The average newborn baby weighs 7½ pounds at birth. But some grow much larger. In fact, there's a technical term for babies weighing more than 8 pounds 13 ounces when they're born.

Called macrosomia, it affects around 8 percent of infants. Most macrosomic babies are born perfectly healthy without complications. But babies born on the bigger side can face some risks. 

Here’s a look at how macrosomia happens, how it might affect you and your baby, and what you can do to reduce your risk or manage the condition if you have it. 

What is macrosomia?

When an infant weighs more than 8 pounds 13 ounces at birth, she’s considered a "big baby" — or one with macrosomia.

Macrosomia can occur when a baby gets more nutrients in utero than she needs, causing her to grow faster and larger than usual. However, some babies are just genetically programmed to be taller or bigger than others. 

Who is at risk for macrosomia or being pregnant with a big baby?

Doctors aren’t always sure why some newborns end up bigger than others, since some women with no discernible risks give birth to babies with macrosomia. But there are some factors that can increase the chance of having a big baby. These include:

How is macrosomia diagnosed?

It can be tough to get a sense of your baby’s exact weight while she’s still in utero. For that reason, macrosomia isn’t officially diagnosed until after you’ve given birth.

But before then, your practitioner can get a sense of whether your baby might be big by looking at your medical history along with:

  • Measuring your fundal height. This is the distance between the top of your growing uterus and your pubic bone. Having a higher-than-expected height could mean a larger baby.

  • Manually feeling your abdomen. Touching your belly can also give a sense of your baby’s overall size. This can be a little uncomfortable for you, but rest assured — it doesn’t hurt the baby.

  • Conducting regular ultrasounds. Ultrasounds can offer a rough idea of your baby’s weight as well as the level of amniotic fluid. Excessive amniotic fluid — a condition known as polyhydramnios — is often associated with macrosomia. Larger babies pee out larger quantities of urine, and more urine means more amniotic fluid.

How can you prevent having an overly big baby?

Macrosomia can be unpredictable, and there’s no guaranteed way to prevent it. The good news is that there are steps you can take to reduce your baby’s risk.

First up? Do what you can to keep your blood sugar in check if you have type 1, type 2 or gestational diabetes. Too-high glucose levels can cause your baby to get more nutrients than she needs, raising the risk for macrosomia.

Talk to your practitioner about the best ways to control your diabetes while pregnant, like eating a diet that’s high in complex carbohydrates and dietary fiber and low in cholesterol, saturated fat and refined sugars.

Take steps to maintain a healthy weight, too, whether you have diabetes or not. If you gained too much during a prior pregnancy or are worried about gaining an excessive amount during this pregnancy, talk to your doctor about safe ways to keep your weight gain steady.

Most pregnant women are advised to fit in at least 30 minutes of low-intensity exercise (like walking, yoga or swimming) five days a week and to follow a healthy diet featuring plenty of whole grains, vegetables, fruits and lean protein.

How can you treat or manage macrosomia if your baby is too big?

Simply put, bigger babies can sometimes be harder to push out. Many moms whose babies have macrosomia are still able to deliver vaginally.

If your baby is growing big, your practitioner will want you to give birth in the hospital in case a forceps delivery, a vacuum-assisted delivery or a C-section become necessary.

There are some cases where your doctor might recommend a C-section in advance though, including:

  • If your doctor suspects your baby is larger than 9 pounds 15 ounces and you have diabetes.

  • If your doctor suspects your baby is larger than 11 pounds and you don’t have diabetes.

  • If you previously delivered a baby with shoulder dystocia — where a baby’s shoulder gets stuck behind the mother’s pelvic bone.

No matter how your baby arrives, she’ll be examined for blood sugar problems or any potential birth injuries. You’ll need to be tested for diabetes, too, if you weren’t diagnosed during pregnancy.  

What are the complications associated with being pregnant with a big baby?

Macrosomia is manageable, but it can pose some potential risks for both moms and babies — particularly when a baby is born weighing more than 9 pounds 15 ounces. 

Babies born vaginally could be more likely to experience an injury like shoulder dystocia, where the baby’s shoulder gets stuck behind the mother’s pelvic bone.

If your practitioner suspects shoulder dystocia, he or she might try to manipulate your baby to encourage her to change positions or have you change positions. If that doesn’t work, your doctor may use a vacuum device or forceps or recommend an emergency C-section. 

The more challenging labor could create some added risks for moms as well.

Bigger babies can sometimes cause vaginal tearing or increased bleeding after delivery. Moms who have previously undergone a C-section also have a higher chance for uterine rupture — a rare but serious complication where the uterus tears open along your scar line.

After a macrosomic baby is born, she's more likely to require a short stay in the NICU for low blood sugar, jaundice or trouble breathing. But keep in mind: Most little ones born with macrosomia have no complications at all and should be able to go home with you as soon as you’re ready. 

Macrosomia has the potential to lead to some complications for both you and your baby. But the condition is manageable, and many macrosomic babies are born perfectly healthy. Just remember to stock up on some larger diapers!