Overview

The term "fetal macrosomia" is used to describe a newborn who's significantly larger than average.

A baby diagnosed with fetal macrosomia has a birth weight of more than 8 pounds, 13 ounces (4,000 grams), regardless of his or her gestational age. About 9 percent of babies born worldwide weigh more than 8 pounds, 13 ounces.

However, the risks associated with fetal macrosomia increase greatly when birth weight is more than 9 pounds 15 ounces (4,500 grams).

Fetal macrosomia may complicate vaginal delivery and could put the baby at risk of injury during birth. Fetal macrosomia also puts the baby at increased risk of health problems after birth.

Symptoms

Fetal macrosomia is difficult to detect and diagnose during pregnancy. Possible signs and symptoms include:

  • Large fundal height. During prenatal visits, your health care provider might measure your fundal height — the distance from the top of your uterus to your pubic bone. A fundal height that measures larger than expected could be a sign of fetal macrosomia.
  • Excessive amniotic fluid (polyhydramnios). Too much amniotic fluid — the fluid that surrounds and protects a baby during pregnancy — might be a sign that your baby is larger than average.

    The amount of amniotic fluid reflects your baby's urine output, and a larger baby produces more urine. Some conditions that increase a baby's size might also increase his or her urine output.

Causes

Fetal macrosomia can be caused by genetic factors as well as maternal conditions, such as obesity or diabetes. Rarely, a baby might have a medical condition that speeds fetal growth.

In some cases, what causes a larger than average birth weight remains unexplained.

Risk factors

Many factors might increase the risk of fetal macrosomia — some you can control, but others you can't.

For example:

  • Maternal diabetes. If you had diabetes before pregnancy (pre-gestational diabetes) or you develop diabetes during pregnancy (gestational diabetes), fetal macrosomia is more likely.

    If your diabetes is poorly controlled, your baby is likely to have larger shoulders and greater amounts of body fat than would a baby whose mother doesn't have diabetes.

  • A history of fetal macrosomia. If you've previously given birth to a baby diagnosed with fetal macrosomia, you're at increased risk of having another baby who has the condition. Also, if you weighed more than 8 pounds, 13 ounces at birth, you're more likely to have a large baby.
  • Maternal obesity. Fetal macrosomia is more likely if you're obese.
  • Excessive weight gain during pregnancy. Gaining too much weight during pregnancy increases the risk of fetal macrosomia.
  • Previous pregnancies. The risk of fetal macrosomia increases with each pregnancy. Up to the fifth pregnancy, the average birth weight for each successive pregnancy typically increases by up to about 4 ounces (113 grams).
  • You're having a boy. Male infants typically weigh slightly more than female infants. Most babies who weigh more than 9 pounds, 15 ounces (4,500 grams) are male.
  • Overdue pregnancy. If your pregnancy continues by more than two weeks past your due date, your baby is at increased risk of fetal macrosomia.
  • Maternal age. Women older than 35 are more likely to have a baby diagnosed with fetal macrosomia.

Fetal macrosomia is more likely to be a result of maternal diabetes, obesity or weight gain during pregnancy than other causes. If these risk factors aren't present and fetal macrosomia is suspected, it's possible that your baby might have a rare medical condition that affects fetal growth.

Your health care provider might recommend prenatal diagnostic tests and perhaps a visit with a genetic counselor, depending on the test results.

Complications

Fetal macrosomia poses health risks for you and your baby — both during pregnancy and after childbirth.

Maternal risks

Possible maternal complications of fetal macrosomia might include:

  • Labor problems. Fetal macrosomia can cause a baby to become wedged in the birth canal, sustain birth injuries, or require the use of forceps or a vacuum device during delivery (operative vaginal delivery). Sometimes a C-section is needed.
  • Genital tract lacerations. During childbirth, fetal macrosomia can cause a baby to injure the birth canal — such as by tearing vaginal tissues and the muscles between the vagina and the anus (perineal muscles).
  • Bleeding after delivery. Fetal macrosomia increases the risk that your uterine muscles won't properly contract after you give birth (uterine atony). This can lead to potentially serious bleeding after delivery.
  • Uterine rupture. If you've had a prior C-section or major uterine surgery, fetal macrosomia increases the risk of uterine rupture — a rare but serious complication in which the uterus tears open along the scar line from the C-section or other uterine surgery. An emergency C-section is needed to prevent life-threatening complications.

Newborn and childhood risks

Possible complications of fetal macrosomia for your baby might include:

  • Lower than normal blood sugar level. A baby diagnosed with fetal macrosomia is more likely to be born with a blood sugar level that's lower than normal.
  • Childhood obesity. Research suggests that the risk of childhood obesity increases as birth weight increases.
  • Metabolic syndrome. If your baby is diagnosed with fetal macrosomia, he or she is at risk of developing metabolic syndrome during childhood.

    Metabolic syndrome is a cluster of conditions — increased blood pressure, a high blood sugar level, excess body fat around the waist or abnormal cholesterol levels — that occur together, increasing the risk of heart disease, stroke and diabetes.

Further research is needed to determine whether these effects might increase the risk of adult diabetes, obesity and heart disease.

Prevention

You might not be able to prevent fetal macrosomia, but you can promote a healthy pregnancy.

For example:

  • Schedule a preconception appointment. If you're considering pregnancy, talk to your health care provider. If you're obese, he or she might also refer you to other health care providers — such as a registered dietitian or an obesity specialist — who can help you make changes in your lifestyle and reach a healthy weight before pregnancy.
  • Monitor your weight. Gaining a healthy amount of weight during pregnancy — often 25 to 35 pounds (about 11 to 16 kilograms) — supports your baby's growth and development. There's no one-size-fits-all approach to pregnancy weight gain, though. Work with your health care provider to determine what's right for you.
  • Manage diabetes. If you had diabetes before pregnancy or you develop gestational diabetes, work with your health care provider to manage the condition. Controlling your blood sugar level is the best way to prevent complications, including fetal macrosomia.
  • Include physical activity in your daily routine. Follow your health care provider's recommendations for physical activity.
April 16, 2015
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