A newborn is considered larger than average if it weighs more than 8lbs, 13oz at birth, no matter how long its gestational period. Roughly 9% of infants are born with fetal macrosomia.
It’s not inherently dangerous to have a large newborn, but the chances of complication are higher when carrying and delivering a large baby. Some infants with macrosomia have an increased risk for childhood metabolic diseases. These risks increase significantly when the newborn weighs more than 9lbs, 15oz.
Signs and Symptoms of Fetal Macrosomia
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 is larger than normal.
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.
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 factors are more or less under your control.
- 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.
- 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. With each pregnancy, your risk for macrosomia increases.
- 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 a high birth weight are greater.
In the U.S., fetal macrosomia is most often linked to diabetes, maternal obesity, and excessive weight gain during pregnancy. There are also some rare genetic conditions that increase the fetal growth rate. Talk to your gynecologist about these conditions and how they may affect your pregnancy.
Carrying and delivering a large baby increases some health and safety risks during birth. Your doctor will work with you to ensure the safest possible delivery.
One of the dangers of natural delivery with macrosomia is that the baby may get stuck in the birth canal. This can injure the baby, 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 instead.
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 baby may tear the vaginal tissue and/or the perineum (the area between the vagina and anus).
A severe but rare risk of fetal macrosomia is rupture of the uterine wall along the scar line of a 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?
Most babies with fetal macrosomia can be delivered vaginally. However, doctors recommend Cesarean delivery in certain cases, such as when:
- You have diabetes, and your baby is estimated to weigh 10lbs or more.
- Your baby is estimated to weigh at least 11lbs.
- You once delivered a baby with shoulder dystocia. Shoulder dystocia 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.