Fetal Macrosomia: is my Baby too big to Deliver?

Fetal Macrosomia: Toward the end of your pregnancy, you may feel like your baby is gaining a pound a day and will never come out! For some women, this concern of having a large baby is a very real one. But what exactly is considered “big” when it comes to babies, and how will your delivery be affected if your baby is on the large side?

The term “macrosomia” refers to developing babies who are considered to weigh more than average. Most obstetric providers will use a cut-off of 4,500 grams (or 9 pounds, 15 ounces) to define fetuses as macrosomic.

The reason your doctor or midwife cares about whether or not your baby may be macrosomic is that there are some increased risks to both these babies and their mothers. We know that women who deliver macrosomic babies are at an increased risk for needing a C-section, having more extensive vaginal lacerations, and losing more blood at the time of delivery.

Being macrosomic can have some negative consequences for the baby, too. The main one that obstetricians and midwives worry about is an increased risk of shoulder dystocia. This is a complication that occurs at the time of a vaginal delivery when a baby’s head delivers but the shoulders become stuck in the birth canal.

Additional delivery maneuvers can be used to help deliver the baby in this scenario, but some injuries may occur as a result. For example, a baby’s clavicle may fracture during delivery and certain nerves in the arm may be damaged, resulting in a condition known as Erb’s palsy. While these injuries often heal with time without any long-term problems, it can be stressful for new parents. Macrosomic babies are also more likely to be overweight later in life as well, which can negatively impact their health.

Some risk factors do exist for Fetal macrosomia. Having had a macrosomic baby in a prior pregnancy does make a woman more likely to deliver another one, as does maternal obesity, excessive weight gain in pregnancy, carrying a male fetus, and being a teen mother. Diabetes (both gestational and type 2) both definitely increase rates of macrosomia, and this is one reason that optimizing blood sugar control in pregnancy is so important.

There is no single perfect test to diagnose macrosomia.  A combination of a physical exam and using ultrasound measurements can help estimate the size of your baby. At best these are educated guesses, however, and no one knows for sure how big your baby will be until he or she is born!

Many expectant mothers may think that a scheduled C-section is the best way to give birth if a large baby is suspected. However, we know that C-sections are not risk-free, and babies who are assumed to be big do not always turn out that way. In order to minimize unnecessary surgeries and to balance the risks and benefits to both mothers and babies, the American Congress of Obstetricians and Gynecologists (ACOG) does not recommend abandoning an attempt at a vaginal delivery until a baby is suspected to be 5,000 grams (or 11 pounds) or more in women who do not have diabetes.

Additionally, ACOG does not recommend inducing a woman early because of concerns of large size because undergoing an induction increases the chance of a woman needing a C-section. Studies have shown that in this scenario delivering earlier does not actually decrease any risks to the baby. In fact, babies who are induced in the early-term period actually are at increased risk for other issues.

In the end, if you or your provider are concerned that your baby may be big, it is important to weigh the risks and benefits over your route and timing of delivery. Understanding that no test is perfect and that many women have given birth successfully to large babies are important points to keep in mind when deciding on how and when to meet your little one.

Sources:

• The American Congress of Obstetricians and Gynecologists
• Practice Bulletin #22: Fetal macrosomia.
  The American Congress of Obstetricians and Gynecologists
• Committee Opinion #561: Nonmedically indicated early-term deliveries.

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