It’s not inherently dangerous to have a large newborn, but the chances of complication are higher when carrying and delivering a large baby. Fetal macrosomia also puts the baby at increased risk of health problems after birth. These risk factors increase significantly when the newborn birth weight is more than 9lbs, 15oz.
Fetal macrosomia may complicate natural delivery and could put the macrosomic baby at risk of injury during birth, as well as the pregnant women giving delivery to a macrosomic infant.
The American College of Obstetricians and Gynecologists (ACOG) defines fetal macrosomia as a newborn is considered larger than average if it weighs more than 8lbs, 13oz at birth, no matter how long its gestational age. Roughly 9% of new infants are macrosomic. But these thresholds are not useful for identifying the preterm macrosomic fetus since they are not based upon population statistics, where normal weight is typically defined as between the 10th and 90th percentile for gestational age.
Signs and Symptoms of Fetal Macrosomia
Unfortunately, the ultrasound techniques do not have high reliability in the detection and prediction of macrosomia and the probability of a correct diagnosis of macrosomia by ultrasound tests is not very high. Often manifests with truncal obesity and therefore the abdominal circumference may be one of the first parameters to increase.
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 may be larger than normal and maybe macrosomic.
The amount of amniotic fluid can be measured to estimate the size of the fetus. If your doctor finds excessive amniotic fluid during a prenatal visit, this may be an indication of fetal macrosomia. Excessive amniotic fluid defined as greater than or equal to 60th percentile for gestational age has recently been associated with macrosomia.
Causes of Fetal 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 maternal risk factors are more or less under your control:
- Maternal 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. High pre-pregnancy body mass index (BMI)
- 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 weighed more than 8lbs, 13oz at birth, then you are more likely to carry a large baby.
- Previous pregnancies. And if you’ve had a macrosomic baby before, you’re more likely to deliver another one the next time around, since the risk of macrosomia increases with each pregnancy.
- The baby is a boy. Male infants tend to weigh more than female infants.
- Overdue pregnancy. If you’re more than two weeks past your due date, the odds of high birth weight are greater.
In the US, fetal macrosomia is most often linked to diabetes, maternal obesity, gestational diabetes mellitus, and excessive weight gain during pregnancy. There are also some rare genetic conditions that increase the fetal growth rate.
Nevertheless, identifying macrosomic fetuses is important given the implications on fetal and maternal pregnancy outcomes. Talk to your gynecologist about these conditions and how they may affect your pregnancy.
Carrying and delivering larger babies increases some health and safety risks in pregnant women and their macrosomic babies. In general, maternal complications such as obese women, poorly controlled diabetes, maternal obesity, and excessive maternal weight gain are all associated with macrosomia and have intermittent periods of hyperglycemia in common. Your OB/GYN will work with you to ensure the safest possible delivery.
One of the dangers of natural delivery with macrosomic births is that the baby may get stuck in the birth canal. This can cause birth injury to the macrosomic infant, and the doctor may need to help him or her out using a vacuum device or forceps. In some cases, a Cesarean delivery is recommended to reduce the risk of complications for the macrosomic infant and the mother.
Although rare, shoulder dystocia is the most serious complication associated with fetal macrosomia. When above average birth weight is the risk is increased to 9.2 to 24 percent in pregnant women without diabetes and to 19.9 to 50 percent in pregnancies complicated by diabetes. However, while macrosomia increases risk, shoulder dystocia also occurs unpredictably in infants of normal birth weight.
Fracture of the clavicle and damage to the nerves of the brachial plexus are the most common fetal injuries associated with macrosomia. In macrosomic infants, clavicular fracture and brachial plexus injury are also at higher risk.
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 macrosomic infant may tear the vaginal tissue and/or the perineum (the area between the vagina and anus).
Severe but rare risk of fetal macrosomia is the rupture of the uterine wall along the scar line of prior surgery (such as a C-section). This is a potentially fatal medical emergency, and the baby must be delivered via C-section immediately.
Will I Need a C-Section?
The role of cesarean delivery in suspected fetal macrosomia remains controversial. While the risk of birth trauma with vaginal delivery is higher with increased birth weight, the mode of delivery by cesarean section reduces but does not eliminate this risk. Most babies with fetal macrosomia can be delivered vaginally. However, doctors recommend Cesarean delivery in certain cases, such as when:
- You have diabetes with impaired glucose tolerance, and your baby’s fetal weight is estimated 10lbs or more.
- Your baby is estimated to weigh at least 11lbs.
- Risk of shoulder dystocia. Previous delivery caused shoulder dystocia, which is a type of obstructed labor in which the baby’s shoulders get stuck behind the pelvic bone after the head has already emerged. To avoid a recurrence, a C-section may be recommended.
Preventing Fetal Macrosomia
Even though there are some cases in which fetal macrosomia occurs for no apparent reason, pregnant women can help reduce the chances by:
- Watching Weight Gain: Although this may be difficult during pregnancy, gaining between 25 to 35 pounds only, the ideal weight gain during pregnancy, is recommended by most doctors. It’s important to note, however, that this is just a general guideline and doesn’t apply to each individual case. A physician should work closely with you in order to find the ideal weight gain amount for your situation.
- Control Diabetes: Controlling blood sugar during pregnancy is one of the best ways to help prevent fetal macrosomia.
- Keep All Prenatal Appointments: Prenatal check-ups are crucial during pregnancy as it allows doctors to examine the pregnancy and run tests to ensure everything is normal. It also helps them prepare for your upcoming birth and the steps needed should you show signs of carrying a large infant.