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.
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:
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.
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:
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 doctor might also ask a parous mother (a woman who has given birth before) how much she thinks the baby weighs.
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:
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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This information is provided by the Cleveland Clinic and is not intended to replace the medical advice of your doctor or healthcare provider. Please consult your healthcare provider for advice about a specific medical condition. This document was last reviewed on: 07/02/2018