What Is Fetal Macrosomia?

Medically Reviewed by Traci C. Johnson, MD on April 23, 2023
3 min read

Big babies, while being very cute, can cause several complications during pregnancy. Babies that weigh more than 8 pounds, 13 ounces when born are diagnosed with a condition called fetal macrosomia.

A baby that is larger than average is at risk of injuring themself and their mother during vaginal birth. A baby with fetal macrosomia may be susceptible to future health problems, such as metabolic syndrome.

Detecting fetal macrosomia is difficult to do during pregnancy. The methods used to predict a baby’s weight during pregnancy are inaccurate. Even ultrasounds can be off by almost 20% when used to determine a baby’s size.

Your fundal height can signal fetal macrosomia. Your doctor will measure the distance between the top of your uterus and your pubic bone to determine your fundal height. The larger the fundal height, the larger your baby might be.

A longer pregnancy usually means you’ll have a larger baby. As the pregnancy continues, the baby continues growing. Once a pregnancy goes beyond 40 weeks, there’s a greater chance of fetal macrosomia.

Too much amniotic fluid may be a sign of a larger baby. The amount of amniotic fluid that surrounds a baby in the uterus reflects the amount of urine the baby produces. Larger babies produce more urine.

Fetal macrosomia is most commonly caused by a mother with uncontrolled diabetes — pregestational or gestational. Higher amounts of sugar in the mother’s system pass through the placenta and converts into fat, leading to a larger baby.

Besides diabetes, other causes of fetal macrosomia are:

  • Having a family history of fetal macrosomia
  • Excessive weight gain during pregnancy
  • Obesity during pregnancy
  • Multiple pregnancies
  • A pregnancy lasting more than 40 weeks
  • A mother with an above-average height and weight
  • Having a male child

The most common factors that determine fetal macrosomia are maternal diabetes, obesity, or weight gain. If these factors aren’t present but the signs of fetal macrosomia are still seen, your baby might have a rare condition that alters their growth.

Fetal macrosomia can cause complications for the baby and mother. Generally, if the baby is estimated to weigh more than 9 pounds, 9 ounces, your doctor will likely suggest a cesarean section (C-section) to avoid any unexpected complications.

A major complication when giving birth to a baby with fetal macrosomia is shoulder dystocia. This occurs during a vaginal delivery when the baby’s shoulders get stuck in the birth canal. This can happen before or during delivery. Shoulder dystocia can be dangerous for the mother and baby.

Shoulder dystocia can lead to the following complications in the baby:

  • Clavicle (collarbone) fractures
  • Fractures in the humerus (upper arm bones)
  • Brachial plexus injury (nerves that connect the spine to the shoulders and arms)

Shoulder dystocia can lead to the following complications in the mother:

  • Excessive bleeding
  • A uterus rupture
  • Vaginal injuries and tearing

Once the baby is born, they may be at risk for other conditions such as.

  • Lower blood sugar levels
  • Childhood obesity
  • Metabolic syndrome that can lead to an increased risk of heart disease, stroke, and diabetes

Fetal macrosomia is unforeseeable and isn’t diagnosed until after the baby is born. Since a baby’s weight can’t be accurately predicted during pregnancy, it’s difficult to predict if your baby will have fetal macrosomia.

A healthy weight can prevent fetal macrosomia. Managing your diet and weight gain before and during pregnancy will help minimize the risk of fetal macrosomia. If you’re considering getting pregnant and you’re obese, you’ll likely be referred to a professional who can help get you to a healthy weight for pregnancy.

It’s normal to put on extra weight during pregnancy. The amount of weight you are allowed to gain depends on your starting weight. Monitor your weight during your pregnancy and let your doctor know if you experience drastic changes in your weight. Your doctor may recommend a diet and exercise regimen to help maintain a healthy weight.

Managing your diabetes minimizes the risk of fetal macrosomia. If you have untreated diabetes, gestational or pregestational, the risk of your baby having fetal macrosomia increases. 

It isn’t necessary to get a C-section if your baby is expected to have fetal macrosomia. Your doctor will monitor your labor carefully for any complications that might arise.

Your doctor may suggest a C-section if you have diabetes, your baby weighs 11 pounds or more, or you’ve given birth to a baby with shoulder dystocia. These factors will put you and your baby at risk, and alternate birth options will likely be suggested.