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.

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:

  • Is obese or gains too much weight during the pregnancy.
  • Has diabetes or glucose intolerance (the body cannot process glucose properly).
  • Is older than 35 or younger than 17.
  • Has already had a large baby.

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