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Fetal Macrosomia

Macrosomia Complications, Fetal Macrosomia, Menopause Center of Los AngelesA newborn is considered larger than average if it weighs more than 8lbs, 13oz at birth, no matter how long its gestational period. Roughly 9% of infants are born with fetal macrosomia.

It’s not inherently dangerous to have a large newborn, but the there are some risks of complications when carrying and delivering a large baby. Some infants with macrosomia may be at increased risk for childhood metabolic diseases. These risks increase significantly when the newborn weighs more than 9lbs, 15oz.

Signs and Symptoms of Fetal Macrosomia

Fundal height

At each prenatal visit, your OB-GYN will measure your fundal height. This is the length between the pubic bone and the very top of your uterus. An abnormally large fundal height could be a sign that the baby is larger than normal.

Amniotic Fluid

The amount of amniotic fluid is often measured to estimate the size of the fetus. If your doctor finds excessive amniotic fluid during a prenatal visit, this may indicate fetal macrosomia.

Causes of Macrosomia

There are many cases when the cause of fetal macrosomia is unknown. However, some conditions do seem to affect the incidence rate of macrosomia and some of these factors are more or less under your control.

  • Diabetes. If the mother has diabetes or develops diabetes while pregnant (gestational diabetes), she is more likely to give birth to a large baby.
  • Maternal obesity.
  • Gaining excessive weight during pregnancy.
  • Maternal age. Women over 35 are more at risk for fetal macrosomia.
  • History of macrosomia. If you have given birth to a large baby in the past, or if you yourself were born larger than 8lbs, 13oz, then you are more likely to carry a large baby.
  • Previous pregnancies. With each pregnancy, your risk for macrosomia increases.
  • The baby is a boy. Male infants tend to weigh more than female infants.
  • Overdue pregnancy. When the baby is more than two weeks past its due date, the odds are higher that it will have a high birth weight.

The most common culprits for fetal macrosomia are diabetes, maternal obesity, and excessive weight gain during pregnancy. There are also some rare genetic conditions that increase the fetal growth rate. Talk to your gynecologist about these conditions and how they may affect your pregnancy.

Macrosomia Complications

High-Risk Obstetrics, Macrosomia, Menopause Center of Los AngelesCarrying and delivering a large baby increases some health and safety risks. Your doctor will work with you to ensure the safest possible delivery if fetal macrosomia is present.

Difficult labor

A baby with macrosomia may get stuck in the birth canal. Sometimes, this causes injuries in the baby and may require the doctor to remove the baby using a vacuum device or forceps. In some cases, the doctor may recommend a Cesarean delivery.


Delivering a large baby through the birth canal may cause a lot of damage to your uterine muscles. If they cannot contract properly after delivery, this is called uterine atony, and could lead to severe bleeding.

Genital tract lacerations

The baby may tear the vaginal tissue and/or the perineum (the area between the vagina and anus)

Uterine rupture

A severe but rare risk of fetal macrosomia is rupture of the uterine wall along the scar line where surgery may have previously been performed (such as a previous Cesarean section). This is a potentially fatal medical emergency and the baby must be delivered via C-section immediately.

Will I Need a C-Section?

Most babies with fetal macrosomia can be delivered vaginally. However, doctors recommend Cesarean delivery in certain cases.

  • You have diabetes and your baby is estimated to weigh 10lbs or more.
  • Your baby is estimated to weigh at least 11lbs.
  • You once delivered a baby with shoulder dystocia. Shoulder dystocia is a complication in which the baby’s shoulders get stuck behind the pelvic bone after the head has already emerged. To avoid this problem from reoccurring, a C-section may be recommended.


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