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Fetal macrosomia is a medical term used to describe a large baby.  The term has been defined in several ways, including a newborn baby weighing 8 lbs. 13 oz. to 9 lbs. 14 oz.  Macrosomia affects approximately 7-12% of all births and can cause maternal and neonatal complications.

Experienced clinicians may use techniques such as “Leopold’s maneuvers” to help determine fetal weight.  The maneuvers consist of four distinct gripping actions performed by a healthcare provider to assess fetal size, including the fundal grip, lateral grip, Pawlick’s grip and pelvic grip.  Discrepancies in fundal height versus fetal age as well as excessive amniotic fluid can help predict the presence of macrosomia. Additionally, fetal growth measurements and estimated fetal weight by ultrasound also indicate whether macrosomia is present.

Risk factors for fetal macrosomia include: family history of macrosomia, maternal obesity, excessive weight gain in pregnancy, a long gestational period, gestational diabetes or class A-C diabetes mellitus, multiparity, having a previous macrosomic child, and being born male.

Certain genetic, racial, and ethnic factors influence birth weight and the risk of macrosomia as well. For example, a study out of the University of California found that babies born to Hispanic mothers have a higher incidence of being macrosomic than those born to mothers of non-Hispanic origin.

The physiological process that contributes to fetal macrosomia varies but most cases are related to metabolic anomalies in the mother or fetus, which usually result in hyperglycemia (an excess of glucose) in the fetus. When hyperglycemia occurs, the fetal metabolism compensates by producing more insulin, insulin-like growth factors, human growth hormone, and other hormones that increase growth. As gestation progresses and the fetus grows it will store the extra glucose as fat and glycogen. Even after birth it is not uncommon for macrosomic babies to have problems with either hypo or hyperglycemia, which can lead to neurological problems including seizures.

Complications from fetal macrosomia include neonatal injury, maternal injury, and cesarean delivery.  The risk of a brachial plexus injury, which can lead to permanent nerve damage, is approximately 20 times higher in a baby born at an excessive birth weight of over 9 lbs. 14 oz. The risk of shoulder dystocia is at least 10 times higher for babies with macrosomia. Macrosomic babies are also at a higher risk of birth trauma and stillbirth. Infants with a birth weight greater than 9 lbs. 14 oz. have almost a three-time greater risk of being admitted into a NICU compared with newborns with a birth weight of less than 8 lbs. 13 oz. Because macrosomic babies frequently prolong labor, they are at an increased risk for neonatal encephalopathy due to a lack of oxygen and blood flow at the time of labor and delivery.

There are also maternal consequences with fetal macrosomia. Macrosomia increases the chances of the need for a C-section, can cause vaginal lacerations, and can increase the risk of maternal bleeding after or during labor. Additionally, macrosomia is related to uterine rupture during a trial of labor after caesarian with the intention of a vaginal birth after caesarian.

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