Fetal Macrosomia: An Overview

Medically Reviewed By Joshua Copel, MD
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Fetal macrosomia occurs when a baby is born at a larger size than average. The condition can increase the risk of birth complications for both the parent and the newborn. Macrosomia is also known as “large for gestational age.” Macrosomia affects babies who weigh more than 9 in 10 babies at the same stage, such as when they are born at full term. The weight of babies born before or after 40 weeks is comparable with other babies born before or after full term.

Fetal macrosomia occurs in around 6–10% of pregnancies. Macrosomia can cause complications before, during, and after birth. It can also have an effect on the length of the labor and the type of delivery.

It is possible to lower the risk of fetal macrosomia by improving various factors of your health. Read on for more information about fetal macrosomia, including how to reduce the risk of complications.

What is fetal macrosomia?

A mother is holding a baby.
Chalit Saphaphak/Stocksy United

Fetal macrosomia affects a baby who weighs more than 4 kilograms (kg), or 8 pounds (lb) 13 ounces (oz), at birth. This compares with the 3.12-kg, or 7-lb, average birth weight of a baby.

Macrosomia is more common among birthing parents who have obesity or diabetes. The condition is also more prevalent among babies assigned male at birth than those assigned female at birth.

A diagnosis can be difficult to make in pregnancy. The only true way to know if a fetus will have macrosomia is by weighing them after birth. However, performing ultrasound scans and measuring the height of the uterus can sometimes help predict how big the baby will be.

Indicators of fetal macrosomia

Fetal macrosomia can be difficult to detect during pregnancy. One indicator, however, is a fundal height larger than a doctor would expect it to be. Fundal height is the distance from the top of the uterus to the pubic bone.

After 20 weeks, the fundal height typically matches the week number in centimeters (cm). For example, at week 25, the height is typically around 25 cm, and at week 33, the height is around 33 cm. A larger fundal height than this could indicate fetal macrosomia.

Another indicator is excessive amniotic fluid, or polyhydramnios. Amniotic fluid is what surrounds and protects the fetus inside the uterus. Having a lot of this fluid could indicate that the fetus will be larger than average at birth. In addition, a larger fetus produces more urine, and the amount of amniotic fluid reflects the fetus’s urine output.

Causes of fetal macrosomia

There are numerous factors that can cause fetal macrosomia. These include:

  • Diabetes in the birthing parent: High blood sugar levels in a pregnant person can cause higher insulin and glucose levels in the fetus, leading to an increase in growth.
  • Obesity: Obesity is a risk factor for diabetes. It leads to a fourfold to 12-fold increase in the risk of fetal macrosomia.
  • Previous fetal macrosomia: People who have previously had large babies have a fivefold to 10-fold higher risk of having another baby with fetal macrosomia.
  • Post-date pregnancy: A gestational age of more than 42 weeks is linked to fetal macrosomia due to the continuous supply of nutrients to the developing fetus.

Male fetuses are more prone to macrosomia than female fetuses. Some congenital conditions are also linked to fetal macrosomia. These include Beckwith-Wiedemann syndrome, Sotos syndrome, fragile X syndrome, and Weaver syndrome.

Learn more about factors that can affect a baby’s birth weight here.

Managing fetal macrosomia

It is still possible to have a vaginal delivery with a fetus who is larger than average. Contact your doctor or midwife to discuss the risks and benefits of having a vaginal birth if there is a possibility of fetal macrosomia.

There are some conditions with which your doctor may recommend delivery by cesarean section. These include:

  • Diabetes in the birthing parent: If you have diabetes before pregnancy or develop gestational diabetes during pregnancy and the fetus has an estimated birth weight of more than 4.5 kg, or 9 lb 15 oz, your doctor may recommend a cesarean delivery.
  • A high suspected birth weight: If you do not have diabetes but the fetus has an estimated birth weight of at least 5 kg, or 11 lb, your doctor will likely recommend a cesarean delivery.
  • A history of shoulder dystocia: Shoulder dystocia happens when the fetus’s shoulder gets stuck behind the pelvic bone during delivery. If you have a history of shoulder dystocia, your doctor may recommend a cesarean delivery to prevent this complication from happening again.

Complications of fetal macrosomia

Complications of fetal macrosomia can be categorized as occurring in the birthing parent or occurring in the fetus.

