having a baby larger than 4kg and 6.1% chance of having a baby larger than 4.5kg. While this is significant, a mother with type I or type II diabetes will work closely with her providers to monitor her pregnancy, blood sugar and baby’s growth.

Macrosomia Myth #3: High BMI Equals Big Baby

A BMI in the overweight or obese range is linked to a greater risk of having a big baby. However, it is a risk factor and not a guarantee. Women with high BMIs are more likely to have gestational diabetes, which is why we see an increased risk of a large baby.

Having a high BMI might put you at risk for pregnancy complications, such as gestational diabetes, big baby and pregnancy induced hypertension, but it does not always equal a high risk situation. The majority of women, regardless of BMI, have babies less than 4kg.

All expectant mothers should do their best to have eat a whole foods diet and maintain healthy activity levels, as considered safe by their healthcare providers. Pregnancy isn’t the time to try fad diets or attempt to lose weight. It is a great time to try lifestyle changes though! Even if your BMI stays the same throughout pregnancy, a good diet and exercise can greatly reduce your risk of gestational diabetes and having a large baby.

Macrosomia Myth #4: A Big Baby Means A High Risk Birth

One of the most important things to remember about the average baby weight is that it is simply that, an average. Many large babies are born without any complications. Some high risk births involve smaller than average babies.

Many women worry about cephalopelvic disproportion (CPD), when baby’s are too large to pass through the pelvis. While we hear about CPD often, true cases are very rare and are associated with pelvic abnormalities (the result of congenital anomalies or pelvic fractures). A large baby does not equal a large head.

Shoulder dystocia, a serious birth complication where the baby’s shoulders are stuck in the mother’s pelvis, is another concern we hear of. While this is something women fear when thinking of birthing a large baby, the overall incidence of dystocia is small, 0.2-2.1%. This rate varies due to varying definitions of dystocia and the rate at which providers report it.

While it is a very serious complication, and the rate of dystocia increases as baby gets larger, it still isn’t a guarantee with a large baby. In many cases, a skilled provider can resolve shoulder dystocia without incident.

Having a big baby can increase the risk of shoulder dystocia and implied CPD, tearing and pelvic floor damage, and the catch all diagnosis failure to progress (FTP). However, there are many things we can do to reduce the risk of those things.

Avoiding unnecessary inductions and assisted births, and remaining upright and mobile during labor and birth can reduce the risk of complications. It is also important to note that it is nearly impossible to diagnose CPD or predict the risk of shoulder dystocia prior to birth.

Macrosomia Myth #5: Scheduled Inductions And C-Sections Are Always Indicated With A Big Baby

It isn’t uncommon to hear of scheduled c-sections or inductions due to a suspected big baby. The problem with this, there’s no way to accurately diagnose a macrosomic baby prior to birth. While some providers recommend scheduled births, there isn’t enough evidence to support this as routine management for suspected big baby.

Many women give birth naturally to large babies without trauma or complications. Yes, risks can increase with larger babies but c-section and inductions also have risks. With it being impossible to accurately diagnose macrosomic babies in utero, it is important to weigh the benefit and risk of spontaneous labor vs scheduled birth.

Are Big Baby Complications Unnecessarily Created?

An interesting study found that women with suspected macrosomic babies had higher rates of complications when compared to women who had unexpected macrosomic babies. This suggests that in our effort to avoid rare complications, we create unnecessary ones. It also means that some of the studies indicating an increased risk of complications for big babies may not be comparable to having physiological birth with a big baby.

• Women who gave birth to a big baby that was suspected prior to birth had a 42% induction rate. While women who gave birth to a big baby that wasn’t suspected had an induction rate of only 14%
• The maternal complication rate for suspected big babies was 17% compared to just 4% when big baby was not suspected
• The c-section rate for suspected big babies was shown at 52% with unsuspected big babies c-section rate being just 17%

Having a big baby can be a concern for some. If you have risk factors, try to reduce them. Keeping in mind that we can control some aspects of pregnancy and birth, but many things are also unpredictable and out of our control.

Here are a few things to consider if you are worried about having a big baby:

1. Maintain a healthy, whole foods and diabetic friendly diet if you have gestational diabetes (GD) or your blood sugar is borderline GD.
2. Remain active to keep blood sugar at healthy levels
3. Choose a provider that doesn’t utilize routine growth scans. Growth scans have a place in obstetrical care, but should be reserved to cases where risk factors are present
4. Keep in mind you cannot accurately diagnose a big baby prior to birth
5. Remain upright and moving during labor and birth
6. Choose a provider with low scheduled induction and c-section rates. While scheduled births are sometimes medically indicated, it should be the exception to their typical care and not their norm