What is fetal macrosomia?
Fetal macrosomia is a condition in which a fetus is larger than average—the birth weight is between 4,000 grams (8 pounds, 13 ounces) and 4,500 grams (9 pounds, 15 ounces). Fetal macrosomia can cause complications for both the baby and the mother during delivery. These risks can be further increased when the weight of the fetus is greater than 4,500 grams.
What is shoulder dystocia?
One of the complications of fetal macrosomia is shoulder dystocia, which occurs when the baby’s head emerges from the womb but the shoulders get stuck inside the mother’s body, requiring additional maneuvers to deliver the baby safely. This is a serious situation that can be more common in large babies.
For the baby, shoulder dystocia can cause fractures in the clavicle (collarbone) or humerus (the bone in the upper arm), or a brachial plexus injury (damage to the nerves that send signals to the arms). In rare cases, it can cause brain damage or death.
For the mother, shoulder dystocia can cause heavy bleeding, a rupture in the uterus, or an injury to the vagina that requires stitches to repair.
When shoulder dystocia occurs, there are several options for delivering the baby safely:
- The doctor can manipulate the baby to change its position.
- The mother can change her position by turning over or by raising her legs up to her belly.
- The baby can be delivered by cesarean section (surgically removed from the mother’s abdomen).
However, research has shown that the value of cesarean sections in dealing with fetal macrosomia is limited, in part because the size of the baby is difficult to predict. Also, some nerve injuries similar to the injuries caused by shoulder dystocia can happen in the womb, before delivery.
Symptoms and Causes
What causes fetal macrosomia?
The longer the pregnancy continues past the due date (especially if the pregnancy extends beyond 40 weeks), the greater the risk of fetal macrosomia, because the baby continues to grow in the womb. Fetal macrosomia is more common in boys.
In addition, fetal macrosomia can occur if the mother:
Diagnosis and Tests
How is fetal macrosomia diagnosed?
During a pregnancy, it’s difficult to know how large the baby will be when he or she is born. Doctors use ultrasound tests to learn what they can about a fetus, including its size. An ultrasound is a diagnostic procedure that transmits high-frequency sound waves through body tissues. The echoes are recorded and transformed into video or photographic images of the internal structures of the body.
To diagnose fetal macrosomia, the doctor will look for:
- The mother’s fundal height (the distance from the top of the uterus to the pubic bone)
- The amount of amniotic fluid, which surrounds the baby inside the womb. If there is an abnormally large amount of amniotic fluid (polyhydramnios), the baby may be larger than average
- The estimated weight of the baby based on ultrasound calculations
The doctor might also ask a parous mother (a woman who has given birth before) how much she thinks the baby weighs.
Can fetal macrosomia be prevented?
Fetal macrosomia is unpredictable. The diagnosis is made only after the baby has been weighed after delivery. Babies can be born larger than average with or without any of the known risk factors.
Promoting good health and a healthy pregnancy can improve the odds:
- See a doctor for regular prenatal care. Frequent visits allow you and your doctor to closely follow your baby’s progress, and also give you a chance to ask questions. Your doctor will make up a schedule for you.
- Watch your weight. Your weight before pregnancy matters, as does how much weight you put on while you are pregnant.
- If you have diabetes, take appropriate steps to manage it. Diabetes before the pregnancy and diabetes that occurs during the pregnancy (gestational diabetes) are risk factors for fetal macrosomia. Gestational diabetes usually goes away after the baby is born, but women who have had it have a greater risk for diabetes later in life.
Outlook / Prognosis
What problems can fetal macrosomia cause after the delivery?
Research suggests that large babies are at greater risk of having low blood sugar (hypoglycemia), being obese in childhood, and developing metabolic syndrome (a cluster of conditions that increase the likelihood of heart disease, stroke and diabetes). Most babies who suffer fractures or nerve damage during delivery recover fully from those injuries.
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