Complications in the birthing parent

One 2016 study reports that 56.3% of people who experienced a macrosomic birth developed at least one complication.

The most common complications include:

  • Postpartum hemorrhage: Macrosomia can lead to uterine overdistention and atony, which are both contributors to postpartum hemorrhages.
  • Perineal trauma: The delivery of babies with macrosomia often occurs with a prolonged second stage of labor and operative vaginal deliveries. This can result in trauma to the perineum, or the area between the genitals and anus.
  • Prolonged second stage of labor: During a prolonged second stage of labor, there is a risk of further complications. These include infection, urinary retention, and hematoma.
  • Uterine atony: This occurs when the uterus does not contract well after the baby and placenta have come out. Contraction of the uterus is the main way the body reduces bleeding after the birth.

Complications in the fetus

The same 2016 study reports that 44.3% of macrosomic births resulted in complications.

Such complications can include:

  • Shoulder dystocia: This is when the fetus’s shoulders get stuck in the vaginal canal after the delivery of the head.
  • Congenital anomalies: Congenital anomalies relate to birth defects, or structural or functional abnormalities. These are more likely to occur in infants of birthing parents who have diabetes.
  • Jaundice: A larger fetus or baby can have an excessive amount of red blood cells. As these break down, the liver can struggle to handle increased amounts of bilirubin, which is a yellowish pigment that results from the typical breakdown of those red blood cells. This can lead to jaundice, or yellowing of the skin, after birth.

Diagnosing fetal macrosomia

A doctor can only make a diagnosis of fetal macrosomia by weighing the newborn after delivery. Predicting fetal macrosomia is often imprecise.

Ultrasound scans can measure fetal body parts, but their accuracy in measuring fetal weights over 4 kg, or 8 lb 13 oz, is low. In fact, an ultrasound scan measuring fetal weight at 4.5 kg, or 9 lb 15 oz, is accurate in predicting birth weight only 33–44% of the time.

Measuring the fundal height is sometimes an option for predicting fetal macrosomia. However, this method is often an unreliable predictor of fetal macrosomia.

A doctor or midwife may also perform a Leopold maneuver to estimate the weight of the fetus. This is a noninvasive technique wherein the doctor or midwife will feel various parts of your abdomen with their fingers and hands to determine the weight of the fetus. This can be as accurate as ultrasound scans in some cases, but it is less accurate in heavier people.

Preventing fetal macrosomia

You may not be able to prevent fetal macrosomia, but some research suggests that exercising during pregnancy and eating a low glycemic diet can help reduce your risk.

Some specific steps you can take to help reduce the likelihood of experiencing fetal macrosomia include:

  • eating a healthy diet
  • eating a low glycemic diet if you have gestational diabetes
  • following the diet that your doctor, midwife, or diabetes care team has prescribed
  • staying active during pregnancy, such as by walking every day

If you have overweight or obesity, schedule a preconception visit to discuss ways of managing your weight before getting pregnant. If you become pregnant, monitor your weight gain. If you are within a moderate weight range, you can expect to gain around 10 kg to 12.5 kg, or 22–26 lb, during pregnancy — mostly after week 20.

Learn more about pregnancy weight gain here.

If you have diabetes before your pregnancy or develop it during your pregnancy, work with your healthcare professional on ways to control your blood sugar levels. Managing your blood sugar levels and keeping them within a healthy range can help reduce the risk of fetal macrosomia and related complications.

Staying active and exercising during your pregnancy can help promote a moderate weight and make you less likely to develop fetal macrosomia. Talk with your doctor or midwife about what exercises might be best for you.

Summary

Fetal macrosomia affects babies who weigh more than average at birth. It is more common among babies who are assigned male at birth and who are born to birthing parents with obesity or diabetes.

Complications of fetal macrosomia can affect both the birthing parent and the baby. Examples include postpartum hemorrhaging, perineal trauma, shoulder dystocia, and jaundice.

If you are concerned about fetal macrosomia, contact your doctor to discuss diet and exercise plans to help manage weight gain during pregnancy. Your doctor will also be able to give you advice on other steps you can take to reduce the likelihood of a macrosomic birth.

Visit our pregnancy hub here.

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Medical Reviewer: Joshua Copel, MD
Last Review Date: 2022 Apr 7
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