In newborn medicine, gestational age and birth weight are our north star. They’re often the starting point for understanding a baby's early clinical course.
A late preterm infant may look surprisingly mature while struggling with feeding or temperature regulation. A small for gestational age newborn may be at increased risk for hypoglycemia or polycythemia. A large for gestational age infant may require closer monitoring after a complicated delivery. Recognizing these patterns helps pediatricians anticipate problems before they arise.
This sample chapter from Hippo Education's MOCA-Peds 2026 eBook covers the common complications associated with SGA, LGA, and late preterm infants, with an emphasis on the concepts most likely to influence clinical decision-making — and show up on boards.
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Pearls
Both small for gestational age (SGA) and large for gestational age (LGA) infants are at high risk for neonatal hypoglycemia.
Late preterm infants (34–36 6/7 weeks) often look term but are physiologically immature, leading to feeding problems, jaundice, and respiratory instability.
Rapid catch-up growth after SGA birth increases future risk for obesity, insulin resistance, and cardiovascular disease.
Maternal diabetes and obesity are the most important modifiable risk factors for LGA birth.
Both extremes of birth size are linked to long-term cardiometabolic disease risk - a U-shaped association.
Overview of growth and maturity at birth
Birth weight and/or length < –2 SD (or <10th percentile) for gestational age
Reflects restricted intrauterine growth from maternal, placental, or fetal factors
Birth weight >90th percentile for gestational age
Macrosomia: >4,000–4,500 g at any gestational age
Most often due to maternal hyperglycemia or obesity
Birth between 34 0/7 and 36 6/7 weeks gestation
May appear mature but exhibit physiologic immaturity in feeding, thermoregulation, and respiration
Small for gestational age (SGA)
Etiology and risk factors
Maternal factors
Chronic hypertension or preeclampsia
Malnutrition or low pre-pregnancy BMI
Smoking, substance use, or chronic infection (e.g., malaria, CMV)
Thyroid dysfunction or other chronic illness
Placental factors
Placental insufficiency, infarction, or abnormal implantation
Poor uterine perfusion
Fetal factors
Chromosomal or congenital syndromes (e.g., Silver-Russell, imprinting disorders)
Congenital infections
Genetic growth disorders
Neonatal complications
Metabolic
Hypoglycemia (~33%) due to low glycogen and fat stores
Hypothermia from limited insulation and poor thermoregulation
Hematologic
Polycythemia secondary to chronic hypoxia
Feeding and growth
Poor feeding and inadequate weight gain
Other
Increased perinatal morbidity and mortality, especially if preterm
Long-term risks and complications
Growth
Delayed if also preterm
Persistent short stature (<–2.5 SD at 2 years or <–2 SD at 3–4 years) → refer to endocrinology
Growth hormone therapy is effective and safe
Metabolic
Rapid postnatal weight gain → ↑ risk for obesity, insulin resistance, and metabolic syndrome
Neurodevelopment
Higher risk of developmental delay, learning difficulties, and ADHD
Greatest risk if microcephalic or perinatal complications
Counseling
Promote balanced nutrition
Avoid excessive caloric intake or rapid catch-up growth
Close developmental surveillance and screening
Large for gestational age (LGA)
Etiology and risk factors
Maternal
Preexisting or gestational diabetes
Obesity and excessive gestational weight gain
Multiparity
Fetal/genetic
Familial tall stature
Male sex
Neonatal complications
Delivery-related
Shoulder dystocia, brachial plexus injury, clavicular fracture
Increased likelihood of cesarean or instrumental delivery
Metabolic
Hypoglycemia due to persistent fetal hyperinsulinemia
Polycythemia, hyperbilirubinemia
Respiratory
Respiratory distress, transient tachypnea, meconium aspiration
Postnatal care
Require glucose monitoring, careful feeding support, and observation for trauma
Long-term risks and complications
Metabolic
Childhood obesity, insulin resistance, type 2 diabetes
Elevated lifetime risk of cardiovascular disease
Other
Possible increased risk of certain cancers (e.g., leukemia, colon, breast)
Prevention
Optimize maternal health before and during pregnancy
Strict glycemic control and weight management reduce risk of LGA births
Late preterm infants
Definition: birth between 34 0/7 and 36 6/7 weeks gestation
Etiology and risk factors
Spontaneous preterm labor or premature rupture of membranes
Medically indicated early delivery for maternal or fetal reasons (e.g., preeclampsia, growth restriction, multiple gestation)
Appear near term but have immature organ systems (lungs, brain, liver, and immune system)
Respiratory
Transient tachypnea, respiratory distress syndrome, apnea
Feeding
Inefficient suck-swallow coordination → poor intake, dehydration, jaundice
Metabolic
Hypoglycemia from limited glycogen stores and poor feeding
Hyperbilirubinemia due to immaturity of UDP-glucuronosyltransferase
Thermoregulatory
Hypothermia due to high surface area-to-mass ratio and immature thermoregulation
Infectious
Increased susceptibility to sepsis due to immune immaturity
Hospital course
Higher rates of readmission within 2 weeks (most commonly for jaundice, poor feeding, or weight loss)
Long-term risks and complications
Neurodevelopment
Slightly increased risk of learning and behavioral difficulties compared with term peers
Growth
Usually normal long-term growth, though early feeding problems may delay initial weight gain
Follow-up
Early postdischarge follow-up (within 48 hours)
Emphasize lactation support and parental education
Practical approach for pediatricians
Identify infant’s growth category using accurate gestational age and birth weight percentiles
Anticipate risks for hypoglycemia, temperature instability, hyperbilirubinemia, and feeding problems
Monitor glucose, feeding tolerance, and growth trajectory closely
Provide clear discharge instructions and early outpatient follow-up, especially for late preterm infants
Refer to endocrinology for persistent growth failure or to developmental services if neurodevelopmental delays emerge.