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Fetal Macrosomia

Although having a low birth weight can pose health risks for a baby, having a high birth weight can also be an issue. Babies with fetal macrosomia, or high birth weight, are at risk for a variety of issues, particularly during labor and delivery. Caregivers should closely monitor a pregnancy in order to detect high birth weight early, because interventions can significantly decrease the risk of complications.

What is fetal macrosomia?

Fetal macrosomia refers to a baby with a birth weight that is particularly high. The exact definition varies, but medical sources usually use 4000g (approximately 8 pounds, 13 ounces) or 4500g (approximately 9 pounds, 15 ounces) as the cutoff point.

of babies in the US weigh more than 8 pounds, 13 ounces when they’re born.
Babies with fetal macrosomia are at an increased risk for problems during labor and delivery. These issues can create serious lifelong consequences for the child. There are also some long-term health risks caused by fetal macrosomia.

What causes fetal macrosomia?

Fetal macrosomia is most commonly a result of various conditions in the mother that affect the baby’s rate of growth during the pregnancy. It’s not always possible to predict exactly which babies will grow to be larger than average. However, there are certain factors that increase the risk of fetal macrosomia, including:

  • Maternal diabetes. Some women develop diabetes during pregnancy (gestational diabetes), while others have diabetes before getting pregnant. Diabetes is a major risk factor for fetal macrosomia. 
  • Maternal obesity. When the mother’s body is larger, it’s more likely that the baby will also grow to be larger.
  • A history of fetal macrosomia. Mothers who have given birth to larger babies in the past are significantly more likely to do so again. In addition, if the mother had fetal macrosomia when she was born, then she’s more likely to give birth to a baby with the condition.
  • Higher weight gain during pregnancy. The Institute of Medicine recommends different weight gain targets during pregnancy, depending on the mother’s weight status before getting pregnant. Women who gain more weight than the recommendations are at a higher risk for fetal macrosomia.
  • Previous pregnancies. The average birth weight for a baby gets larger with each successive pregnancy. This is believed to be related to the stretching of the tissues.
  • Postterm pregnancy. This refers to a pregnancy that’s gone beyond the expected date of delivery (EDD), or “due date.” In a pregnancy that has extended for more than two weeks beyond the EDD, there is a greater that the baby will have fetal macrosomia.
  • Older maternal age. Women over the age of 35 are more likely to have a baby with fetal macrosomia.
  • Medical conditions in the baby. While fetal macrosomia is much more commonly associated with factors like maternal diabetes, obesity, or excessive weight gain, there are also some rare genetic conditions that can cause the baby to grow more quickly than average.

In cases with more risk factors, doctors should closely monitor the pregnancy so that fetal macrosomia can be detected early.

How is fetal macrosomia diagnosed?

It’s impossible to know the exact birth weight of a baby until after the birth. Instead, doctors can use various methods to estimate the size of the baby, including:

  • Fundal height, which is a measurement of the size of the uterus from the front of the pubic bone to the top of the uterus. A larger pregnant uterus suggests a large fetus, although this is not always accurate.
  • Weight gain during pregnancy. Although this is an imprecise measurement of the size of the baby, increased weight gain by the mother is an indication that the baby may be larger than average.
  • Leopold’s maneuvers, which are a series of physical examination techniques developed by a German physician many years ago. 
  • Ultrasound measurements of the baby’s size. If doctors are worried that the baby is large, then they may do an ultrasound in the third trimester, taking a variety of measurements to estimate the size of the baby. However, measuring the size of the baby using ultrasound is difficult, and studies have shown that the measurements differ from a baby’s actual birth weight by up to 15 to 20%.
  • Measurement of the amount of amniotic fluid by ultrasound. If the amount of amniotic fluid is higher than normal, this may be an indication that the mother’s blood sugars are not well-controlled, and the baby is at risk for fetal macrosomia. 

The American College of Obstetricians and Gynecologists recommends using a combination of fundal height and Leopold’s maneuvers to diagnose fetal macrosomia before birth. Surprisingly, according to ACOG, measuring the baby’s size by ultrasound has been found to be no more accurate than Leopold’s maneuvers.

The mother’s weight gain should also be monitored, to increase the chances of detecting fetal macrosomia in time to intervene and prevent serious complications. The recommendations for weight gain depend on the mother’s prepregnancy weight status. The most recent recommendations are that a mother should gain a total of:

28 to 40
pounds if she’s underweight
25 to 35
pounds if she’s of normal weight
15 to 25
pounds if she’s overweight
11 to 20
pounds if she’s obese
If the mother’s weight gain during pregnancy is higher than these targets, then there’s a higher risk that her baby will be born with fetal macrosomia.

What are the complications of fetal macrosomia?

