Fetal Macrosomia: Everything You Need to Know

Reviewed by Olga Adereyko, MD, Primary Care Physician, General Practitioner, Medical Consultant

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What is fetal macrosomia?

The fetal macrosomia definition applies to newborns weighing more than eight pounds 13 ounces (regardless of gestational age). They comprise roughly nine percent of all births worldwide and can pose serious medical consequences for women in labor. The situation is even more concerning when the newborn weighs nine pounds 15 ounces or more.

Macrosomia complications include difficulties with vaginal delivery for mom and complex medical issues for the baby. At times, fetal macrosomia might be unavoidable, especially if a genetic predisposition is to blame. However, practicing good prenatal care and adopting healthy habits may reduce your chances for fetal macrosomia.

Symptoms of fetal macrosomia

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If your doctor observes signs that your fetus is larger than average, they’ll likely order a macrosomia ultrasound. Warning signs include:

• Unusual fundal height

As part of your prenatal checkups, your fundal height is routinely measured to track fetal growth. It represents the distance between the top of your uterus (or fundus) and your pubic bone. 

• Overabundance of 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.  

Causes of fetal macrosomia

Although in some instances, there’s no discernible reason behind the condition, one of the most common fetal macrosomia causes is genetics. Couples who are physically larger naturally produce bigger babies. As the mother, your overall health also comes into play. If you’ve been diagnosed with diabetes (type 1, type 2, or gestational) or struggle with obesity, your child has a greater chance of developing fetal macrosomia. In rare circumstances, they may have a medical issue that speeds up their growth.

Contributing factors for fetal macrosomia

Each pregnancy is unique. So you might exhibit some or even all of the typical neonatal macrosomia risks and still give birth to an average-sized baby. These signs include:

• Poorly controlled diabetes

Obesity and/or untreated diabetes (and elevated blood glucose levels) could produce a fetus with large shoulders and a lot of body fat. These two chronic conditions, in particular, have often been linked to fetal macrosomia.

• History of macrosomia

If you’ve already given birth to one child weighing more than eight pounds 13 ounces, you’re quite likely to have another. 

• Excessive weight gain

Most doctors recommend putting on between 25 and 35 pounds during pregnancy (if you were previously of average weight). But if you had a BMI between 25 and 30 prior to conception, you should only gain 15 to 25 pounds, or you’ll up your chances of neonatal macrosomia.

• Prior pregnancies

After your first child, each one born tends to weigh a bit more. The average birth weight for each subsequent pregnancy increases by roughly four ounces.

• Carrying a baby boy

Boys are frequently born larger than girls. As a result, severe cases of fetal macrosomia (birth weight of nine pounds 15 ounces or more) are usually boys.

• Going past your due date

Not surprisingly, pregnancies that pass the 40-week mark might produce heavier-than-average newborns.

• Advanced age

If you’re pregnant and older than 35, your child carries a greater chance for macrosomia.

Fortunately, if you’ve remained at a normal weight (before and during pregnancy), and haven’t been diagnosed with diabetes, fetal growth should progress as expected. But if your baby appears large for their gestational age, your doctor may recommend tests or begin monitoring you more closely. 

Consequences of macrosomia

Complications arising from fetal macrosomia can directly affect the health of both mother and baby. 

Complications for the mother

• Difficulties with delivery

Your doctor may need to use forceps or a vacuum suction apparatus to prevent your fetus from getting stuck in the vaginal canal. Alternatively, you might require an emergency C-section.

• Vaginal lacerations 

A baby with fetal macrosomia is capable of tearing or damaging the birth canal or your perineum (the area between your vagina and anus) during delivery.

• Post-delivery bleeding or hemorrhaging 

Fetal macrosomia makes it difficult for your uterine muscles to properly contract and return to pre-pregnancy size and position, sometimes leading to serious bleeding.

• Uterine rupture

If you’ve had previous C-sections, your uterus could tear open along the scar line, warranting surgery or even a partial hysterectomy. It’s a potentially fatal condition for both you and your newborn if it’s not immediately addressed.

Complications for the baby

Macrosomia complications can affect your child throughout life, and include:

• Low blood sugar 

When blood glucose levels drop, a body can no longer function normally. 

• Childhood obesity

Research indicates that higher birth weight increases the likelihood of obesity.

• Metabolic syndrome 

This set of medical conditions increases their chances of diabetes, heart attack, and stroke. This is due to the presence of high blood sugar, high blood pressure, abnormal cholesterol, and excess body fat around the waist.

Macrosomia complications

Aside from the fetal macrosomia complications listed above, a condition known as shoulder dystocia could be life-threatening for both you and your baby. 

Fetuses with macrosomia tend to get their shoulders stuck as their heads emerge from the womb during delivery. Shoulder dystocia could even break the collarbone, the humerus bones in their upper arms, or cause brain and nerve damage. In very rare cases, shoulder dystocia can be fatal.

Takeaway

Seeking quality prenatal care and maintaining a healthy pregnancy weight may help reduce your chances of fetal macrosomia. Take active steps to avoid serious medical complications for you and your child arising from macrosomia. If your fetus is measuring larger than normal, consult your doctor about the proper way to address the problem.

History of updates

Current version (18 December 2019)

Reviewed by Olga Adereyko, MD, Primary Care Physician, General Practitioner, Medical Consultant

Published (13 April 2020)