Large for Gestational Age (LGA): What You Need to Know

Large for gestational age (LGA) is a term that doctors use to describe fetuses and newborn babies who are larger than the typical size for their gestational age. Also known as macrosomia (meaning large body) LGA is defined and diagnosed by comparing the baby’s weight to a standard fetal growth chart, which shows the range of weights for babies at different gestational ages. If a baby’s weight falls above the 90th percentile on the chart, she is classified as LGA. There are several reasons that a baby may be LGA, including genetic causes and maternal obesity, but maternal diabetes mellitus is the most common reason. This can be either pregestational diabetes (diabetes that was present, even before pregnancy) or gestational diabetes (diabetes that manifests only during pregnancy and subsides when pregnancy ends). LGA babies are at risk for complications during labor and delivery and after birth, so it is important for pregnant women with diabetes to receive proper screening.

Women with poorly controlled pregestational diabetes are at increased risk of having not only a large baby, but also a baby with other congenital anomalies, including congenital heart disease and neural tube defects. If you are diabetic, it is thus important for you to receive counseling on strict glycemic control before becoming pregnant, and to take a high dose of folic acid before conception to reduce the risk of neural tube defects. You should also receive serum screening tests and ultrasounds early in the pregnancy to evaluate the fetus for congenital anomalies.

Gestational and pregestational diabetes both elevate the risk of neonatal complications, notably shoulder dystocia (if the baby is delivered vaginally) and neonatal hypoglycemia (low blood sugar).

Shoulder dystocia is a condition on which the baby’s shoulder gets stuck and pulled during the trip through the birth canal. This can lead to injury to a group of nerves called the brachial plexus, but it is preventable by simply delivering the baby through cesarean section. This delivery should be planned for a time a couple of weeks prior to term, typically around 38 weeks gestation, to avoid spontaneous labor. As for neonatal hypoglycemia, this develops because, in utero, the fetus adapts to the high concentrations of glucose (blood sugar) coming from the mother via the placenta by producing more insulin. During birth, when the umbilical cord is clamped, suddenly excessive sugar is gone from the neonatal blood, so the neonate’s higher-than-normal insulin levels cause his own sugar level to drop too low. This can be easily treated and usually the baby will have good sugar control within a matter of days and will not require more treatment.

Hyperinsulinemia in LGA also increase the risk of newborn respiratory distress. This is because hyperinsulinemia suppresses the release of corticosteroids, which stimulate certain cells in the lungs to release of a soapy substance called surfactant, which is necessary for the alveoli (air sacs) to expand easily when the baby inhales. When the quantity and quality of surfactant is not ready to support the neonate’s lungs, the neonate may develop neonatal respiratory distress syndrome. However, known about LGA during pregnancy, doctors can give corticosteroids to the mother to held the fetal lungs mature in advance of delivery.

LGA infants also often have polycythemia, meaning that their red blood cell volume is increased compared to a normal-sized newborn. It is not fully understood why this occurs, but it is thought to be due to increased levels of erythropoietin stimulating red blood cell production. One consequence of polycythemia in LGA infants is hyperbilirubinemia, or jaundice. This occurs when there is excess bilirubin in the bloodstream, causing the infant’s skin and sclera to take on a yellow appearance. The most common cause of jaundice in LGA infants is increased red blood cell breakdown, which releases more bilirubin into the bloodstream. Jaundice is important to identify and treat in all newborns because excess bilirubin can cause brain damage, a condition called kernicterus. Phototherapy is the main treatment for jaundice, in which the infant is placed under blue UV lights to help increase bilirubin excretion. If the bilirubin levels are extremely high, exchange transfusion may be required.

Screening of pregnant women for gestational diabetes typically involves a glucose challenge test in which the mother is given 50 gram of sugar as a syrup and her blood sugar levels are then tested after one hour. This is performed at 24 to 28 weeks of pregnancy. If the blood glucose is above a certain level, the patient will need to undergo a diagnostic test to confirm the diagnosis of gestational diabetes. Usually, this test will be a glucose tolerance test in which you drink more sugar, 75 gram, and you do that after an 8-hour fast. If your blood glucose remains above a certain level after an hour, that’s when the doctor diagnoses gestational diabetes.

In mothers with gestational diabetes it is important to focus on strict glycemic control through lifestyle modifications such as maintaining a healthy diet and exercising regularly. If these measures are not sufficient to achieve adequate glucose control, regular insulin administration may be necessary. Pregnant women with any type of diabetes are also monitored closely for both maternal and fetal complications, and should undergo antenatal ultrasound scans every 2 to 4 weeks starting at 28 weeks of pregnancy to monitor fetal growth and amniotic fluid volume. It is important to identify whether a fetus is small or large for gestational age prenatally in order to guide the management plan for the newborn during labor and after birth.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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