Large For Gestational Age

Large for gestational age (LGA) describes full-term or post-term infants that are born of high birth weight.

The term LGA or large for gestational age is defined by birth weight above the 90th percentile for their gestational age and gender. In infants with birth weight above the 97th percentile in their gestational age and gender group, research has shown that greater risk of long-term health complications and fatal outcomes are present in LGA infants.

Specifically, large for gestational age can be characterized by macrosomia, referring to a fetal growth beyond a certain threshold (threshold ranging from a body weight of 4,000 grams to above 5,000 grams). Experts in Obstetrics and Gynecology currently use a grading system to evaluate LGA infants, where their birth weight may help identify risks associated with their birth, including labor complications of both mother and child, potential long-term health complications of the neonate and infant mortality.

Presentation

Complications

Neonatal

Common risks in LGA babies include shoulder dystocia, hypoglycemia, brachial plexus injuries, metatarsus adductus, hip subluxation and talipes calcaneovalgus, due to intrauterine deformation.

Shoulder dystocia occurs when the anterior shoulder becomes impacted on the maternal pubic symphysis during birth. The doctor or midwife will try to push the baby's anterior shoulder downward to pass through the birth canal and clear the woman's pubic symphysis. This can be difficult if the child is LGA, since the birth canal is 10 cm when fully dilated for most women and there may not be much room to move the baby. If shoulder dystocia occurs, there are various maneuvers which can be performed by the birth attendant to try to deliver the shoulders. These generally involve trying to turn the shoulders into the oblique, using suprapubic pressure to disimpact the anterior shoulder from above the pubic symphysis, or delivering the posterior arm first. If these do not resolve the situation, the provider may intentionally snap the baby's clavicle (bone that holds shoulder in place) in a procedure called cleidotomy in order to displace the shoulder and allow the child to be delivered. Other methods to deliver the baby as a last resort when all else have failed are the Zavanelli maneuver and symphysiotomy. The Zavanelli maneuver involves flexing and pushing the fetal head back into the birth canal, and an emergency cesarean section is then performed. Symphysiotomy allows childbirth by surgically dividing the pubic bone to widen the pelvis and it is performed after a failed Zavanelli maneuver. Newborns with shoulder dystocia are at risk of temporary or permanent nerve damage to the baby's arm, or other injuries such as humeral fracture.

In non-diabetic women, shoulder dystocia happens 0.65% of the time in babies that weigh less than 8 pounds 13 ounces (4,000 g), 6.7% of the time in babies that weigh 8 pounds 13 ounces (4,000 g) to 9 pounds 15 ounces (4,500 g), and 14.5% of the time in babies that weigh more than 9 pounds 15 ounces (4,500 g). In diabetic women, shoulder dystocia happens 2.2% of the time in babies that weigh less than 8 pounds 13 ounces (4,000 g), 13.9% of the time in babies that weigh 8 pounds 13 ounces (4,000 g) to 9 pounds 15 ounces (4,500 g), and 52.5% of the time in babies that weigh more than 9 pounds 15 ounces (4,500 g). Although big babies are at higher risk for shoulder dystocia, most cases of shoulder dystocia happen in smaller babies because there are many more small and normal-size babies being born than big babies. Researchers have been unable to predict who will have shoulder dystocia and who will not.

LGA babies are at higher risk of hypoglycemia in the neonatal period, independent of whether the mother has diabetes. Hypoglycemia, as well as hyperbilirubinemia and polycythemia, occur as a result of hyperinsulinemia in the fetus.

High birth weight may impact the baby in the long term. Macrosomic neonates are at a higher risk of being overweight and obese than their normal-weight counterparts later in life. Studies have shown that the long-term overweight risk is doubled when the birth weight is greater than 4,000 g. The risk of having type 2 diabetes mellitus in adult life is 19% higher among macrosomic babies with birth weights heavier than 4,500 g compared to those with birth weights between 4,000 g and 4,500 g.

Maternal

Maternal complications in pregnancies with macrosomia include emergency cesarean section, postpartum hemorrhage and obstetric anal sphincter injury. The risk of maternal complications in pregnancies with newborns weighing between 4,000 g and 4,500 g is two-fold greater than in pregnancies without macrosomia. In pregnancies with newborns weighing over 4,500 g, the risk is approximately three-fold greater.

Risk factors

Maternal diabetes

One of the primary risk factors of LGA is poorly-controlled maternal diabetes, particularly gestational diabetes (GD), as well as preexisting diabetes mellitus (DM) (preexisting type 2 is associated more with macrosomia, while preexisting type 1 can be associated with microsomia). The risk of having a macrosomic fetus is three times greater in mothers with diabetes than those without diabetes. DM increases maternal plasma glucose levels as well as insulin, stimulating fetal growth of subcutaneous fat. The LGA newborn exposed to maternal DM usually only has an increase in weight, not a change in body length or head size.

Genetics

Genetics plays a role in having an LGA baby. Taller, heavier parents tend to have larger babies. Genetic disorders of overgrowth (e.g. Beckwith–Wiedemann syndrome, Sotos syndrome, Perlman syndrome, Simpson-Golabi-Behmel syndrome ) are often characterized by macrosomia.

