Macrosomia

We hear a lot about low birth weight in babies because the consequences of the problem can be so severe. Low birth weight also gets a lot of attention because it often involves risk factors that we are capable of controlling—maternal nutrition, drug and alcohol use, prenatal care, smoking, exposure to environmental toxins, etc.

What we don’t hear quite as much about is the problem of high-birth-weight babies. But high birth weight, known dually as Macrosomia or Large for Gestational Age (LGA) can introduce its own set of risks and complications, both for the mother and the baby, making the circumstance something that we should all be aware of.

The term macrosomia sounds ominous, but it translates simply as “large body,” and it refers to a baby that is above a certain weight threshold at or before delivery. A baby is generally considered to be macrosomic if its weight is above 8 pounds, 13 ounces at birth or before.

That weight is nearly two pounds above the average birth weight of about seven pounds. Worldwide, macrosomia as defined in this way is quite common; about 9 percent of babies worldwide weigh in above that threshold at birth.

The risks associated with macrosomia increase as the baby gets heavier, and if the baby’s weight is above 9 pounds, 15 ounces, the risks are significantly higher.

Causes and Risk Factors

Diabetes in the mother can cause a baby to have both a larger skeletal structure and more than the typical amount of body fat, and diabetes is a significant risk factor for macrosomia whether the mother had diabetes before she became pregnant or if she developed diabetes during her pregnancy.

History is also a factor. If the mother has had several pregnancies, the chance that she’ll have a macrosomic pregnancy increases with each baby; each successive baby up to the fifth one tends to be about 4 ounces heavier than the previous baby. If any of the previous babies were macrosomic, the next one has a good chance of being macrosomic, too. And if the mother herself was a macrosomic baby, she has more of a chance of delivering an atypically large baby herself.

The pregnancy can also be an indicator of risk. Women over 35 are more likely to have a macrosomic baby than are younger women. If the mother gains an excessive amount of weight during her pregnancy, her baby is likely to be large, and if she goes more than two weeks past her due date before giving birth, the baby has plenty of time to grow to an atypically large size.

Risks to the mother

The most immediate and obvious risks to the mother in having a macrosomic baby occur during the delivery itself. Vaginal delivery of an atypically large baby is inherently more risky than delivery of a baby in the typical weight range. There’s a greater chance that the baby won’t be able to fit easily through the birth canal, and it may become wedged so that it cannot be delivered normally. In these cases, the use of forceps or a vacuum device may be required—a risky and scary procedure—and the chance that an unplanned C-section may become necessary is greater.

There’s also chance of injury to the mother during the delivery of a macrosomic baby. The chance of vaginal and perineal tearing or laceration is greater than in a typical delivery, and there’s also an increased chance of uterine rupture in women who have had previous C-sections or uterine surgeries. Finally, there’s also an increased risk that the mother’s uterus won’t contract as it should after delivery, a situation that can result in potentially dangerous post-delivery bleeding.

Risks to the baby

The macrosomic baby is also at risk. The same potential complications that are a risk to the mother during delivery can also affect the baby. A large baby may sustain injuries, including broken bones, both as a result of the difficult trip through the birth canal and because of the mechanical steps taken by medical personnel during the delivery.

The baby is also at risk of some of the same problems that affect all overweight people. Babies with high birth weights are at greater risk of having abnormally high blood sugar levels at birth, and they have a higher risk of developing conditions such as high blood pressure, high cholesterol levels, excessive body fat, obesity and high blood sugar later in childhood. These conditions, in turn, increase the risk of the child having future problems with diabetes, heart disease and stroke.

Treatment

The “treatment” involved with macrosomia is initially concerned with delivering the baby. Your doctor won’t necessarily rule out the possibility that you can delivery your baby vaginally, but he or she will likely insist that you deliver the baby in a hospital. The doctor will probably also avoid inducing labor artificially, because induction has been shown to increase the likelihood that a C-section will be required.

In some cases, your doctor might suggest that you deliver your macrosomic baby via a planned C-section. If you have diabetes, or if your baby is estimated to weigh more than 11 pounds, this will likely be the case. One of the biggest risks with a macrosomic delivery is a situation, called shoulder dystocia, in which the baby’s shoulder gets stuck behind the mother’s pelvic bone; it makes vaginal delivery difficult and potentially dangerous, and it often results in collar bone fractures in the baby. If you’ve experienced shoulder dystocia in a previous delivery, your doctor will probably recommend that you deliver a subsequent macrosomic baby via C-section.

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