Macrosomia is a term that describes a baby who is born much larger than average for their gestational age, which is the number of weeks in the uterus. Babies with macrosomia weigh over 8 pounds, 13 ounces.

On average, babies weigh between 5 pounds, 8 ounces (2,500 grams) and 8 pounds, 13 ounces (4,000 grams). Babies with macrosomia are in the 90th percentile or higher in weight for their gestational age if born at term.

Macrosomia can cause a difficult delivery, and increase the risks for a cesarean delivery (C-section) and injury to the baby during birth. Babies born with macrosomia are also more likely to have health problems such as obesity and diabetes later in life.

About 9 percent of all babies are born with macrosomia.

Causes of this condition include:

  • diabetes in the mother
  • obesity in the mother
  • genetics
  • a medical condition in the baby

You’re more likely to have a baby with macrosomia if you:

  • have diabetes before you get pregnant, or develop it during your pregnancy (gestational diabetes)
  • start out your pregnancy obese
  • gain too much weight while pregnant
  • have high blood pressure during pregnancy
  • have had a previous baby with macrosomia
  • are more than two weeks past your due date
  • are over 35 years of age

The main symptom of macrosomia is a birth weight of more than 8 pounds, 13 ounces — regardless of whether the baby was born early, on time, or late.

Your doctor will ask about your medical history and past pregnancies. They can check your baby’s size during pregnancy, however this measurement isn’t always accurate.

Methods to check the baby’s size include:

  • Measuring the height of the fundus. The fundus is the length from the top of the mother’s uterus to her pubic bone. A larger than normal fundal height could be a sign of macrosomia.
  • Ultrasound. This test uses sound waves to view an image of the baby in the uterus. Although it’s not completely accurate at predicting the birth weight, it can estimate whether the baby is too large in the womb.
  • Check the amniotic fluid level. Too much amniotic fluid is a sign that the baby is producing excess urine. Larger babies produce more urine.
  • Nonstress test. This test measures your baby’s heartbeat when he or she moves.
  • Biophysical profile. This test combines the nonstress test with an ultrasound to check your baby’s movements, breathing, and the level of amniotic fluid.

Macrosomia can cause these problems during delivery:

  • the baby’s shoulder may get stuck in the birth canal
  • the baby’s clavicle or another bone gets fractured
  • labor takes longer than normal
  • forceps or vacuum delivery is needed
  • cesarean delivery is needed
  • the baby doesn’t get enough oxygen

If your doctor thinks your baby’s size could cause complications during a vaginal delivery, you may need to schedule a cesarean delivery.

Macrosomia can cause complications to both the mother and baby.

Problems with the mother include:

  • Injury to the vagina. As the baby is delivered, he or she can tear the mother’s vagina or the muscles between the vagina and anus, the perineal muscles.
  • Bleeding after delivery. A large baby can prevent the muscles of the uterus from contracting like they should after delivery. This can lead to excess bleeding.
  • Uterine rupture. If you’ve had a past cesarean delivery or uterine surgery, the uterus can tear during delivery. This complication could be life-threatening.

Problems with the baby that may arise include:

  • Obesity. Babies born at a heavier weight are more likely to be obese in childhood.
  • Abnormal blood sugar. Some babies are born with lower than normal blood sugar. Less often, blood sugar is high.

Babies born large are at risk for these complications in adulthood:

They’re also at risk of developing metabolic syndrome. This cluster of conditions includes high blood pressure, high blood sugar, excess fat around the waist, and abnormal cholesterol levels. As the child gets older, metabolic syndrome can increase their risk for conditions like diabetes and heart disease.

If tests during your pregnancy show that your baby is larger than normal, here are a few questions to ask your doctor:

  • What can I do to stay healthy during my pregnancy?
  • Will I need to make any changes to my diet or activity level?
  • How might macrosomia affect my delivery? How could it affect my baby’s health?
  • Will I need to have a cesarean delivery?
  • What special care will my baby need after birth?

Your doctor may recommend a cesarean delivery as necessary to ensure a healthy delivery. Inducing labor early so that the baby is delivered before its due date, hasn’t been shown to make a difference in the outcome.

Babies born large should be monitored for health conditions like obesity and diabetes as they grow. By managing preexisting conditions and your health during pregnancy, as well as monitoring the health of your baby into adulthood, you may be able to help prevent complications that can arise from macrosomia.