Other risk factors

• Gestational age: pregnancies that go beyond 40 weeks increase incidence of an LGA infant
• Fetal sex: male infants tend to weigh more than female infants
• Obesity prior to pregnancy and maternal weight gain above recommended guidelines during pregnancy
• Multiparity: giving birth to previous LGA infants vs. non-LGA infants
• Frozen embryo transfer as fertility treatment, as compared with fresh embryo transfer or no artificial assistance

Diagnosis

Diagnosing fetal macrosomia cannot be performed until after birth, as evaluating a baby's weight in the womb may be inaccurate. While ultrasound has been the primary method for diagnosing LGA, this form of fetal weight assessment remains imprecise, as the fetus is a highly variable structure in regards to density and weight— no matter the gestational age. Ultrasonography involves an algorithm that incorporates biometric measurements of the fetus, such as biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL), to calculate the estimated fetal weight (EFW). Variability of fetal weight estimations has been linked to differences due to sensitivity and specificity of ultrasound algorithms as well as to the individual performing the ultrasound examination.

In addition to sonography, fetal weight can also be assessed using clinical and maternal methods. Clinical methods for estimating fetal weight involves measuring the mother's symphysis-fundal height and performing Leopold's maneuvers, which can help with determining the fetus position in utero in addition to size. However, as this method relies heavily on practitioner experience and technique, it does not provide an accurate and definite diagnosis of an LGA infant and only would only serve as a potential indication of suspected macrosomia. Fetal weight can also be estimated through a mother's subjective assessment of the fetus size, but this method is dependent on a mother's experience with past pregnancies and may not be clinically useful. There are new methods being studied for their accuracy in predicting fetal weight, such as measuring fetal soft tissue, but more research needs to be done to find a consistent, reliable method.

Management

Induction of labor at or near term for women with a baby of suspected macrosomia has been proposed as a treatment method, as it stops fetal growth and results in babies with a lower birth weight, fewer bone fractures, and less incidence of shoulder dystocia. However, this method could increase the number of women with perineal tears, and failed inductions can prompt the need for emergency cesarean sections. LGA babies are more than two times likely to be delivered by Cesarean section, compared to infants under 4000 grams (below the threshold of macrosomia). Predicting a baby’s weight can be inaccurate and women could be worried unnecessarily, and request their labor to be induced without a medical reason. Doctors disagree whether women should be induced for suspected macrosomia and more research is needed to find out what is best for women and their babies.

Elective cesarean section has also been presented as a potential delivery method for infants of suspected macrosomia, as it can serve to prevent possible birth trauma. However, the American College of Obstetricians and Gynecologists recommends that cesarean delivery should only be considered if the fetus is an estimated weight of at least 5,000 grams in non-diabetic mothers and at least 4,500 grams in diabetic mothers. A number needed to treat analysis determined that approximately 3,700 women with suspected fetal macrosomia would have to undergo an unnecessary cesarean section in order to prevent one incident of brachial plexus injuries secondary to shoulder dystocia.

Management of gestational diabetes through dietary modifications and anti-diabetic medications has been shown to decrease the incidence of LGA. The use of metformin to control maternal blood glucose levels has shown to be more effective than using insulin alone in reducing the likelihood of fetal macrosomia. There is a 20% lower chance of having an LGA baby when using metformin to manage diabetes compared to using insulin.

Modifiable risk factors that increase the incidence of LGA births, such as gestational weight gain above recommended BMI guidelines, can be managed with lifestyle modifications, including maintaining a balanced diet and exercising. Such interventions can help mothers achieve the recommended gestational weight and lower the incidence of fetal macrosomia in obese and overweight women. The World Health Organization also recommends that mothers aim for their recommended BMI prior to conception. In general, obese mothers or women with excessive gestational weight gain may have higher risk of pregnancy complications (ranging from LGA, shoulder dystocia, etc.).

Epidemiology

In healthy pregnancies without pre-term or post-term health complications, large for gestational age, or fetal macrosomia have been observed to affect around 12% of newborns. By comparison, women with gestational diabetes are at an increased risk of giving birth to LGA babies, where ~15-45% of neonates may be affected. In 2017, the National Center of Health Statistics found that 7.8% of live-born infants born in the United States meet the definition of macrosomia, where their birth weight surpasses the threshold of 4000 grams (above ~8.8 pounds). Women in Europe and the United States tend to have higher pre-term body weight and have increased gestational weight during pregnancy compared to women in east Asia. Thus, women in Europe and the United States, with higher gestational weight gain, tend to have higher associated risk of LGA infants, macrosomia and cesarean. In European countries, the prevalence of births of newborns weighing between 4,000 g and 4,499 g is 8% to 21%, and in Asian countries the prevalence is between 1% and 8%. In general, rates of LGA infants have increased 15-25% in many countries including the United States, Canada, Germany, Denmark, Scotland and more in the past 20-30 years, suggesting an increase in LGA births worldwide.