What Causes Macrosomia?

When a baby is much larger than average, it can cause childbirth complications. Learn about what causes fetal macrosomia and how it can be diagnosed

Macrosomia is a term used to describe a newborn who is much larger and weighs more than average. On average, babies weigh between 5 pounds, 8 ounces, and 8 pounds, 13 ounces. Babies with fetal macrosomia weigh more than 8 pounds, 13 ounces.

Vaginal delivery of such a big baby is often difficult and carries risks for both mother and baby. Babies born with the condition are also at higher risk of developing health problems later in life.

In general, genetic factors and maternal conditions can cause fetal macrosomia. Below are 8 potential causes of macrosomia:

1. Maternal diabetes: Fetal macrosomia more likely occurs if you had diabetes before pregnancy (pregestational diabetes) or if you develop diabetes during pregnancy (gestational diabetes). If your diabetes isn't well controlled, your baby is likely to have larger shoulders and higher amounts of body fat than a baby whose mother doesn't have diabetes.
2. Maternal obesity: Fetal macrosomia is more likely to occur if you're obese or severely overweight. Gaining too much weight during pregnancy also increases the risk of fetal macrosomia. This is because all nutrients that the fetus receives come directly from the mother’s blood. If your blood has too much glucose, the fetus’ pancreas senses high glucose levels and produces more insulin, converting extra glucose to fat. The combination of high blood glucose levels from the mother and high insulin levels in the fetus results in large deposits of fat that cause the fetus to grow excessively large.
3. History of fetal macrosomia in previous delivery: If you've previously given birth to a large baby, you're at an increased risk of having another large baby. Moreover, if you weighed more than 8 pounds 13 ounces at birth, you're more likely to have a large baby.
4. Previous pregnancies: The risk of fetal macrosomia increases with each pregnancy. Up to the fifth pregnancy, the average birth weight for each successive pregnancy typically increases by up to about 4 ounces (113 grams).
5. Having a boy: Male infants typically weigh slightly more than female infants. Most babies who weigh more than 9 pounds 15 ounces (4,500 grams) are male.
6. Overdue pregnancy: If your pregnancy continues by more than 2 weeks past your due date, your baby is at an increased risk of fetal macrosomia.
7. Maternal age: Women over age 35 are more likely to have a baby with fetal macrosomia.
8. Rare medical conditions: If the above risk factors aren't present and your doctor suspects fetal macrosomia, your baby could have a rare medical condition that affects fetal growth. If a rare medical condition is suspected, depending on test results your doctor may recommend prenatal diagnostic tests and a genetic counselor visit.

Fetal macrosomia is often very hard to diagnose during pregnancy. However, there are a few tests that can be performed to identify symptoms early. In addition to taking your medical history and measuring the baby’s size during pregnancy, your doctor may use the following methods to check for macrosomia: 

• Ultrasound: An ultrasound can help a doctor determine the size of a baby’s head, abdomen, and femur. These measurements can predict the baby’s weight with some degree of accuracy.
• Height of the fundus: The fundal height, measured from the top of the pubic bone to the top of the uterus, usually corresponds with the length of the pregnancy. If the measurement of the fundus seems unusually large for the number of weeks of pregnancy, the baby may be larger than normal.
• Too much amniotic fluid: Amniotic fluid in your uterus surrounds and protects your baby and is linked to the amount of urine they’re producing. An excessive amount sometimes indicates a larger fetus.
• Antenatal testing: If your doctor suspects fetal macrosomia, they may perform antenatal testing, such as a non-stress test or a fetal biophysical profile, to monitor your baby's health.

To prevent any birth complications or injuries, your doctor may recommend the following if they detect fetal macrosomia:

• Scheduled cesarean delivery (making an incision in your uterus and abdomen to deliver your baby)
• Early induction in some cases 

The best thing you can do is eat healthy and stay active during your pregnancy. This will help you maintain a healthy weight, lower your risk of gestational diabetes, and potentially lower your risk of having a baby with macrosomia.