For mothers of babies with fetal macrosomia, there is an increased risk of complications during delivery, including:

  • Injuries to the mother’s body during delivery, such as significant tearing of the muscles of the perineum or even the anus.
  • Uterine rupture, in which the uterus becomes torn. This can lead to severe and life-threatening bleeding.
  • Postpartum hemorrhage, or excessive bleeding after the birth. When the uterus is excessively stretched because of the baby’s large size, the muscles may not be able to contract after the birth. This uterine muscle contraction is necessary to stop bleeding.
  • The need for assisted delivery (using forceps or vacuum extractors) or delivery by Cesarean section, which can both increase the damage to the mother’s body related to the birth. If complications develop during labor and delivery, an emergency Cesarean section may be necessary.
Pregnant woman in pain

Babies with fetal macrosomia are also an increased risk of medical problems, including:

  • Shoulder dystocia, when the baby’s shoulder gets stuck in the birth canal. This increases the risk of birth injuries, such as damage to the nerves of the arm or fractures of the collarbone.
  • Birth asphyxia, in which the baby is deprived of oxygen during birth. This can lead to severe and lifelong complications, due to damage to the brain and other organs.
  • Medical problems later in life, including childhood obesity and metabolic syndrome (a combination of high blood pressure, high blood sugar, abnormal cholesterol levels, and increased body fat).

It’s important for the medical team to be aware of the risks that fetal macrosomia can present, so they can take steps to manage the risks and achieve the best possible outcomes for both mother and baby.

The foot of an infant

How is fetal macrosomia treated?

The management of fetal macrosomia may involve two main steps. During the pregnancy, treatments may be helpful to manage risk factors and decrease the risk of macrosomia. 

  • In pregnant women with diabetes, keeping blood sugar under control helps to avoid fetal macrosomia. The better controlled the mother’s diabetes is, the less likely the baby is to grow too large. Careful blood sugar management is crucial to help prevent serious issues for both the baby and the mother. 
  • Monitoring the mother’s weight gain, and counseling her on how to stay healthy during pregnancy, is also an important part of prenatal care. The American College of Obstetricians and Gynecologists recommends that all pregnant women be counseled about appropriate weight gain, and studies have shown that this simple intervention can significantly reduce the risk of fetal macrosomia and improve outcomes. Unfortunately, studies have shown that about one-third of pregnant women don’t receive counseling from their providers about weight gain.

The delivery should also be carefully managed in order to reduce the chances of significant injuries to the mother and/or the baby.

  • In cases where the baby is suspected to be very large, or if the mother has significant risk factors (such as diabetes), then doctors may recommend a scheduled Cesarean delivery. This can help to avoid the significant risks that vaginal delivery can pose in the setting of fetal macrosomia.
  • When a baby is suspected to be large, and vaginal delivery is chosen, medical providers should be ready to intervene if problems start to develop during the labor and delivery process. If the baby becomes stuck in the birth canal or experiences significant distress, then it may be necessary to perform a Cesarean section quickly.
  • Medical providers sometimes recommend inducing labor at or near full term, so that the baby will be somewhat smaller. However, this approach carries a significant risk of needing a Cesarean section anyway, because inductions of labor are often unsuccessful when the mother’s body is not yet ready to give birth. In addition, induction of labor for fetal macrosomia has not consistently been shown to significantly reduce the risks associated with the condition. As a result, induction of labor is typically not recommended for fetal macrosomia.

What if the medical team misses the signs of fetal macrosomia?

It’s important for doctors to evaluate and recognize the signs of fetal macrosomia. If they miss these signs, then they may fail to intervene in time to avoid significant complications. If the healthcare team isn’t aware that the baby is large, then they may take longer to intervene during a problematic vaginal birth, leading to injuries to both the baby and the mother.

In some cases, providers don’t realize that the baby has fetal macrosomia, and they don’t intervene in time. This can lead to serious injuries to the baby, such as brachial plexus injury. It may also cause oxygen deprivation during birth, which can have lifelong consequences for the child.

Medical providers have a duty to recognize and manage the risks of labor and delivery, including the extra risks created by fetal macrosomia. If they fail to perform this duty, then they may be held liable for the harm that they cause. If you believe that medical providers failed to provide you with proper medical care for fetal macrosomia, and you and/or your child experienced harm as a result, then you should contact an attorney to discuss your case. Due to the statute of limitations, you have a limited amount of time to file a claim, so don’t delay in seeking legal advice.

Written by:
Birth Injury Center Team

The Birth Injury Center aims to create informational web content and guides to help women and their families seeking support and guidance for birth injuries caused by medical negligence. All of the content published across The Birth Injury Center website has been thoroughly investigated and approved by medical expert Natalie Speer, RNC-OB, Attorney Ryan Mahoney.