Overview

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

A baby who is diagnosed as having fetal macrosomia weighs more than 8 pounds, 13 ounces (4,000 grams), regardless of his or her gestational age. About 9% of babies worldwide weigh more than 8 pounds, 13 ounces.

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 can 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 can be difficult to detect and diagnose during pregnancy. 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 larger than expected fundal height could be a sign of fetal macrosomia.
  • Excessive amniotic fluid (polyhydramnios). Having 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 cause a baby to be larger might also increase his or her urine output.

Causes

Genetic factors and maternal conditions such as obesity or diabetes can cause fetal macrosomia. Rarely, a baby might have a medical condition that makes him or her grow faster and larger.

Sometimes it's unknown what causes a baby to be larger than average.

Risk factors

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

For example:

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

    If your diabetes isn't well 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 large baby, you're at increased risk of having another large baby. 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).
  • 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.

If a rare medical condition is suspected, 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 (shoulder dystocia), 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 during labor — 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 and 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. Research shows that exercising during pregnancy and eating a low-glycemic diet can reduce the risk of macrosomia.

For example:

  • Schedule a preconception appointment. If you're considering pregnancy, talk with your health care provider. If you're obese, you might also be referred to another health care provider — such as a registered dietitian or an obesity specialist — who can help you 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) if you have a normal pre-pregnancy weight — supports your baby's growth and development. Women who weigh more when they get pregnant will have lower recommended pregnancy weight gain. Work with your health care provider to determine what's right for you.
  • Manage diabetes. If you had diabetes before pregnancy or if 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.
  • Be active. Follow your health care provider's recommendations for physical activity.

May 29, 2020
  1. American College of Obstetricians and Gynecologists. Practice Bulletin No. 216: Macrosomia. Obstetrics & Gynecology. 2020; doi:10.1097/AOG.0000000000003606.
  2. Abramocwicz JS, et al. Fetal macrosomia. https://www.uptodate.com/contents/search. Accessed March 17, 2020.
  3. Dashe JS, et al. Society for Maternal-Fetal Medicine (SMFM) Consult Series #46: Evaluation and management of polyhydramnios. American Journal of Obstetrics and Gynecology. 2018; doi:10.1016.j.ajog.2018.07.016.
  4. Copel JA, et al. Fetal macrosomia. In: Obstetric Imaging: Fetal Diagnosis and Care. 2nd. ed. Elsevier. 2018. https://www.clinicalkey.com. Accessed March 17, 2020.
  5. Gleason CA, et al. Antepartum fetal assessment. In: Avery's Diseases of the Newborn. 10th ed. Elsevier. 2018. https://www.clinicalkey.com. Accessed March 17, 2020.
  6. Mandy GT. Large for gestational age newborn. https://www.uptodate.com/contents/search. Accessed March 17, 2020.
  7. Wick MJ (expert opinion). Mayo Clinic. March 17, 2020.

Related

Associated Procedures