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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!
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- WhatToExpect.com, Your Newborn’s Weight: Normal Gains and Losses and What the Average Baby Weighs, August 2020.
- WhatToExpect.com, Type 2 Diabetes During Pregnancy, February 2021.
- WhatToExpect.com, How Does Gestational Diabetes (GD) Affect Your Pregnancy and Baby?, December 2020.
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- Stanford Children’s Health, Large for Gestational Age, 2022.
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From your baby's first day, doctors will keep track of weight, length, and head size. Growth is a good indicator of general health. Babies who are growing well are generally healthy, while poor growth can be a sign of a problem.
How Big Are Newborns?
Newborns come in a range of healthy sizes. Most babies born between 37 and 40 weeks weigh somewhere between 5 pounds, 8 ounces (2,500 grams) and 8 pounds, 13 ounces (4,000 grams).
Newborns who are lighter or heavier than the average baby are usually fine. But they might get extra attention from the doctors and nurses after delivery to make sure there are no problems.
Different things can affect a baby's size at birth. The length of the pregnancy is important. Babies born around their due date or later tend to be larger than those born earlier.
Other factors include:
- Size of parents. Big and tall parents may have larger-than-average newborns; short and petite parents may have smaller-than-average newborns.
- Multiple births. If you have twins, triplets, or more, you can count on your babies being a bit small. Multiples have to share their growing space in the uterus, and they're often born early, which leads to small size at birth.
- Birth order. First babies are sometimes smaller than brothers or sisters born later.
- Gender. Girls tend to be smaller, boys larger, but the differences are slight at birth.
- Mom's health during pregnancy. Things that can lead to a lower birth weight include a mother with high blood pressure or heart problems; or one who used cigarettes, alcohol, or illegal drugs during the pregnancy. If the mother has diabetes or is obese, the baby may have a higher birth weight.
- Nutrition during pregnancy. Good nutrition is vital for a baby's growth — before and after birth. A poor diet during pregnancy can affect how much a newborn weighs and how the infant grows. Gaining a lot of weight can make a baby more likely to be born bigger than average.
- Baby's health. Medical problems, including some birth defects and some infections during the pregnancy, can affect a child's birth weight and later growth.
What About Preemies?
Premature babies generally are smaller and weigh less than other newborns. A preemie's weight will largely depend on how early he or she was born. The time an infant missed being in the womb was growing time, so the baby has to do that growing after birth.
Many pre-term babies are classified as having "low birth weight" or "very low birth weight." In medical terms:
- Low birth weight means a baby weighs less than 5 pounds, 8 ounces (2,500 grams) at birth. That's the case for about 1 in every 12 babies in the United States, so it's quite common.
- Very low birth weight means a baby weighs less than 3 pounds, 5 ounces (1,500 grams).
Most babies with low birth weight or very low birth weight were born prematurely.
Premature babies get special medical attention right away after they're born. A specialist called a neonatologist may help care for them. Many preemies spend time in the neonatal intensive care unit (NICU) while they get medical care.
Is Bigger Better?
A baby with chubby cheeks and dimpled thighs once was many people's picture of a healthy newborn. But a baby born much larger than average may have special medical problems that need attention.
Some very large babies — especially those born to mothers with diabetes, including gestational diabetes — may have problems for a few days keeping blood sugar levels up. They might need extra feedings or even IV (given into a vein) to keep those levels from falling too low.
Will My Baby Lose Weight?
Yes, at first. Babies are born with some extra fluid, so it's normal for them to drop a few ounces when they lose that fluid in the first few days of life. A healthy newborn is expected to lose 7% to 10% of the birth weight, but should regain that weight within the first 2 weeks or so after birth.
During their first month, most newborns gain weight at a rate of about 1 ounce (30 grams) per day. They generally grow in height about 1 to 1½ inches (2.54 to 3.81 centimeters) during the first month. Many newborns go through a period of rapid growth when they are 7 to 10 days old and again at 3 and 6 weeks.
pShould I Be Concerned?
Newborns are so small, and it can be hard to know if your baby is gaining weight the way he or she should. You may worry that your baby has lost too much weight in the first few days or isn't taking enough breast milk or formula. If so, talk to your doctor, who may ask you about:
- How many feedings a day your baby gets. A breastfed baby may feed about 8 or more times in a 24-hour period; formula-fed babies usually eat less often, perhaps every 3 to 4 hours. A lactation (breastfeeding) counselor can make suggestions to increase comfort and improve technique, if a mom needs extra help.
- How much your baby eats at each feeding. A baby generally nurses for at least 10 minutes, should be heard to swallow after 3 or 4 sucks, and should seem satisfied when done. At this age, formula-fed babies may drink up to 3 to 4 ounces (90 to 120 milliliters) at a time.
- How often your baby pees. A breastfed baby may have only 1 or 2 wet diapers a day until the mother's milk comes in. Expect about 6 wet diapers by 3 to 5 days of age for all babies. After that, babies should have at least 6 to 8 wet diapers a day.
- How many bowel movements your baby has each day, and what they're like. Newborns may have only one poopy diaper a day at first. Poop is dark and tarry the first few days, then becomes soft or loose and greenish-yellow by about 3 to 4 days. Newborns usually have several poopy diapers a day if breastfed and fewer if formula-fed.
What Else Should I Know?
Being small or large at birth doesn't mean a baby will be small or large later in childhood or as an adult. Plenty of tall teens began life as small babies, and the biggest baby in the family can grow up to be a petite adult.
By the time they're adults, kids tend to resemble their parents in size. Genetics, as well as good nutrition and your attention, will play a large part in how your baby grows in the years to come.
Whether your baby starts out large, small, or average, in the next few months you can expect your little one to keep growing fast.