Macrosomia is the term used to describe larger-than-average babies. Average newborns weigh around seven pounds, eight ounces. According to the American College of Obstetricians and Gynecologists and the World Health Organization, newborns weighing more than eight pounds, thirteen ounces (4,000 g) are considered to be macrosomic. The rates of macrosomia worldwide vary significantly. In the United States, it has been estimated that up to ten percent of newborn babies may be macrosomic.

Healthcare providers may have a hard time determining whether a baby is macrosomic while he or she is still in the womb. Still, doctors look for signs during the pregnancy to help them estimate fetal size. Measurements taken during ultrasound examinations can provide a more accurate idea of how big a baby is, but generally everyone must wait until the birth to find out how much a baby actually weighs.

Women carrying a larger-than-average baby may be at risk for complications during delivery. The risk of complications increases if the pregnant mother has diabetes or if the baby’s estimated weight is more than eleven pounds (5,000 g). If your doctor suspects your fetus is larger than average, you may be monitored more closely for complications that might require induction or cesarean delivery. In most cases, you will not be induced or scheduled for a c-section just on the suspicion that your baby is macrosomic.

Macrosomia Causes

There are a variety of factors that can contribute to a large baby. Although genetics may be the most significant factor determining your baby’s growth and development, size can also be influenced by:

It’s important to know that even if you have one or more of the risk factors above, you can still give birth to a normal-sized baby. Often, big babies are born to mothers with none of the risk factors listed above. Unfortunately, the variation of birth weight remains unexplained, as many macrosomic babies are born to mothers with no identifiable risk factors.

Macrosomia Diagnosis

A diagnosis of macrosomia cannot be made until after the baby is born. This is because prenatal assessments such as clinical evaluations (for example, measuring the mother’s belly) and ultrasound examinations remain imprecise. Another definition of macrosomia is a birth weight above the 90th percentile, corrected for gestational age and sex. Although your doctor may suspect macrosomia on the basis of prenatal findings, you will probably not require any treatment. In some pregnant women with diabetes and suspected macrosomia, dietary adjustments may decrease the likelihood of having a baby with a higher than average birth weight. It is not known, however, whether dietary restrictions during pregnancy in obese women without diabetes can prevent macrosomia.

Risks of a Macrosomic Baby

Although macrosomia is a recognized cause of fetal and maternal morbidity and mortality, serious complications associated with this condition are rare. Fortunately, most women have no significant problems with having a large baby. During delivery, macrosomia can occasionally lead to a broken collarbone (clavicle) for the baby. The mother may suffer from excessive tearing in her perineum, and infections are always a concern if a tear occurs.

When birth weight is estimated to be more than nine pounds, four ounces, the risks associated with a macrosomia baby are increased. Cesarean delivery is common for babies of this birth weight. Shoulder dystocia is another serious complication associated with macrosomia, but it is rare and can also occur in infants of normal birth weight.

Macrosomia Treatment

In most cases, no treatment is needed for a baby with macrosomia. Problems that may occur for the mother include the ripping of the perineum at the time of delivery, the need to reduce blood sugar levels, and the need to lower blood pressure. Rest and relaxation after delivery are an important part of post-partum treatment and recovery, and any infections that may occur are generally treated with antibiotics.

If a prenatal ultrasound shows the baby has macrosomia, then your healthcare provider may recommend a c-section close to your due date. However, this recommendation depends on how high the estimated birth weight is and whether any other complications, such as diabetes, are present. The American College of Obstetricians and Gynecologists guidelines state that a cesarean delivery may be considered when the weight of the fetus is more than eleven pounds in a mother without diabetes and more than nine pounds, four ounces in a mother with diabetes. Although the risk of birth injury from vaginal delivery rises with increased birth weight, vaginal delivery is not contraindicated in pregnant women with estimated fetal weights up to eleven pounds, as long as the pregnancy is not complicated by diabetes.

There is no evidence that induction of labor can minimize or prevent birth injury from suspected macrosomia or improve maternal or fetal outcomes.

You may be monitored closely if your healthcare provider suspects macrosomia, but it’s important to know that in most cases macrosomia is not a life-threatening condition for either mom or baby.

This page was last updated on 06/2017

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