Having a baby who's bigger than average for their gestational age? Here's what to expect when it comes to fetal macrosomia.
What is fetal macrosomia?
Macrosomia means "large body" and is used to describe a newborn who's much larger than average. (The average newborn weighs about 7 pounds.)
There isn't a single definition for macrosomia. Historically, babies with macrosomia weigh more than 8 pounds, 13 ounces (4,000 grams) or more than 9 pounds, 15 ounces (4,500 grams) at birth. Macrosomic babies are more likely to have a difficult delivery. But the risk of complications is significantly greater when a baby is born weighing more than 9 pounds, 15 ounces.
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The Centers for Disease Control and Prevention (CDC) estimates that 6 percent of infants born in 2021 weighedbetween 8 pounds 13 ounces and 9 pounds 14 ounces at birth, and less than 1 percent weighed 9 pounds 15 ounces or more.
How will I know if my baby is macrosomic?
It's difficult to tell how big your baby is while they're still in the womb, but your healthcare provider may suspect macrosomia if you're measuring large for dates. Also, because bigger babies produce more amniotic fluid, excessive amniotic fluid (polyhydramnios) might be a sign.
What causes macrosomia?
Some women are just genetically predisposed to have larger babies, and birth weight also tends to increase with each successive pregnancy.
Most women who have a baby weighing more than 9 pounds have no risk factors, but macrosomia may be more likely if:
- You have unmanaged high blood sugar levels from diabetes or gestational diabetes
- You have obesity
- You're tall
- You've gained too much weight during pregnancy
- You've already had a large baby. If you previously delivered a macrosomic baby, you're five to 10 times more likely to have another large baby.
- You're more than two weeks past your due date
- You were large for gestational age (LGA) yourself
- You're over age 35
- You have certain genetic abnormalities or syndromes (such as Sotos syndrome or Beckwith-Wiedemann syndrome
Also, male babies are more often macrosomic than females. And mothers who are white, American Indian, or Samoan are more likely to have macrosomic babies than women of other ethnicities. A study of mothers with gestational diabetes found that Latino women had macrosomic babies more often than Black women.
How does a big baby affect delivery?
With a big baby, you have a greater chance of a difficult vagin*l delivery. You may also have an increased risk of preterm birth, perineal tearing, and blood loss.
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Also, if you've had a previous C-section or major uterine surgery, a large baby would increase your risk of uterine rupture, a rare but dangerous complication.
A large baby also means you're more likely to have an assisted vagin*l delivery or a cesarean. Although it's difficult to determine a baby's exact size before birth, your doctor may want to schedule a C-section if you're measuring large or have other risk factors for macrosomia.
The American College of Obstetrics and Gynecologists (ACOG) doesn't recommendinducing labor early for suspected macrosomia, since this doesn't have any proven benefit.
Can macrosomia cause problems for my baby?
If your baby is macrosomic, there's a higher risk for birth injury and some complications. But most of the possible complications usually resolve with no long-term consequences.
There's a small chance of shoulder dystocia, a rare but potentially serious complication in which the baby's shoulder gets caught behind your pubic bone, causing the baby to get stuck in the birth canal during delivery.
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This situation is a medical emergency. Your healthcare provider will need to do some maneuvering or perform an episiotomy to get your baby out safely.
In rare cases, your baby could end up with a broken collarbone or upper arm bone. (The treatment is to immobilize the arm as much as possible until the fracture heals.) A more serious complication of shoulder dystocia is nerve damage to the arm on the side where the shoulder was trapped.
A macrosomic baby is also at higher risk for:
- Low blood sugar
- Lower Apgar score
- Childhood obesity
- Metabolic syndrome in childhood, which can increase the risk of heart disease, diabetes, and stroke.
- Breathing problems immediately after birth
What is recovery like after giving birth to a big baby?
If you had a perineal tear or an episiotomy, be sure to follow your provider's instructions for perineal care, and watch for signs of infection.
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If you had gestational diabetes, your blood glucose levels should return to normal after birth. But you still have an increased risk of developing diabetes in the future, so within a few months of your baby's birth, schedule a follow-up appointment with your provider to be tested for postpartum diabetes or other problems with glucose metabolism.
Can I prevent macrosomia?
There are things you can do to reduce the risk:
- Start pregnancy at a healthy weight if possible. Try to lose weight before becoming pregnant if you're overweight.
- Do your best to maintain a healthy pregnancy weight.
- If you have diabetes or develop gestational diabetes, stick to your provider's recommendations to control your blood sugar.
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